High-magnification Faraday rotation imaging and analysis of X-pinch implosion dynamics

An X-pinch load driven by an intense current pulse (>100 kA in ∼100 ns) can result in the formation of a small radius, runaway compressional micro-pinch. A micro-pinch is characterized by a hot (>1 keV), current-driven (>100 kA), high-density plasma column (near solid density) with a small neck diameter (1-10 µm), a short axial extent (<1 mm), and a short duration (≲1 ns). With material pressures often well into the multi-Mbar regime, a micro-pinch plasma often radiates an intense, sub-ns burst of sub-keV to multi-keV x rays. A low-density coronal plasma immediately surrounding the dense plasma neck could potentially shunt current away from the neck and thus reduce the magnetic drive pressure applied to the neck. To study the current distribution in the coronal plasma, a Faraday rotation imaging diagnostic (1064 nm) capable of producing simultaneous high-magnification polarimetric and interferometric images has been developed for the MAIZE facility at the University of Michigan. Designed with a variable magnification (1-10×), this diagnostic achieves a spatial resolution of ∼35 µm, which is useful for resolving the ∼100-μm-scale coronal plasma immediately surrounding the dense core. This system has now been used on a reduced-output MAIZE (100-200 kA, 150 ns) to assess the radial distribution of drive current immediately surrounding the dense micro-pinch neck. The total current enclosed was found to increase as a function of radius, r, from a value of ≈50±25 kA at r ≈ 140 µm (at the edge of the dense neck) to a maximal value of ≈150±75 kA for r ≥ 225 µm. This corresponds to a peak magnetic drive pressure of ≈75±50 kbar at r ≈ 225 µm. The limitations of these measurements are discussed in the paper.

Stereotactic MR-guided on-table adaptive radiation therapy (SMART) for borderline resectable and locally advanced pancreatic cancer: A multi-center, open-label phase 2 study

BACKGROUND AND PURPOSE

Radiation dose escalation may improve local control (LC) and overall survival (OS) in select pancreatic ductal adenocarcinoma (PDAC) patients. We prospectively evaluated the safety and efficacy of ablative stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) for borderline resectable (BRPC) and locally advanced pancreas cancer (LAPC). The primary endpoint of acute grade ≥ 3 gastrointestinal (GI) toxicity definitely related to SMART was previously published with median follow-up (FU) 8.8 months from SMART. We now present more mature outcomes including OS and late toxicity.

MATERIALS AND METHODS

This prospective, multi-center, single-arm open-label phase 2 trial (NCT03621644) enrolled 136 patients (LAPC 56.6 %; BRPC 43.4 %) after ≥ 3 months of any chemotherapy without distant progression and CA19-9 ≤ 500 U/mL. SMART was delivered on a 0.35 T MR-guided system prescribed to 50 Gy in 5 fractions (biologically effective dose10 [BED10] = 100 Gy). Elective coverage was optional. Surgery and chemotherapy were permitted after SMART.

RESULTS

Mean age was 65.7 years (range, 36-85), induction FOLFIRINOX was common (81.7 %), most received elective coverage (57.4 %), and 34.6 % had surgery after SMART. Median FU was 22.9 months from diagnosis and 14.2 months from SMART, respectively. 2-year OS from diagnosis and SMART were 53.6 % and 40.5 %, respectively. Late grade ≥ 3 toxicity definitely, probably, or possibly attributed to SMART were observed in 0 %, 4.6 %, and 11.5 % patients, respectively.

CONCLUSIONS

Long-term outcomes from the phase 2 SMART trial demonstrate encouraging OS and limited severe toxicity. Additional prospective evaluation of this novel strategy is warranted.

A Multi-Institutional Phase 2 Trial of Ablative 5-Fraction Stereotactic Magnetic Resonance-Guided On-Table Adaptive Radiation Therapy for Borderline Resectable and Locally Advanced Pancreatic Cancer

PURPOSE

Magnetic resonance (MR) image guidance may facilitate safe ultrahypofractionated radiation dose escalation for inoperable pancreatic ductal adenocarcinoma. We conducted a prospective study evaluating the safety of 5-fraction Stereotactic MR-guided on-table Adaptive Radiation Therapy (SMART) for locally advanced (LAPC) and borderline resectable pancreatic cancer (BRPC).

METHODS AND MATERIALS

Patients with LAPC or BRPC were eligible for this multi-institutional, single-arm, phase 2 trial after ≥3 months of systemic therapy without evidence of distant progression. Fifty gray in 5 fractions was prescribed on a 0.35T MR-guided radiation delivery system. The primary endpoint was acute grade ≥3 gastrointestinal (GI) toxicity definitely attributed to SMART.

RESULTS

One hundred thirty-six patients (LAPC 56.6%, BRPC 43.4%) were enrolled between January 2019 and January 2022. Mean age was 65.7 (36-85) years. Head of pancreas lesions were most common (66.9%). Induction chemotherapy mostly consisted of (modified)FOLFIRINOX (65.4%) or gemcitabine/nab-paclitaxel (16.9%). Mean CA19-9 after induction chemotherapy and before SMART was 71.7 U/mL (0-468). On-table adaptive replanning was performed for 93.1% of all delivered fractions. Median follow-up from diagnosis and SMART was 16.4 and 8.8 months, respectively. The incidence of acute grade ≥3 GI toxicity possibly or probably attributed to SMART was 8.8%, including 2 postoperative deaths that were possibly related to SMART in patients who had surgery. There was no acute grade ≥3 GI toxicity definitely related to SMART. One-year overall survival from SMART was 65.0%.

CONCLUSIONS

The primary endpoint of this study was met with no acute grade ≥3 GI toxicity definitely attributed to ablative 5-fraction SMART. Although it is unclear whether SMART contributed to postoperative toxicity, we recommend caution when pursuing surgery, especially with vascular resection after SMART. Additional follow-up is ongoing to evaluate late toxicity, quality of life, and long-term efficacy.

Carbon Footprint of Photon Therapy

PURPOSE/OBJECTIVE(S)

Rising carbon dioxide levels have hazardous impact on human health and climate change. This study estimates the energy utilization of radiation therapy and estimates corresponding carbon footprint.

MATERIALS/METHODS

Patients treated between 07/2020 and 06/2021 using a Varian LINAC system were evaluated. Power draw was directly measured in 1 second increments, from the LINAC machine and the radiation department facility. Patients treated were reviewed for number of fractions and beam duration during each fraction. kWh power consumption was calculated per fraction and per treatment course. Patient commute distance was evaluated using Google Maps. EPA calculator was used to calculate CO2 equivalent (CO2e) footprint.

RESULTS

There were 176 patients treated for 191 treatment courses. Total of 4,517 fractions were delivered (avg 23.6, range 1 to 48). Average BeamOn time was 141 seconds per fraction (22 to 310 sec); electron plans (n = 8) 29 sec, 2D/3D (n = 63) 77 sec, and IMRT (n = 120) 182 sec. BeamOn power draw was ∼36.3 kW. Power consumption per fraction was 1.4 kWh (0.2 to 3.0 kWh), and per course 37.5 kWh (0.9 to 109.1 kWh). LINAC was otherwise in standby mode 71% of time (∼7.3 kW) and ON/ready mode 25% (∼12.5 kW). Patient BeamOn time accounted for 2% of total its time and 8% of its power use during the year. Incremental standby/ready energy contribution per treatment course was 410 kWh (78,332/191). The building power draw was 12.5 kW, for incremental contribution of 576 kWh per course. Average patient travel distance was 9.6 miles (0.7 miles to 73.5 miles), resulting in 711-mile round trip per course (6 miles to 5620 miles). Corresponding carbon footprint for BeamOn time was 0.6 kg CO2e per fraction (electrons 0.1 kg, 2D/3D 0.3 kg, and IMRT 1.1 kg per fx). The treatment course footprint was 16.2 kg CO2e (0.4 kg to 47.2 kg). The attributed standby/ready LINAC footprint was 177.8 kg CO2e per course. The attributed facility footprint was further 250.0 kg CO2e per course. The attributed commute footprint was further 286.3 kg CO23 per course (2.4 kg to 2264.0 kg). The entire treatment course was 730.4 kg CO2e. As such, single fraction was 0.1% of the total CO2e course (0.01% to 0.27%), BeamOn was 2.3% (0.1% to 7.1%), LINAC standby/ready was 28.0% (6.5% to 41.3%), building was 39.4% (9.2% to 58%), and commute was 30.3% (0.6% to 82.9%). The entire program generated 139.5 metric tons of CO2e during the year.

CONCLUSION

Linear accelerator BeamOn carbon footprint varied as a function of technique but was only 2% of the overall patient treatment footprint. The LINAC standby electricity contributed 28%, building electricity 39%, and commute 30%, though this varied significantly per patient, as a function of number of fractions and treatment distance. Fixed CO2e contribution from the standby LINAC and the building draw accounted for ∼67% of the footprint. However, radiation oncologists can potentially impact the overall carbon footprint by prescribing fewer fractions to lower total commute, as clinically indicated.

Defining the Role of Intensity Modulation in Electron Conformal Therapy for the Treatment of Head and Neck Cancer

PURPOSE/OBJECTIVE(S)

The use of bolus electron conformal therapy (BECT) in the treatment of cancers of the head and neck is often limited by an inability to reduce dosimetric hot spots resulting from surface or tissue heterogeneity. In this study, we examined the potential benefits of using a recently patented, low-cost form of Intensity Modulation for electron therapy (IM-BECT) to reduce treatment hotspots in patients undergoing electron beam therapy for cancer of the Head and Neck (HN).

MATERIALS/METHODS

The treatment plans from twelve patients with HN cancer previously treated with BECT were identified (treatment energies ranged from 6-18 MeV and field sizes of 36-625 cm2). Each case included the treatment targets and at least one primary OAR that were defined by the original treating radiation oncologist. Additionally, a target + 2 cm rind structure was created for analysis of the dose deposition in areas immediately surrounding the target volume as a measure of conformality. Each patient plan was transferred into a novel IM-BECT planning software and each case was recomputed as per the original prescription, gantry, couch, collimator angles, and original clinically used bolus. Next, each case was replanned with the inclusion of intensity modulation, as well as a new custom conformal bolus that was designed for optimized range compensation. The patient plans were then normalized such that 100% equals the prescription dose value and then transferred to a plan analysis software for comparison of the target coverage/dose and OAR dose.

RESULTS

Comparison of the BECT and IM-BECT treatment plans demonstrated that IM-BECT was able to significantly reduce dosimetric hotspots for this cohort of patients undergoing radiation therapy for HN cancer, bringing the average maximum dose down from 130.6% to only 120.6% (p = 0.044, paired t-test). Moreover, the impact of IM-BECT appeared to be most substantial in the patients with the highest baseline maximum dose. For patients who had a hotspot of 125% or greater, the hotspot was on average reduced by 19%. Further dosimetric analysis demonstrated a small resultant increase in the low dose deposition to the surrounding normal tissues. For BECT, the average primary OAR mean dose and Target+2cm rind mean dose were 27.5% and 60.0%, respectively. For IM-BECT, the average primary OAR mean dose and Target+2cm rind mean dose increased slightly to 30.9% and 64.6%, respectively [Primary OAR mean (P = 0.0008), and Target+2cm rind mean (P = 0.0001), paired t-test].

CONCLUSION

IM-BECT is an effective method of reducing dosimetric hotspots in patients undergoing radiation therapy for cancer of the HN. This improvement came at the expense of a small increase in dose to the underlying tissues. This retrospective planning study represents the first example of IM-BECT being applied to real patient cases and suggests further development of IM-BECT is warranted.

Comparison of the Direct Power Consumption for Different Linac Treatment Parameters in External Beam Radiation Therapy

PURPOSE/OBJECTIVE(S)

The negative impact of climate change on the environment has led to increasing concern around the world. Relatively little is known about the contribution of radiation oncology systems to carbon emissions. This study measures the energy utilization of Linac-based EBRT for different treatment parameters and techniques, which was then converted to power use (kWh) and ultimately a carbon footprint of various photon treatment approaches.

MATERIALS/METHODS

A Varian TrueBeam was evaluated. The direct power consumption was measured using a Fluke 1736 Power Logger, recorded in one-second intervals for one week. Different photon and electron beam energies, dose rates, treatment techniques (3D, IMRT, gated), and imaging types (kV, MV) were evaluated. Clinical treatment plans were reviewed, and average treatment times used to determine kWh. IRB approval was obtained. The Greenhouse Gas Equivalencies Calculator was used to determine kg of CO2.

RESULTS

Power draw for 6 MV, 10 MV, and 18 MV at a rep rate of 600 MU/min was 31.6, 32.0, and 27.5 kW, respectively. The power draw for end of range dose rates for 6 MV (60, 600), 6 FFF (400, 1400), 10 FFF (400, 2400), and 6 MeV (100, 1000) were (22.2, 31.6 kW), (23.7, 31.6 kW), (19.6, 32.1 kW), and (22.6, 23.6 kW), respectively. 6 MV open 2D fields, modulated IMRT fields, and gated/beam hold fields had similar power draw at 31.6, 31.7, and 31.9 kW, respectively. Portal MV imaging with 2.5 MV beam was 32.5 kW, while CBCT and kV/kV imaging was ∼11 kW and not distinguishable from baseline power fluctuations. A total of 191 delivered treatment plans were reviewed. Electron plans (n = 8) treatment time was on average 29 sec (16 sec STDEV) per fraction and 635 sec (185 sec STDEV) per course, 2D/3D plans (n = 63) were 77 sec (32 sec STDEV) and 1004 sec (1002 sec STDEV), and IMRT plans (n = 120) were 182 sec (60 sec STDEV) and 5351 sec (2401 sec STDEV), respectively. The kWh per treatment for electron, 2D/3D, and IMRT plans were 0.19, 0.68, and 1.6 kWh, respectively. The CO2 equivalent for electrons, 2D/3D, and IMRT techniques are 0.08, 0.29, and 0.7 kg.

CONCLUSION

There was some variability in the power draw for different energies and different dose rates but was relatively stable around 32 kW. Power consumption for clinical therapy is a result of kW power draw multiplied by duration of beam delivery, which for our patients varied significantly as a function of technique and number of fractions delivered. Reduction in radiation oncology carbon footprint will likely be driven more by number of fractions and type of treatment technique, and length of patient commute, rather than beam energy and dose rate selection.

Adapting to the Adaptive Radiation Workflow: Incorporating Video Sign Out for Improved Safety and Efficiency as Part of Magnetic Resonance Image Guided Adaptive Radiation

 : Magnetic resonance image guided adaptive radiation therapy (MRgART) represents a significant improvement in our ability to deliver therapeutic radiation. However, for the process of MRgART to be carried out safely and efficiently, the covering radiation oncologist must be aware of all aspects of a patient's case, because they will be required to recontour and replan the patient before each treatment. In this report, we will demonstrate our initial experience with a video sign-out process to convey the detailed level of information required for the covering physician to treat patients safely and effectively with MRgART. We then describe our optimized video sign-out process to allow for other centers to adopt a similar approach.

Research productivity of radiation therapy physics faculty in the United States

Purpose

Research productivity metrics are important for decisions regarding hiring, retention, and promotion in academic medicine, and these metrics can vary widely among different disciplines. This article examines productivity metrics for radiation therapy physicists (RTP) in the United States.

Methods and materials

Database searches were performed for RTP faculty at US institutions that have RTP residencies accredited by the Commission on Accreditation of Medical Physics Education Programs (CAMPEP). Demographics, academic rank, number of publications, academic career length, Hirsch index (h‐index), m‐quotient, and history of National Institutes of Health (NIH) funding as a principal investigator (PI) were collected for each RTP. Logistic regression was performed to determine the probability of academic rank as a function of h‐index and m‐quotient. Statistical tests used included the Wilcoxon ranked sum test and the Pearson χ² test.

Results

A total of 1038 faculty and staff were identified at 78 institutions with CAMPEP‐accredited residencies. The average RTP academic career duration is 13.5 years, with 46.7 total publications, h‐index of 10.7, and m‐quotient of 0.66. Additionally, 10.5% of RTP have a history of NIH funding as a PI. Large disparities were found in academic productivity of doctoral‐prepared physicists compared to those with a terminal master's degree. For differences in junior and senior faculty, statistical tests yielded significance in career duration, number of publications, h‐index, and m‐quotient. Gender disparities were identified in the overall distribution of RTP consistent with the membership of the American Association of Physicists in Medicine. Further gender disparities were found in the number of doctoral‐prepared RTP and physicists in senior faculty roles.

Conclusions

This manuscript provides objective benchmark data regarding research productivity of academic RTP. These data may be of interest to faculty preparing for promotion, and also to institutional leadership.

Retrospective comparison of percutaneous balloon pericardiotomy with pericardiocentesis versus pericardiocentesis alone for management of symptomatic pericardial effusions

Background

Cardiac tamponade is a potentially life-threatening complication of pericardial effusion. Pericardiocentesis with drainage is the mainstay of treatment for patients with pericardial effusion and cardiac tamponade. Percutaneous balloon pericardiotomy (PBP) is an adjunct to pericardiocentesis that may alleviate the risk for recurrent effusion and repeat procedures. However, the efficacy of PBP plus pericardiocentesis compared to pericardiocentesis alone is not clear.

Purpose

We sought to determine whether PBP plus pericardiocentesis was associated with less recurrence of pericardial effusion than pericardiocentesis alone.

Methods

We conducted a single-centre retrospective analysis of patients ≥18 years old with non-iatrogenic pericardial effusion undergoing either pericardiocentesis alone or PBP plus pericardiocentesis for the first time. For PBP, a balloon was advanced over a guidewire until it crossed the pericardium and was then inflated until the balloon profile was fully expanded. Type of balloon used, and single or double balloon technique were left up to the operator. Recurrent pericardial effusion was defined as a large pericardial effusion on echocardiogram, pericardial effusion that caused hemodynamic compromise, or pericardial effusion that necessitated another intervention to drain at any time after initial procedure. Risk factors for recurrent pericardial effusion were also assessed.

Results

There were 208 patients who underwent pericardiocentesis, with 33 patients receiving PBP plus pericardiocentesis. In all patients, the rate of recurrent pericardial effusion was 15.9% and 15.2%, respectively (p=0.92). In patients with a cancer diagnosis at time of procedure, the rate of recurrent pericardial effusion was 17.8% and 16.7%, respectively (p=0.89). In patients with malignant pericardial effusion as confirmed by cytology, the rate of recurrent pericardial effusion was 20.4% and 13.3%, respectively (p=0.72). Patients with a connective tissue disease (CTD) had an increased odds ratio (OR) of recurrent pericardial effusion when compared to patients without a CTD (OR 3.19, 95% CI 1.31–7.77).

Conclusions

The results of this study suggest that PBP plus pericardiocentesis offers no significant benefit over pericardiocentesis alone at preventing recurrent pericardial effusion. This finding was true in all sub-groups, including patients with cancer and patients with malignant pericardial effusion. Patients with a CTD were three times more likely than patients without a CTD of having a recurrent pericardial effusion, independent of treatment strategy.

Funding Acknowledgement

Type of funding sources: None.

1253 Unlocking Growth Options in Surgical Education and Training During the Pandemic

Introduction

The COVID-19 pandemic brought widespread disruption to structured surgical education and training. The knee-jerk reaction is often pessimism about surgical training’s future, particularly in the Improved Surgical Training (IST) pilot’s context. However, Einstein famously once said, “In the midst of every crises lies great opportunity”. Unlocking growth during periods of high uncertainty is a premise of real options theory; one utilised by supply chain managers and decision scientists, but novel to medical education. This study explores the growth options that have resulted from new operational models during the pandemic.

Method

Using a qualitative case study approach, data were obtained from interviews with core surgical trainees across Scotland. Data coding and inductive thematic analysis were undertaken.

Results

Forty-six trainees participated. Analysis from trainees’ perspective revealed: unexpected fulfilment from redeployment to non-surgical specialties, benefits to personal development from the unintended broad-based training across surgical specialties, improved collaborative teamworking between specialties and allied healthcare professionals, and enhanced supervised learning opportunities. Institutional growth options reported by trainees included: rapid uptake of telemedicine and digital technology, implementation of single hospital episode encounters for minor conditions, streamlined processes in theatre and acute admissions, and changes in working culture towards rationalising and teamworking.

Conclusions

Growth options have been deliberately and unintentionally unlocked due to individual and institutional adaptions and innovations in response to the exogenous disruption. While some changes may be temporary, hopefully structured reflection on these changes and responders to them will drive surgical education and training into a new sustainable and resilient post-pandemic era.

Impact of Radiation Oncology Alternative Payment Model on Community Cancer Centers

PURPOSE

An episode-based payment model, the Radiation Oncology Alternative Payment Model (RO-APM), is scheduled to go into effect in January 2022. This article investigates the effects of RO-APM on hospital-based and freestanding community centers.

METHODS

Historical Medicare data used to generate the RO-APM base rates were reviewed. A sensitivity analysis was performed to show how the RO-APM reimbursements compare with current reimbursements for commonly accepted treatment schedules and with current reimbursements at a large community practice.

RESULTS

The RO-APM base rates represent a 2.2% decrease in overall Medicare reimbursement. Freestanding centers have historically billed at higher rates than hospital-based centers, however, and the RO-APM base rates represent a 6% decrease in global reimbursement for freestanding centers. The sensitivity analysis showed that, except for proton therapy, moderately hypofractionated treatment schedules will receive comparable reimbursement under RO-APM. Treatments using higher numbers of fractions of intensity-modulated radiation therapy or protons will see larger decreases in reimbursement. Application of the RO-APM base rates to the 2020 Medicare treatments in our health care network would result in small changes in expected reimbursement, but our sensitivity analysis indicated that Medicare reimbursement reductions could be as large as 23%.

CONCLUSION

Compared with historical Medicare reimbursement, RO-APM base rates provide lower reimbursement for many common treatment scenarios, and this will have a larger effect on centers that use complex treatment techniques and longer fractionation schedules or have a large Medicare population.

Variability in commercially available deformable image registration: A multi-institution analysis using virtual head and neck phantoms

PURPOSE

The purpose of this study was to evaluate the performance of three common deformable image registration (DIR) packages across algorithms and institutions.

METHODS AND MATERIALS

The Deformable Image Registration Evaluation Project (DIREP) provides ten virtual phantoms derived from computed tomography (CT) datasets of head-and-neck cancer patients over a single treatment course. Using the DIREP phantoms, DIR results from 35 institutions were submitted using either Velocity, MIM, or Eclipse. Submitted deformation vector fields (DVFs) were compared to ground-truth DVFs to calculate target registration error (TRE) for six regions of interest (ROIs). Statistical analysis was performed to determine the variability between each DIR software package and the variability of users within each algorithm.

RESULTS

Overall mean TRE was 2.04 ± 0.35 mm for Velocity, 1.10 ± 0.29 mm for MIM, and 2.35 ± 0.15 mm for Eclipse. The MIM mean TRE was significantly different than both Velocity and Eclipse for all ROIs. Velocity and Eclipse mean TREs were not significantly different except for when evaluating the registration of the cord or mandible. Significant differences between institutions were found for the MIM and Velocity platforms. However, these differences could be explained by variations in Velocity DIR parameters and MIM software versions.

CONCLUSIONS

Average TRE was shown to be <3 mm for all three software platforms. However, maximum errors could be larger than 2 cm indicating that care should be exercised when using DIR. While MIM performed statistically better than the other packages, all evaluated algorithms had an average TRE better than the largest voxel dimension. For the phantoms studied here, significant differences between algorithm users were minimal suggesting that the algorithm used may have more impact on DIR accuracy than the particular registration technique employed. A significant difference in TRE was discovered between MIM versions showing that DIR QA should be performed after software upgrades as recommended by TG-132.

Whole brain radiotherapy with hippocampal sparing using Varian HyperArc

The purpose of this work was to evaluate using Varian HyperArc as a planning and treatment solution for whole brain radiotherapy (WBRT) with hippocampal sparing following Radiation Therapy Oncology Group (RTOG) 0933 dosimetric criteria. Ten patients previously treated for intracranial lesions were retrospectively planned for WBRT with hippocampal sparing using HyperArc and a 2-arc coplanar VMAT technique. The whole brain and hippocampus were delineated on fused MRI and CT datasets. The planning target volume (PTV), defined as the whole brain excluding the hippocampal avoidance region, was prescribed 30 Gy in 10 fractions. Plans were evaluated using dosimetric parameters which included the volume of 105% of the prescription dose (V105%) and the maximum dose to the PTV, and the minimum dose to the hippocampus. The planning time, delivery time, and delivery quality assurance (QA) results were also evaluated. Statistical significance was performed between the HyperArc and coplanar VMAT metrics using the Wilcoxon signed-rank test with a significance level of 0.05. All plans met RTOG 0933 dosimetric criteria. HyperArc plans demonstrated significant improvements in PTV dosimetric quality which included a reduced V105% of 6 ± 7% and decreased maximum dose of 1.3 ± 0.3 Gy, compared to coplanar VMAT. Significant OAR sparing was also found for HyperArc plans that included a decreased minimum dose to the hippocampus of 0.3 ± 0.3 Gy. Coplanar VMAT plans resulted in significantly shorter planning and delivery times, compared to HyperArc, by 2.4 minutes and 1.5 minutes, respectively. No significant difference was found between the delivery QA results. This study demonstrated using Varian HyperArc as a planning and treatment solution for WBRT with hippocampal sparing following RTOG 0933 dosimetric criteria. The primary advantages of WBRT with hippocampal sparing using HyperArc, compared to coplanar VMAT, are the gains in OAR sparing and reduced high dose volumes to the PTV.

Evaluation of cine imaging during multileaf collimator and gantry motion for real-time magnetic resonance guided radiation therapy

Purpose

Real‐time magnetic resonance guided radiation therapy (MRgRT) uses 2D cine imaging for target tracking. This work evaluates the percent image uniformity (PIU) and spatial integrity of cine images in the presence of multileaf collimator (MLC) and gantry motion in order to simulate sliding window and volumetric modulated arc therapy (VMAT) conditions.

Methods

Percent image uniformity and spatial integrity of cine images were measured (1) during MLC motion, (2) as a function of static gantry position, and (3) during gantry rotation. PIU was calculated according to the ACR MRI Quality Control Manual. Spatial integrity was evaluated by measuring the geometric distortion of 16 measured marker positions (10 cm or 15.225 cm from isocenter).

Results

The PIU of cine images did not vary by more than 1% from static linac conditions during MLC motion and did not vary by more than 3% during gantry rotation. Banding artifacts were present during gantry rotation. The geometric distortion in the cine images was less than 0.88 mm for all points measured throughout MLC motion. For all static gantry positions, the geometric distortion was less than 0.88 mm at 10 cm from isocenter and less than 1.4 mm at 15.225 cm from isocenter. During gantry rotation, the geometric distortion remained less than 0.92 mm at 10 cm from isocenter and less than 1.60 mm at 15.225 cm from isocenter.

Conclusion

During MLC motion, cine images maintained adequate PIU, and the geometric distortion of points within 15.225 cm from isocenter was less than the 1 mm threshold necessary for real‐time target tracking and gating. During gantry rotation, PIU was negatively affected by banding artifacts, and spatial integrity was only maintained within 10 cm from isocenter. Future work should investigate the effects imaging artifacts have on real‐time target tracking during MRgRT.

Effect of Proposed Episode-Based Payment Models on Advanced Radiotherapy Procedures

PURPOSE

An episode-based payment model, the Radiation Oncology Alternative Payment Model (RO-APM), has been proposed for Medicare reimbursement of radiation services provided to oncology patients. RO-APM may have significant impact on reimbursement for specific patient populations.

METHODS

This investigation compares historical fee-for-service technical reimbursement estimates at a large hospital-based system to the RO-APM for advanced radiotherapy treatment of specific cancer types. These advanced techniques, stereotactic radiosurgery (SRS), stereotactic body radiotherapy (SBRT), online-adaptive SBRT, and proton therapy, were specifically chosen because they are resource intensive and are correspondingly among the most expensive radiation oncology procedures. A total of 203 Medicare patients were analyzed.

RESULTS

RO-APM base-rate reimbursements were similar for SRS and were 38%-47% higher for SBRT. The proposed rates were 1%-31% lower for online-adaptive SBRT, and 48%-71% lower for proton therapy.

CONCLUSION

These data suggest that the RO-APM may have the desired effect of encouraging shorter courses of radiotherapy, such as SBRT. However, emerging technologies that require large capital and operating investments may see an overall significant reduction in proposed reimbursement.

Intrafraction motion during frameless radiosurgery using Varian HyperArcTM and BrainLab ElementsTM immobilization systems

Commercial systems such as Varian HyperArcTM and BrainLab Elements MultiMetTM have been developed that allow radiosurgery treatment of multiple brain metastases using a single isocenter. Each software package places increased demands on frameless immobilization and requires the use of a specific immobilization system: the QFix-Encompass system for Varian and the BrainLab frameless-mask system for BrainLab. At our institution, patients receiving traditional radiosurgery (one isocenter per target lesion) were treated using both immobilization systems. Intrafraction motion was determined for each patient using multiple cone-beam CT scans and the same image-registration software during treatment. There were no statistically-significant differences in mean absolute translational shifts between the two mask systems, with a mean 3D-vector motion of approximately 0.43 mm for both systems. There were also no statistically-significant differences in the mean absolute rotational shifts between the two mask systems. Although the average residual errors were insignificant between the mask systems, special attention should be paid to individual maximum shifts with both systems. Large maximum rotational misalignments could present significant misalignment of lesions as distance increases from the isocenter. Finally, large maximum shifts highlight the need for real-time monitoring of patient movement during radiosurgery of multiple lesions using a single isocenter.

Optimization of switch diagnostics on the MAIZE linear transformer driver

The MAIZE Linear Transformer Driver consists of 40 capacitor-switch-capacitor "bricks" connected in parallel. When these 40 bricks are charged to ±100-kV and then discharged synchronously, the MAIZE facility generates a 1-MA current pulse with a 100-ns rise time into a matched load impedance. Discharging each of the capacitors in a brick is carried out by the breakdown of a spark-gap switch, a process that results in the emission of light. Monitoring this output light with a fiber optic coupled to a photomultiplier tube (PMT) and an oscilloscope channel provides information on switch performance and timing jitter-whether a switch fired early, late, or in phase with the other switches. However, monitoring each switch with a dedicated detector-oscilloscope channel can be problematic for facilities where the number of switches to be monitored (e.g., 40 on MAIZE) greatly exceeds the number of detector-oscilloscope channels available. The technique of using fibers to monitor light emission from switches can be optimized by treating a PMT as a binary digit or bit and using a combinatorial encoding scheme, where each switch is monitored by a unique combination of fiber-PMT-oscilloscope channels simultaneously. By observing the unique combination of fiber-PMT-oscilloscope channels that are turned on, the prefiring or late-firing of a single switch on MAIZE can be identified by as few as six PMT-oscilloscope channels. The number of PMT-oscilloscope channels, N, required to monitor X switches can be calculated by 2^N = X + 1, where the number "2" is selected because the PMT-oscilloscope acts as a bit. In this paper, we demonstrate the use of this diagnostic technique on MAIZE. We also present an analysis of how this technique could be scaled to monitor the tens of thousands of switches proposed for various next generation pulsed power facilities.

Commissioning an in-room mobile CT for adaptive proton therapy with a compact proton system

Purpose

To describe the commissioning of AIRO mobile CT system (AIRO) for adaptive proton therapy on a compact double scattering proton therapy system.

Methods

A Gammex phantom was scanned with varying plug patterns, table heights, and mAs on a CT simulator (CT Sim) and on the AIRO. AIRO‐specific CT‐stopping power ratio (SPR) curves were created with a commonly used stoichiometric method using the Gammex phantom. A RANDO anthropomorphic thorax, pelvis, and head phantom, and a CIRS thorax and head phantom were scanned on the CT Sim and AIRO. Clinically realistic treatment plans and nonclinical plans were generated on the CT Sim images and subsequently copied onto the AIRO CT scans for dose recalculation and comparison for various AIRO SPR curves. Gamma analysis was used to evaluate dosimetric deviation between both plans.

Results

AIRO CT values skewed toward solid water when plugs were scanned surrounded by other plugs in phantom. Low‐density materials demonstrated largest differences. Dose calculated on AIRO CT scans with stoichiometric‐based SPR curves produced over‐ranged proton beams when large volumes of low‐density material were in the path of the beam. To create equivalent dose distributions on both data sets, the AIRO SPR curve's low‐density data points were iteratively adjusted to yield better proton beam range agreement based on isodose lines. Comparison of the stoichiometric‐based AIRO SPR curve and the “dose‐adjusted” SPR curve showed slight improvement on gamma analysis between the treatment plan and the AIRO plan for single‐field plans at the 1%, 1 mm level, but did not affect clinical plans indicating that HU number differences between the CT Sim and AIRO did not affect dose calculations for robust clinical beam arrangements.

Conclusion

Based on this study, we believe the AIRO can be used offline for adaptive proton therapy on a compact double scattering proton therapy system.

The Mobius AIRO mobile CT for image-guided proton therapy: Characterization & commissioning

PURPOSE

The purpose of this study was to characterize the Mobius AIRO Mobile CT System for localization and image-guided proton therapy. This is the first known application of the AIRO for proton therapy.

METHODS

Five CT images of a Catphan® 504 phantom were acquired on the AIRO Mobile CT System, Varian EDGE radiosurgery system cone beam CT (CBCT), Philips Brilliance Big Bore 16 slice CT simulator, and Siemens SOMATOM Definition AS 20 slice CT simulator. DoseLAB software v.6.6 was utilized for image quality analysis. Modulation transfer function, scaling discrepancy, geometric distortion, spatial resolution, overall uniformity, minimum uniformity, contrast, high CNR, and maximum HU deviation were acquired. Low CNR was acquired manually using the CTP515 module. Localization accuracy and CT Dose Index were measured and compared to reported values on each imaging device. For treatment delivery systems (Edge and Mevion), the localization accuracy of the 3D imaging systems were compared to 2D imaging systems on each system.

RESULTS

The AIRO spatial resolution was 0.21 lp mm-1 compared with 0.40 lp mm-1 for the Philips CT Simulator, 0.37 lp mm-1 for the Edge CBCT, and 0.35 lp mm-1 for the Siemens CT Simulator. AIRO/Siemens and AIRO/Philips differences exceeded 100% for scaling discrepancy (191.2% and 145.8%). The AIRO exhibited higher dose (>27 mGy) than the Philips CT Simulator. Localization accuracy (based on the MIMI phantom) was 0.6° and 0.5 mm. Localization accuracy (based on Stereophan) demonstrated maximum AIRO-kV/kV shift differences of 0.1 mm in the x-direction, 0.1 mm in the y-direction, and 0.2 mm in the z-direction.

CONCLUSIONS

The localization accuracy of AIRO was determined to be within 0.6° and 0.5 mm despite its slightly lower image quality overall compared to other CT imaging systems at our institution. Based on our study, the Mobile AIRO CT system can be utilized accurately and reliably for image-guided proton therapy.

Orthogonal image pairs coupled with OSMS for noncoplanar beam angle, intracranial, single-isocenter, SRS treatments with multiple targets on the Varian Edge radiosurgery system

Purpose

To characterize the accuracy of noncoplanar image guided radiation therapy with the Varian Edge radiosurgery system for intracranial stereotactic radiosurgery (SRS) treatments by assessing the accuracy of kV/kV orthogonal pair registration with Optical Surface Monitoring System (OSMS) monitoring relative to cone beam computed tomography (CT).

Methods and materials

A Computerized Imaging Reference System head phantom and Encompass SRS Immobilization System were used to determine collision-free space for orthogonal image pairs (kV/kV) for couch rotations (CRs) of 45°, 30°, 15°, 345°, 330°, and 315°. Couch-induced shifts were measured using kV/kV orthogonal image pairs, OSMS, and cone beam CT. The kV/kV image pairs and OSMS localization accuracy was also assessed with respect to cone beam CT.

Results

Mean orthogonal image pair differences for 315°, 330°, 345°, 15°, 30°, and 45° CRs were ≤±0.60 mm and ±0.37°. OSMS localization accuracy was ≤±0.25 mm and ±0.20°. Correspondingly, kV/kV localization accuracy was ≤±0.30 mm and ±0.5°. Shift differences for various image pairs at all CRs were ≤±1.10 mm and ±0.7°. Cone beam CT deviation was 0.10 mm and 0.00° without patient motion or CR.

Conclusion

Based on our study, CR-induced shifts with the Varian Edge radiosurgery system will not produce noticeable dosimetric effects for SRS treatments. Thus, replacing cone beam CT with orthogonal kV/kV pairs coupled with OSMS at the treatment couch angle could reduce the number of cone beam CT scans that are acquired during a standard SRS treatment while providing an accurate and safe treatment with negligible dosimetric effects on the treatment plan.

Benchmarking of five commercial deformable image registration algorithms for head and neck patients

Benchmarking is a process in which standardized tests are used to assess system performance. The data produced in the process are important for comparative purposes, particularly when considering the implementation and quality assurance of DIR algorithms. In this work, five commercial DIR algorithms (MIM, Velocity, RayStation, Pinnacle, and Eclipse) were benchmarked using a set of 10 virtual phantoms. The phantoms were previously developed based on CT data collected from real head and neck patients. Each phantom includes a start of treatment CT dataset, an end of treatment CT dataset, and the ground‐truth deformation vector field (DVF) which links them together. These virtual phantoms were imported into the commercial systems and registered through a deformable process. The resulting DVFs were compared to the ground‐truth DVF to determine the target registration error (TRE) at every voxel within the image set. Real treatment plans were also recalculated on each end of treatment CT dataset and the dose transferred according to both the ground‐truth and test DVFs. Dosimetric changes were assessed, and TRE was correlated with changes in the DVH of individual structures. In the first part of the study, results show mean TRE on the order of 0.5 mm to 3 mm for all phantoms and ROIs. In certain instances, however, misregistrations were encountered which produced mean and max errors up to 6.8 mm and 22 mm, respectively. In the second part of the study, dosimetric error was found to be strongly correlated with TRE in the brainstem, but weakly correlated with TRE in the spinal cord. Several interesting cases were assessed which highlight the interplay between the direction and magnitude of TRE and the dose distribution, including the slope of dosimetric gradients and the distance to critical structures. This information can be used to help clinicians better implement and test their algorithms, and also understand the strengths and weaknesses of a dose adaptive approach.

PACS number(s): 87.57.nj, 87.55.dk, 87.55.Qr

Role of TrkB in the anxiolytic-like and antidepressant-like effects of vagal nerve stimulation: Comparison with desipramine

A current hypothesis regarding the mechanism of antidepressant (AD) action suggests the involvement of brain-derived neurotrophic factor (BDNF). Consistent with this hypothesis, the receptor for BDNF (and neurotrophin 4/5 (NT-4/5)), Tropomyosin-related kinase B (TrkB), is activated in rodents by treatment with classical AD drugs. Vagal nerve stimulation (VNS), a therapy for treatment resistant depression (TRD), also activates TrkB in rodents. However, the role of this receptor in the therapeutic effects of VNS is unclear. In the current study, the involvement of TrkB in the effects of VNS was investigated in rats using its inhibitor, K252a. Anxiolytic-like and AD-like effects were analyzed using the novelty suppressed feeding test (NSFT) and forced swim test (FST), respectively. K252a blocked the anxiolytic-like effect of chronic VNS treatment and the AD-like effect of acute VNS treatment. By contrast, blocking TrkB did not prevent either the anxiolytic-like or AD-like effect of chronic treatment with desipramine (DMI), a selective noradrenergic reuptake inhibitor; it did, however, block the acute effect of DMI in the FST. To examine whether the activation of TrkB caused by either VNS or DMI is ligand-dependent, use was made of TrkB-Fc, a molecular scavenger for ligands of TrkB. Intraventricular administration of TrkB-Fc blocked the acute activation of TrkB induced by either treatment, indicating that treatment-induced activation of this receptor is ligand-dependent. The behavioral results highlight differences in the involvement of TrkB in the chronic effects of an AD drug and a stimulation therapy as well as its role in acute versus chronic effects of DMI.

A margin-based analysis of the dosimetric impact of motion on step-and-shoot IMRT lung plans

Purpose

Intrafraction motion during step-and-shoot (SNS) IMRT is known to affect the target dosimetry by a combination of dose blurring and interplay effects. These effects are typically managed by adding a margin around the target. A quantitative analysis was performed, assessing the relationship between target motion, margin size, and target dosimetry with the goal of introducing new margin recipes.

Methods

A computational algorithm was used to calculate 1,174 motion-encoded dose distributions and DVHs within the patient’s CT dataset. Sinusoidal motion tracks were used simulating intrafraction motion for nine lung tumor patients, each with multiple margin sizes.

Results

D_95% decreased by less than 3% when the maximum target displacement beyond the margin experienced motion less than 5 mm in the superior-inferior direction and 15 mm in the anterior-posterior direction. For target displacements greater than this, D_95% decreased rapidly.

Conclusions

Targets moving in excess of 5 mm outside the margin can cause significant changes to the target. D_95% decreased by up to 20% with target motion 10 mm outside the margin, with underdosing primarily limited to the target periphery. Multi-fractionated treatments were found to exacerbate target under-coverage. Margins several millimeters smaller than the maximum target displacement provided acceptable motion protection, while also allowing for reduced normal tissue morbidity.

Real-time tumor tracking in the lung using an electromagnetic tracking system

PURPOSE

To describe the first use of the commercially available Calypso 4D Localization System in the lung.

METHODS AND MATERIALS

Under an institutional review board-approved protocol and an investigational device exemption from the US Food and Drug Administration, the Calypso system was used with nonclinical methods to acquire real-time 4-dimensional lung tumor tracks for 7 lung cancer patients. The aims of the study were to investigate (1) the potential for bronchoscopic implantation; (2) the stability of smooth-surface beacon transponders (transponders) after implantation; and (3) the ability to acquire tracking information within the lung. Electromagnetic tracking was not used for any clinical decision making and could only be performed before any radiation delivery in a research setting. All motion tracks for each patient were reviewed, and values of the average displacement, amplitude of motion, period, and associated correlation to a sinusoidal model (R(2)) were tabulated for all 42 tracks.

RESULTS

For all 7 patients at least 1 transponder was successfully implanted. To assist in securing the transponder at the tumor site, it was necessary to implant a secondary fiducial for most transponders owing to the transponder's smooth surface. For 3 patients, insertion into the lung proved difficult, with only 1 transponder remaining fixed during implantation. One patient developed a pneumothorax after implantation of the secondary fiducial. Once implanted, 13 of 14 transponders remained stable within the lung and were successfully tracked with the tracking system.

CONCLUSIONS

Our initial experience with electromagnetic guidance within the lung demonstrates that transponder implantation and tracking is achievable though not clinically available. This research investigation proved that lung tumor motion exhibits large variations from fraction to fraction within a single patient and that improvements to both transponder and tracking system are still necessary to create a clinical daily-use system to assist with actual lung radiation therapy.

Clinical evaluation of interfractional variations for whole breast radiotherapy using 3-dimensional surface imaging

PURPOSE

To evaluate the impact of 3-dimensional (3D) surface imaging on daily patient setup for breast radiotherapy.

MATERIALS AND METHODS

Fifty patients undergoing treatment for whole breast radiotherapy were setup daily using an AlignRT system (VisionRT, London, UK) for 3D surface-based alignment. Daily alignments were performed against a reference surface topogram and shifts from skin marks were recorded daily. This investigation evaluated the following: (1) the performance of the surface-based imaging system for daily breast alignment; (2) the absolute displacements between setup with skin marks and setup with the surface-based imaging system; and (3) the dosimetric effect of daily alignments with skin marks versus surface-based alignments.

RESULTS

Displacements from 1258 treatment fractions were analyzed. Sixty percent of those fractions (749) were reviewed against MV portal imaging in order to assess the performance of the AlignRT system. Daily setup errors were given as absolute displacements, comparing setup marks against shifts determined using the surface-based imaging system. Averaged over all patients, the mean displacements were 4.1 ± 2.6 mm, 2.7 ± 1.4 mm, and 2.6 ± 1.2 mm in the anteroposterior (AP), superoinferior (S/I), and left-right (L/R) directions, respectively. Furthermore, the standard deviation of the random error (σ) was 3.2 mm, 2.2 mm, and 2.2 mm in the A/P, S/I, and L/R directions, respectively.

CONCLUSIONS

Daily alignment with 3D surface imaging was found to be valuable for reducing setup errors when comparing with patient alignment from skin marks. The result of the surface-based alignments specifically showed that alignment with skin marks was noticeably poor in the anteroposterior directions. The overall dosimetric effect of the interfractional variations was small, but these variations showed a potential for increased dose deposition to both the heart and lung tissues. Although these interfractional variations would not negatively affect the quality of patient care for whole breast radiotherapy, it may require an increase in PTV margin, especially in cases of partial breast irradiation.

An endorectal balloon reduces intrafraction prostate motion during radiotherapy

PURPOSE

To investigate the effect of endorectal balloons (ERBs) on intrafraction and interfraction prostate motion during radiotherapy.

METHODS AND MATERIALS

Thirty patients were treated with intensity-modulated radiotherapy, to a total dose of 80 Gy in 40 fractions. In 15 patients, a daily-inserted air-filled ERB was applied. Prostate motion was tracked, in real-time, using an electromagnetic tracking system. Interfraction displacements, measured before each treatment, were quantified by calculating the systematic and random deviations of the center of mass of the implanted transponders. Intrafraction motion was analyzed in timeframes of 150 s, and displacements >1 mm, >3 mm, >5 mm, and >7 mm were determined in the anteroposterior, left-right, and superoinferior direction, and for the three-dimensional (3D) vector. Manual table corrections, made during treatment sessions, were retrospectively undone.

RESULTS

A total of 576 and 567 tracks have been analyzed in the no-ERB group and ERB group, respectively. Interfraction variation was not significantly different between both groups. After 600 s, 95% and 98% of the treatments were completed in the respective groups. Significantly fewer table corrections were performed during treatment fractions with ERB: 88 vs. 207 (p = 0.02). Intrafraction motion was significantly reduced with ERB. During the first 150 s, only negligible deviations were observed, but after 150 s, intrafraction deviations increased with time. This resulted in cumulative percentages of 3D-vector deviations >1 mm, >3 mm, >5 mm, and >7 mm that were 57.7%, 7.0%, 0.7%, and 0.3% in the ERB-group vs. 70.2%, 18.1%, 4.6%, and 1.4% in the no-ERB group after 600 s. The largest reductions in the ERB group were observed in the AP direction. These data suggest that a 5 mm CTV-to-PTV margin is sufficient to correct for intrafraction prostate movements when using an ERB.

CONCLUSIONS

ERB significantly reduces intrafraction prostate motion, but not interfraction variation, and may in particular be beneficial for treatment sessions longer than 150 s.

Expanding the use of real-time electromagnetic tracking in radiation oncology

In the past 10 years, techniques to improve radiotherapy delivery, such as intensity‐modulated radiation therapy (IMRT), image‐guided radiation therapy (IGRT) for both inter‐ and intrafraction tumor localization, and hypofractionated delivery techniques such as stereotactic body radiation therapy (SBRT), have evolved tremendously. This review article focuses on only one part of that evolution, electromagnetic tracking in radiation therapy. Electromagnetic tracking is still a growing technology in radiation oncology and, as such, the clinical applications are limited, the expense is high, and the reimbursement is insufficient to cover these costs. At the same time, current experience with electromagnetic tracking applied to various clinical tumor sites indicates that the potential benefits of electromagnetic tracking could be significant for patients receiving radiation therapy. Daily use of these tracking systems is minimally invasive and delivers no additional ionizing radiation to the patient, and these systems can provide explicit tumor motion data. Although there are a number of technical and fiscal issues that need to be addressed, electromagnetic tracking systems are expected to play a continued role in improving the precision of radiation delivery.

PACS number: 87.63.‐d

An evaluation of intrafraction motion of the prostate in the prone and supine positions using electromagnetic tracking

PURPOSE

To evaluate differences in target motion during prostate irradiation in the prone versus supine position using electromagnetic tracking to measure prostate mobility.

MATERIALS/METHODS

Twenty patients received prostate radiotherapy in the supine position utilizing the Calypso Localization System® for prostate positioning and monitoring. For each patient, 10 treatment fractions were followed by a session in which the patient was repositioned prone, and prostate mobility was tracked. The fraction of time that the prostate was displaced by >3, 5, 7, and 10mm was calculated for each patient, for both positions (400 tracking sessions).

RESULTS

Clear patterns of respiratory motion were seen in the prone tracks due to the influence of increased abdominal motion. Averaged over all patients, the prostate was displaced >3 and 5mm for 37.8% and 10.1% of the total tracking time in the prone position, respectively. In the supine position, the prostate was displaced >3 and 5mm for 12.6% and 2.9%, respectively. With both patient setups, inferior and posterior drifts of the prostate position were observed. Averaged over all prone tracking sessions, the prostate was displaced >3mm in the posterior and inferior directions for 11.7% and 9.5% of the total time, respectively.

CONCLUSIONS

With real-time tracking of the prostate, it is possible to study the effects of different setup positions on the prostate mobility. The percentage of time the prostate moved >3 and 5mm was increased by a factor of three in the prone versus supine position. For larger displacements (>7 mm) no difference in prostate mobility was observed between prone and supine positions. To reduce rectal toxicity, radiotherapy in the prone position may be a suitable alternative provided respiratory motion is accounted for during treatment. Acute and late toxicity results remain to be evaluated for both patient positions.

Megavoltage Computed Tomography Image-based Low-dose Rate Intracavitary Brachytherapy Planning for Cervical Carcinoma

Initial results of megavoltage computed tomography (MVCT) brachytherapy treatment planning are presented, using a commercially available helical tomotherapy treatment unit and standard low dose rate (LDR) brachytherapy applicators used for treatment of cervical carcinoma. The accuracy of MVCT imaging techniques, and dosimetric accuracy of the CT based plans were tested with in-house and commercially-available phantoms. Three dimensional (3D) dose distributions were computed and compared to the two dimensional (2D) dosimetry results. Minimal doses received by the 2 cm3 of bladder and rectum receiving the highest doses (DB2cc and DR2cc, respectively) were computed from dose-volume histograms and compared to the doses computed for the standard ICRU bladder and rectal reference dose points. Phantom test objects in MVCT image sets were localized with sub-millimetric accuracy, and the accuracy of the MVCT-based dose calculation was verified. Fifteen brachytherapy insertions were also analyzed. The ICRU rectal point dose did not differ significantly from DR2cc (p=0.749, mean difference was 24 cGy ± 283 cGy). The ICRU bladder point dose was significantly lower than the DB2cc (p=0.024, mean difference was 291 cGy ± 444 cGy). The median volumes of bladder and rectum receiving at least the corresponding ICRU reference point dose were 6.1 cm3 and 2.0 cm3, respectively. Our initial experience in using MVCT imaging for clinical LDR gynecological brachytherapy indicates that the MVCT images are of sufficient quality for use in 3D, MVCT-based dose planning.

Severe airflow obstruction and eosinophilic lung disease after Stevens-Johnson syndrome

Respiratory involvement is a frequent complication of Stevens-Johnson syndrome (SJS). However, there are very few convincing reports of persistent pulmonary sequelae, as demonstrated by spirometry, radiology and pathology. The current study presents a case of a 13-yr-old female with T-cell acute lymphocytic leukaemia who developed persistent, severe, obstructive lung disease following an episode of SJS. A lung biopsy demonstrated bronchiolar submucosal fibrosis consistent with constrictive bronchiolitis, as well as eosinophilic micro-abscesses, which, to the current authors' knowledge, has not been previously described. The present study illustrates specific histopathological features that highlight a possible association between Stevens-Johnson syndrome, constrictive bronchiolitis and eosinophilic micro-abscesses. The eosinophils may be associated with permanent mucosal damage, as seen in the present case, by releasing mediators that have a pro-fibrogenetic role. However, further investigation is warranted.

Circulating filarial antigen in serum and hydrocele fluid from individuals living in an endemic area for bancroftian filariasis

This study examined circulating filarial antigen by monoclonal antibody Og4C3-enzyme-linked immunosorbent assay (ELISA) from 114 men with hydrocele, living in an endemic area. Nocturnal blood and hydrocele fluid were collected and examined for microfilaria. ELISA was performed on serum and hydrocele fluid for detection of antigen. Amongst 114 cases, 5(4.4%) showed microfilaria in blood but none in fluid. ELISA was positive in 13(11.40%) serum and 5 (4.4%) fluid samples. All five fluid antigen positive cases were positive for antibodies and showed microfilaria in blood. These findings emphasize the use of circulating filarial antigen detection and alternative usage of hydrocele fluid for diagnosis of filariasis.

Skeletal absorbed fractions for electrons in the adult male: considerations of a revised 50-microm definition of the bone endosteum

In 1995, the International Commission on Radiological Protection (ICRP) issued ICRP Publication 70 which provided an extensive update to the physiological and anatomical reference data for the skeleton of adults and children originally issued in ICRP Publication 23. Although ICRP Publication 70 has been a valuable document in the development of reference voxel computational phantoms, additional guidance is needed for dose assessment in the skeletal tissues beyond that given in ICRP Publication 30. In this study, a computed tomography (CT) and micro-CT-based model of the skeletal tissues is presented, which considers (1) a 50-microm depth in marrow for the osteoprogenitor cells, (2) electron escape from trabecular spongiosa to the surrounding cortical bone, (3) cortical bone to trabecular spongiosa cross-fire for electrons and (4) variations in specific absorbed fraction with changes in bone marrow cellularity for electrons. A representative data set is given for electron dosimetry in the craniofacial bones of the adult male.

CT volumetry of the skeletal tissues

Computed tomography (CT) is an important and widely used modality in the diagnosis and treatment of various cancers. In the field of molecular radiotherapy, the use of spongiosa volume (combined tissues of the bone marrow and bone trabeculae) has been suggested as a means to improve the patient-specificity of bone marrow dose estimates. The noninvasive estimation of an organ volume comes with some degree of error or variation from the true organ volume. The present study explores the ability to obtain estimates of spongiosa volume or its surrogate via manual image segmentation. The variation among different segmentation raters was explored and found not to be statistically significant (p value >0.05). Accuracy was assessed by having several raters manually segment a polyvinyl chloride (PVC) pipe with known volumes. Segmentation of the outer region of the PVC pipe resulted in mean percent errors as great as 15% while segmentation of the pipe's inner region resulted in mean percent errors within approximately 5%. Differences between volumes estimated with the high-resolution CT data set (typical of ex vivo skeletal scans) and the low-resolution CT data set (typical of in vivo skeletal scans) were also explored using both patient CT images and a PVC pipe phantom. While a statistically significant difference (p value <0.002) between the high-resolution and low-resolution data sets was observed with excised femoral heads obtained following total hip arthroplasty, the mean difference between high-resolution and low-resolution data sets was found to be only 1.24 and 2.18 cm3 for spongiosa and cortical bone, respectively. With respect to differences observed with the PVC pipe, the variation between the high-resolution and low-resolution mean percent errors was a high as approximately 20% for the outer region volume estimates and only as high as approximately 6% for the inner region volume estimates. The findings from this study suggest that manual segmentation is a reasonably accurate and reliable means for the in vivo estimation of spongiosa volume. This work also provides a foundation for future studies where spongiosa volumes are estimated by various raters in more comprehensive CT data

Linear regression model for predicting patient-specific total skeletal spongiosa volume for use in molecular radiotherapy dosimetry

The toxicity of red bone marrow is widely considered to be a key factor in restricting the activity administered in molecular radiotherapy to suboptimal levels. The assessment of marrow toxicity requires an assessment of the dose absorbed by red bone marrow which, in many cases, requires knowledge of the total red bone marrow mass in a given patient. Previous studies demonstrated, however, that a close surrogate-spongiosa volume (combined tissues of trabecular bone and marrow)-can be used to accurately scale reference patient red marrow dose estimates and that these dose estimates are predictive of marrow toxicity. Consequently, a predictive model of the total skeletal spongiosa volume (TSSV) would be a clinically useful tool for improving patient specificity in skeletal dosimetry.

METHODS

In this study, 10 male and 10 female cadavers were subjected to whole-body CT scans. Manual image segmentation was used to estimate the TSSV in all 13 active marrow-containing skeletal sites within the adult skeleton. The age, total body height, and 14 CT-based skeletal measurements were obtained for each cadaver. Multiple regression was used with the dependent variables to develop a model to predict the TSSV.

RESULTS

Os coxae height and width were the 2 skeletal measurements that proved to be the most important parameters for prediction of the TSSV. The multiple R(2) value for the statistical model with these 2 parameters was 0.87. The analysis revealed that these 2 parameters predicted the estimated the TSSV to within approximately +/-10% for 15 of the 20 cadavers and to within approximately +/-20% for all 20 cadavers in this study.

CONCLUSION

Although the utility of spongiosa volume in estimating patient-specific active marrow mass has been shown, estimation of the TSSV in active marrow-containing skeletal sites via patient-specific image segmentation is not a simple endeavor. However, the alternate approach demonstrated in this study is fairly simple to implement in a clinical setting, as the 2 input measurements (os coxae height and width) can be made with either pelvic CT scanning or skeletal radiography.

An assessment of bone marrow and bone endosteum dosimetry methods for photon sources

The rather complex and microscopic histological structure of the skeletal system generally limits one's ability to accurately model this tissue during dosimetric evaluations. Consequently, various assumptions must be made to evaluate the absorbed dose from external and internal photons to the radiosensitive tissues of the red (or haematopoietically active) bone marrow and the osteogenic tissues of the skeletal endosteum. These various methods for photon skeletal dosimetry have not been inter-compared, partly due to the lack of a realistic reference model that can provide a high-resolution three-dimensional geometry for secondary electron particle transport. In the present study, the paired-image radiation transport (PIRT) model developed by Shah et al (2005 J. Nucl. Med. 45 344) was utilized to evaluate the absorbed dose per incident photon fluence to these skeletal regions from idealized parallel beams of monoenergetic photons. The PIRT model results were then used as a local reference against which absorbed doses via other methods were compared. For red bone marrow dosimetry, four approximate techniques were considered: (1) the dose response function method (DRF method) presented in ORNL/TM-8381, (2) the mass-energy absorption coefficient ratio method (two-parameter MEAC method), (3) the MEAC method with the additional use of energy-dependent dose enhancement factors from King and Spiers (1985 Br. J. Radiol. 58 345) (three-parameter MEAC method), and (4) the three-parameter MEAC method applied at the voxel level through the use image-specific CT numbers (CTN method). For the bone endosteum (i.e., bone surfaces), two approximate techniques were compared: (1) the DRF method for bone surfaces and (2) the homogeneous bone approximation (HBA) method. In each case, the local reference standard was assumed to be that of the PIRT model. Four different ex vivo bone specimens with distinctively different internal structures were used in the study: the cranium, the lumbar vertebra, the os coxae and the left middle rib, each excised from a 66 year male cadaver (body mass index, 22.7 kg m(-2)). High-resolution CT images of these skeletal sites were used to construct computational voxel models for Monte Carlo radiation transport. Study results indicated that skeletal sites with thick cortical regions and thick trabeculae such as in the cranium provide considerable beam attenuation at low photon energies, which is not properly accounted for in methods based on a homogeneous skeletal tissue structure (DRF, MEAC, HBA). For bone marrow dose assessment, the CTN method showed the best agreement with PIRT model results over a broad range of photon energies, while the HBA method showed better agreement with the PIRT model in assessing bone endosteum dose at energies above 100 keV. Bone surface doses were better approximately by the DRF method at energies below 50 keV. Considerable secondary electron escape at photon energies over 1-3 MeV were accounted for in RBM dose assessment only in the PIRT model, as the other methods presume either an infinite expanse of spongiosa (DRF) or the existence of charge-particle equilibrium (MEAC, CTN).

Chord‐based versus voxel‐based methods of electron transport in the skeletal tissues

Anatomic models needed for internal dose assessment have traditionally been developed using mathematical surface equations to define organ boundaries, shapes, and their positions within the body. Many researchers, however, are now advocating the use of tomographic models created from segmented patient computed tomography (CT) or magnetic resonance (MR) scans. In the skeleton, however, the tissue structures of the bone trabeculae, marrow cavities, and endosteal layer are exceedingly small and of complex shape, and thus do not lend themselves easily to either stylistic representations or in-vivo CT imaging. Historically, the problem of modeling the skeletal tissues has been addressed through the development of chord-based methods of radiation particle transport, as given by studies at the University of Leeds (Leeds, U.K.) using a 44-year male subject. We have proposed an alternative approach to skeletal dosimetry in which excised sections of marrow-intact cadaver spongiosa are imaged directly via microCT scanning. The cadaver selected for initial investigation of this technique was a 66-year male subject of nominal body mass index (22.7 kg m(-2)). The objectives of the present study were to compare chord-based versus voxel-based methods of skeletal dosimetry using data from the UF 66-year male subject. Good agreement between chord-based and voxel-based transport was noted for marrow irradiation by either bone surface or bone volume sources up to 500-1000 keV (depending upon the skeletal site). In contrast, chord-based models of electron transport yielded consistently lower values of the self-absorbed fraction to marrow tissues than seen under voxel-based transport at energies above 100 keV, a feature directly attributed to the inability of chord-based models to account for nonlinear electron trajectories. Significant differences were also noted in the dosimetry of the endosteal layer (for all source tissues), with chord-based transport predicting a higher fraction of energy deposition than given by voxel-based transport (average factor of about 1.6). The study supports future use of voxel-based skeletal models which (1) permit nonlinear electron trajectories across the skeletal tissues, (2) do not rely on mathematical algorithms for treating the endosteal tissue layer, and (3) do not implicitly assume independence of marrow and bone trajectories as is the case for chord-based skeletal models.

A comparison of skeletal chord-length distributions in the adult male

In radiation protection, skeletal dose estimates are required for the tissues of the hematopoietically active bone marrow and the osteogenic cells of the trabecular and cortical endosteum. Similarly, skeletal radiation dose estimates are required in therapy nuclear medicine in order to develop dose-response functions for myelotoxicity where active bone marrow is generally the dose-limiting organ in cancer radioimmunotherapy. At the present time, skeletal dose models in both radiation protection and medical dosimetry are fundamentally reliant on a single set of chord-length distribution measurements performed at the University of Leeds in the late 1970's for a 44-y-old male subject. These distributions describe the relative frequency at which linear pathlengths are seen across both the marrow cavities and bone trabeculae in seven individual bone sites: vertebrae (cervical and lumbar), proximal femur (head and neck), ribs, cranium (parietal bone), and pelvis (iliac crest). In the present study, we present an alternative set of chord-length distribution data acquired within a total of 14 skeletal sites of a 66-y-old male subject. The University of Florida (UF) distributions are assembled via 3D image processing of microCT scans of physical sections of trabecular spongiosa at each skeletal site. In addition, a tri-linear interpolation Marching Cube algorithm is employed to smooth the digital surfaces of the bone trabeculae while chord-length measurements are performed. A review of mean chord lengths indicate that larger marrow cavities are noted on average in the UF individual for the cervical vertebrae (1,038 vs. 910 microm), lumbar vertebrae (1,479 vs. 1,233 microm), ilium (1,508 vs. 904 microm), and parietal bone (812 vs. 389 microm), while smaller marrow cavities are noted in the UF individual for the femoral head (1,043 microm vs. 1,157 microm), the femoral neck (1,454 microm vs. 1,655 microm), and the ribs (1,630 microm vs. 1,703 microm). The mean chord-lengths for the bone trabeculae show close agreement for both individuals in the ilium (approximately 240 microm) and cervical vertebrae (approximately 280 microm). Thicker trabeculae were seen on average in the UF individual for the femoral head (ratio of 1.50), femoral neck (ratio of 1.10), lumbar vertebrae (ratio of 1.29), and ribs (ratio of 1.14), while thinner trabeculae were seen on average in the UF individual for the parietal bone of the cranium (ratio of 0.92). In two bone sites, prominent discrepancies in chord distribution shape were noted between the Leeds 44-y-old male and the UF 66-y-old male: (1) the bone trabeculae in the ribs, and (2) the marrow cavities and bone trabeculae within the cranium.

Accounting for beta‐particle energy loss to cortical bone via paired‐image radiation transport (PIRT)

Current methods of skeletal dose assessment in both medical physics (radionuclide therapy) and health physics (dose reconstruction and risk assessment) rely heavily on a single set of bone and marrow cavity chord-length distributions in which particle energy deposition is tracked within an infinite extent of trabecular spongiosa, with no allowance for particle escape to cortical bone. In the present study, we introduce a paired-image radiation transport (PIRT) model which provides a more realistic three-dimensional (3D) geometry for particle transport in the skeletal site at both microscopic and macroscopic levels of its histology. Ex vivo CT scans were acquired of the pelvis, cranial cap, and individual ribs excised from a 66-year male cadaver (BMI of 22.7 kg m(-2)). For the three skeletal sites, regions of trabecular spongiosa and cortical bone were identified and segmented. Physical sections of interior spongiosa were taken and subjected to microCT imaging. Voxels within the resulting microCT images were then segmented and labeled as regions of bone trabeculae, endosteum, active marrow, and inactive marrow through application of image processing algorithms. The PIRT methodology was then implemented within the EGSNRC radiation transport code whereby electrons of various initial energies are simultaneously tracked within both the ex vivo CT macroimage and the CT microimage of the skeletal site. At initial electron energies greater than 50-200 keV, a divergence in absorbed fractions to active marrow are noted between PIRT model simulations and those estimated under existing techniques of infinite spongiosa transport. Calculations of radionuclide S values under both methodologies imply that current chord-based models may overestimate the absorbed dose to active bone marrow in these skeletal sites by 0% to 27% for low-energy beta emitters (33P, 169Er, and 177Lu), by approximately 4% to 49% for intermediate-energy beta emitters (153Sm, 186Re, and 89Sr), and by approximately 14% to 76% for high-energy beta emitters (32p, 188Re, and 90Y). The PIRT methodology allows for detailed modeling of the 3D macrostructure of individual marrow-containing bones within the skeleton thus permitting improved estimates of absorbed fractions and radionuclide S values for intermediate-to-high energy beta emitters.

A paired-image radiation transport model for skeletal dosimetry

Toxicity of the hematopoietically active bone marrow continues to be a primary limitation in radionuclide therapies of cancer. Improved techniques for patient-specific skeletal dosimetry are thus crucial to the development of dose-response relationships needed to optimize these therapies (i.e., avoid both marrow toxicity and tumor underdosing). Current clinical methods of skeletal dose assessment rely heavily on a single set of bone and marrow cavity chord-length distributions in which particle energy deposition is tracked within an infinite extent of trabecular spongiosa, with no allowance for particle escape to cortical bone. In the present study, we introduce a paired-image radiation transport (PIRT) model that can provide a more realistic 3-dimensional geometry for particle transport of the skeletal site at both microscopic and macroscopic levels of its histology.

METHODS

Ex vivo CT scans were acquired of the lumbar vertebra and right proximal femur excised from a 66-y male cadaver (body mass index, 22.7 kg m(-2)). For both skeletal sites, regions of trabecular spongiosa and cortical bone were identified and segmented. Physical sections of interior spongiosa were then taken and subjected to nuclear magnetic resonance (NMR) microscopy. Voxels within the resulting NMR microimages were segmented and labeled into regions of bone trabeculae, endosteum, active marrow, and inactive marrow. The PIRT methodology was then implemented within the EGSnrc radiation transport code, whereby electrons of various initial energies are simultaneously tracked within both the ex vivo CT macroimage and the NMR microimage of the skeletal site.

RESULTS

At electron initial energies greater than 50-200 keV, a divergence in absorbed fractions to active marrow is noted between PIRT model simulations and those estimated under infinite spongiosa transport techniques. Calculations of radionuclide S values under both methodologies imply that current chord-based models used in clinical skeletal dosimetry can overestimate dose to active bone marrow in these 2 skeletal sites by approximately 4%-23% for low-energy beta-emitters ((33)P, (169)Er, and (177)Lu), by approximately 4%-25% for intermediate-energy beta-emitters ((153)Sm, (186)Re, and (89)Sr), and by approximately 11%-30% for high-energy beta-emitters ((32)P, (188)Re, and (90)Y).

CONCLUSION

The PIRT methodology allows for detailed modeling of the 3D macrostructure of individual marrow-containing bones within the skeleton, thus permitting improved estimates of absorbed fractions and radionuclide S values for intermediate-to-high beta-emitters.

A hyperboliod representation of the bone-marrow interface within 3D NMR images of trabecular bone: applications to skeletal dosimetry

Recent advances in physical models of skeletal dosimetry utilize high-resolution NMR microscopy images of trabecular bone. These images are coupled to radiation transport codes to assess energy deposition within active bone marrow irradiated by bone- or marrow-incorporated radionuclides. Recent studies have demonstrated that the rectangular shape of image voxels is responsible for cross-region (bone-to-marrow) absorbed fraction errors of up to 50% for very low-energy electrons (<50 keV). In this study, a new hyperboloid adaptation of the marching cube (MC) image-visualization algorithm is implemented within 3D digital images of trabecular bone to better define the bone-marrow interface, and thus reduce voxel effects in the assessment of cross-region absorbed fractions. To test the method, a mathematical sample of trabecular bone was constructed, composed of a random distribution of spherical marrow cavities, and subsequently coupled to the EGSnrc radiation code to generate reference values for the energy deposition in marrow or bone. Next, digital images of the bone model were constructed over a range of simulated image resolutions, and coupled to EGSnrc using the hyperboloid MC (HMC) algorithm. For the radionuclides 33P, 117mSn, 131I and 153Sm, values of S(marrow<--bone) estimated using voxel models of trabecular bone were shown to have relative errors of 10%, 9%, <1% and <1% at a voxel size of 150 microm. At a voxel size of 60 microm, these errors were 6%, 5%, <1% and <1%, respectively. When the HMC model was applied during particle transport, the relative errors on S(marrow<--bone) for these same radionuclides were reduced to 7%, 6%, <1% and <1% at a voxel size of 150 microm, and to 2%, 2%, <1% and <1% at a voxel size of 60 microm. The technique was also applied to a real NMR image of human trabecular bone with a similar demonstration of reductions in dosimetry errors.

Adipocyte spatial distributions in bone marrow: implications for skeletal dosimetry models

Few studies have been conducted to quantify the spatial distributions of adipocytes in the marrow cavities of trabecular bone. Nevertheless, such data are needed for the development of 3-dimensional (3D) voxel skeletal models where marrow cellularity is explicitly considered as a model parameter for dose assessment. In this investigation, bone marrow biopsies of the anterior iliac crest were examined to determine the size distribution of adipocyte cell clusters, the percentage of perimeter coverage of trabecular surfaces, and the presence or absence of adipocyte density gradients in the marrow space, all as a function of the biopsy marrow cellularity (5%-95%).

METHODS

Biopsy slides from 42 patients were selected as designated by the hematopathologist as either normocellular or with no evidence of disease. Still-frame video image captures were made of 1-3 regions of interest per biopsy specimen, with subsequent image analysis of adipocyte spatial characteristics performed via a user-written MATLAB routine.

RESULTS

A predictable shift was found in cluster size with decreasing marrow cellularity from single adipocytes to clusters of >or=3 cells; the percentage of 2-cell clusters remained relatively constant with changing cellularity. Also, a nonlinear increase in trabeculae perimeter coverage was found with increasing fat tissue fraction at marrow cellularities between 50% and 80%. Finally, it was demonstrated that only in the range of 20%-50% marrow cellularity was a slight gradient in adipocyte concentration indicated with adipocytes localized preferentially toward the trabecular surfaces.

CONCLUSION

Electron transport simulations were conducted in 4 different 3D voxel models of trabecular bone for sources localized in the active marrow (TAM), bone volume (TBV), bone endosteum (TBE), and bone surfaces (TBS). Voxel model simulations demonstrated that absorbed fractions to active marrow given by the ICRP 30 model (MIRDOSE2) are exceedingly conservative for both TBV and TBS sources, except in the case of high-energy particles (>500 keV) at high values of marrow cellularity (>70%). Values of both phi(TAM<--TBV) and phi(TAM<--TBS) given by the Eckerman and Stabin model (MIRDOSE3) were shown to be reasonably consistent with 3D voxel model simulations at the reference cellularity of 25%, except in the case of low-energy emitters (<100 keV) on the bone surfaces.

Voxel effects within digital images of trabecular bone and their consequences on chord-length distribution measurements

Chord-length distributions through the trabecular regions of the skeleton have been investigated since the early 1960s. These distributions have become important features for bone marrow dosimetry; as such, current models rely on the accuracy of their measurements. Recent techniques utilize nuclear magnetic resonance (NMR) microscopy to acquire 3D images of trabecular bone that are then used to measure 3D chord-length distributions by Monte Carlo methods. Previous studies have shown that two voxel effects largely affect the acquisition of these distributions within digital images. One is particularly pertinent as it dramatically changes the shape of the distribution and reduces its mean. An attempt was made to reduce this undesirable effect and good results were obtained for a single-sphere model using minimum acceptable chord (MAC) methods (Jokisch et al 2001 Med. Phys. 28 1493-504). The goal of the present work is to extend the study of these methods to more general models in order to better quantify their consequences. First, a mathematical model of a trabecular bone sample was used to test the usefulness of the MAC methods. The results showed that these methods were not efficient for this simulated bone model. These methods were further tested on a single voxelized sphere over a large range of voxel sizes. The results showed that the MAC methods are voxel-size dependent and overestimate the mean chord length for typical resolutions used with NMR microscopy. The study further suggests that bone and marrow chord-length distributions currently utilized in skeletal dosimetry models are most likely affected by voxel effects that yield values of mean chord length lower than their true values.

Surface area overestimation within three-dimensional digital images and its consequence for skeletal dosimetry

The most recent methods for trabecular bone dosimetry are based on Monte Carlo transport simulations within three-dimensional (3D) images of real human bone samples. Nuclear magnetic resonance and micro-computed tomography have been commonly used as imaging tools for studying trabecular microstructure. In order to evaluate the accuracy of these techniques for radiation dosimetry, a previous study was conducted that showed an overestimate in the absorbed fraction of energy for low-energy electrons emitted within the marrow space and irradiating the bone trabeculae. This problem was found to be related to an overestimate of the surface area of the true bone-marrow interface within the 3D digital images, and was identified as the surface-area effect. The goal of the present study is to better understand how this surface-area effect occurs in the case of single spheres representing individual marrow cavities within trabecular bone. First, a theoretical study was conducted which showed that voxelization of the spherical marrow cavity results in a 50% overestimation of the spherical surface area. Moreover, this overestimation cannot be reduced through a reduction in the voxel size (e.g., improved image resolution). Second, a series of single-sphere marrow cavity models was created with electron sources simulated within the sphere (marrow source) and outside the sphere (bone trabeculae source). The series of single-sphere models was then voxelized to represent 3D digital images of varying resolution. Transport calculations were made for both marrow and bone electron sources within these simulated images. The study showed that for low-energy electrons (<100 keV), the 50% overestimate of the bone-marrow interface surface area can lead to a 50% overestimate of the cross-absorbed fraction. It is concluded that while improved resolution will not reduce the surface area effects found within 3D image-based transport models, a tenfold improvement in current image resolution would compensate the associated errors in cross-region absorbed fractions for low-energy electron sources. Alternatively, other methods of defining the bone-marrow interface, such as with a polygonal isosurface, would provide improvements in dosimetry without the need for drastic reductions in image voxel size.

Skeletal dosimetry via NMR microscopy: investigations of sample reproducibility and signal source

Nuclear magnetic resonance microscopy has been used for several years as a means of quantifying the 3D microarchitecture of the cancellous regions of the skeleton. These studies were originally undertaken for the purpose of developing non-invasive techniques for the early detection of osteoporosis and other bone structural changes. Recently, nuclear magnetic resonance microscopy has also been used to acquire this same 3D data for the purpose of both (1) generating chord length data across bone trabeculae and marrow cavities and (2) generating 3D images for direct coupling to Monte Carlo radiation transport codes. In both cases, one is interested in the reproducibility of the dosimetric data obtained from nuclear magnetic resonance microscopy. In the first of two studies, a trabecular bone sample from the femoral head of a 51-y-old male cadaver was subjected to repeated image acquisition, image processing, image coupling, and radiation transport simulations. The resulting absorbed fractions at high electron energies (4 MeV) were shown to vary less than 4% among four different imaging sessions of the same sample. In a separate study, two femoral head samples were imaged under differing conditions of the NMR signal source. In the first case, the samples were imaged with intact marrow. These samples were then subjected to marrow digestion and immersed in Gd-doped water, which then filled the marrow cavities. Energy-dependent absorbed fraction profiles for both the marrow-intact and marrow-free samples showed essentially equivalent results. These studies thus provide encouragement that skeletal dosimetry models of improved patient specificity can be achieved via NMR microscopy in vivo.

Site-specific variability in trabecular bone dosimetry: considerations of energy loss to cortical bone

With continual advances in radionuclide therapies, increasing emphasis is being placed on improving the patient specificity of dose estimates to marrow tissues. While much work has been focused on determining patient-specific assessments of radionuclide uptake in the skeleton, few studies have been initiated to explore the individual variability of absorbed fraction data for electron and beta-particle sources in various skeletal sites. The most recent values of radionuclide S values used in clinical medicine continue to utilize a formalism in which electrons are transported under a trabecular bone geometry of infinite extent. No provisions are thus made for the fraction of energy lost to the cortical bone cortex of the skeletal site and its surrounding tissues. In the present study, NMR microscopy was performed on trabecular bone samples taken from the femoral head and humeral proximal epiphysis of three subjects: a 51-year male, an 82-year female, and an 86-year female. Following image segmentation and coupling to EGS4, electrons were transported within macrostructural models of the various skeletal sites that explicitly include the spatial extent of the spongiosa, as well as the thickness of the surrounding cortical bone. These energy-dependent profiles of absorbed fractions to marrow tissues were then compared to transport simulations made within an infinite region of spongiosa. Ratios of mean absorbed fraction, as weighted by the beta energy spectra, under both transport methodologies were then assembled for the radionuclides 32P and 90Y. These ratios indicate that corrections to existing radionuclide S values for 32P can vary by as much as 5% for the male, 6% for the 82-year female, and 8% for the 86-year female. For the higher-energy beta spectrum of 90Y, these same corrections can reach 8%, 10%, and 11%, respectively.

Considerations of marrow cellularity in 3-dimensional dosimetric models of the trabecular skeleton

Dose assessment to active bone marrow is a critical feature of radionuclide therapy treatment planning. Skeletal dosimetry models currently used to assign radionuclide S values for clinical marrow dose assessment are based on bone and marrow cavity chord-length distributions. Accordingly, these models cannot explicitly consider energy loss to inactive marrow (adipose tissue) during particle transport across the trabecular marrow space (TMS). One method to account for this energy loss is to uniformly scale the resulting TMS absorbed fractions by reference values of site-specific marrow cellularity. In doing so, however, the resulting absorbed fractions for self-irradiation of the trabecular active marrow (TAM) do not converge to unity at low electron source energies. This study attempts to address this issue by using nuclear magnetic resonance microscopy images of trabecular bone to define 3-dimensional (3D) dosimetric models in which explicit spatial distributions of adipose tissue are introduced.

METHODS

Cadaveric sources of trabecular bone were taken from both the femoral heads and humeral epiphyses of a 51-y-old male subject. The bone sites were sectioned and subsequently imaged at a proton resonance frequency of 200 MHz (4.7 T) using a 3D spin-echo pulse sequence. After image segmentation, voxel clusters of adipocytes were inserted interior to the marrow cavities of the binary images, which were then coupled to the EGS4 radiation transport code for simulation of active marrow electron sources.

RESULTS

Absorbed fractions for self-irradiation of the TAM were tabulated for both skeletal sites. Substantial variations in the absorbed fraction to active marrow are seen with changes in marrow cellularity, particularly in the energy range of 100-500 keV. These variations are seen to be more dramatic in the humeral epiphysis (larger marrow volume fraction) than in the femoral head.

CONCLUSION

Results from electron transport in 3D models of the trabecular skeleton indicate that current methods to account for marrow cellularity in chord-based models are incomplete. At 10 keV, for example, the Eckerman and Stabin model underestimates the self-absorbed fraction to active marrow by 75%. At 1 MeV, the model of Bouchet et al. overestimates this same value by 40%. In the energy range of 20-200 keV, neither model accurately predicts energy loss to the active bone marrow. Thus, it is proposed that future extensions of skeletal dosimetry models use 3D transport techniques in which explicit delineation of active and inactive marrow is feasible.