Thermoluminescent dosimeters (TLD-100) for absorbed dose measurements in alpha-emitting radionuclides

Early works that used thermoluminescent dosimeters (TLDs) to measure absorbed dose from alpha particles reported relatively high variation (10%) between TLDs, which is undesirable for modern dosimetry applications. This work outlines a method to increase precision for absorbed dose measured using TLDs with alpha-emitting radionuclides by applying an alpha-specific chip factor (CF) that individually characterizes the TLD sensitivity to alpha particles. Variation between TLDs was reduced from 21.8% to 6.7% for the standard TLD chips and 7.9% to 3.3% for the thin TLD chips. It has been demonstrated by this work that TLD-100 can be calibrated to precisely measure the absorbed dose to water from alpha-emitting radionuclides.

Investigation of a measurement-based dosimetry approach to beta particle-emitting radiopharmaceutical therapy nuclides across tissue interfaces

OBJECTIVE

In this work, we present and evaluate a technique for performing interface measurements of beta particle-emitting radiopharmaceutical therapy agents in solution. APPROACH: Unlaminated EBT3 film was calibrated for absorbed dose to water using a NIST matched x-ray beam. Custom acrylic source phantoms were constructed and placed above interfaces comprised of bone, lung, and water equivalent materials. The film was placed perpendicular to these interfaces and measurements for absorbed dose to water using solutions of 90Y and 177Lu were performed and compared to Monte Carlo absorbed dose to water estimates simulated with EGSnrc. Surface and depth dose profile measurements were also performed. MAIN RESULTS: Surface absorbed dose to water measurements agreed with predicted results within 3.6 % for 177Lu and 2.2 % for 90Y. The agreement between predicted and measured absorbed dose to water was better for 90Y than 177Lu for depth dose and interface profiles. In general, agreement within k = 1 uncertainty bounds was observed for both radionuclides and all interfaces. An exception to this was found for the bone to water interface for 177Lu due to the increased sensitivity of the measurements to imperfections in the material surfaces. SIGNIFICANCE: This work demonstrates the feasibility of using radiochromic film for performing absorbed dose to water measurements on beta-emitting radiopharmaceutical therapy agents across material interfaces.

Preclinical Models of Anal Cancer Combined-Modality Therapy

INTRODUCTION

There have been no significant changes in anal cancer treatment options in 4 decades. In this study, we highlight two preclinical models designed to assess anal cancer treatments.

MATERIALS AND METHODS

Transgenic K14E6/E7 mice were treated with 7, 12-dimethylbenz(a)anthracene until anal tumors developed. Mice were treated with localized radiation in addition to chemotherapy (combined-modality therapy [CMT]) and compared to no treatment control (NTC). K14E6/E7 mouse anal spheroids with and without Pik3ca mutations were isolated and treated with vehicle, LY3023414 (LY3) (a drug previously shown to be effective in cancer prevention), CMT, or CMT + LY3.

RESULTS

In the in vivo model, there was a significant increase in survival in the CMT group compared to the NTC group (P = 0.0392). In the ex vivo model, there was a significant decrease in the mean diameter of CMT and CMT + LY3-treated spheroids compared to vehicle (P ≤ 0.0001). For LY3 alone compared to vehicle, there was a statistically significant decrease in spheroid size in the K14E6/E7 group without mutation (P = 0.0004).

CONCLUSIONS

We have provided proof of concept for two preclinical anal cancer treatment models that allow for the future testing of novel therapies for anal cancer.

Non-contact scintillator imaging dosimetry for total body irradiation in radiotherapy

Objective.The goal of this work was to assess the potential use of non-contact scintillator imaging dosimetry for tracking delivery in total body irradiation (TBI).Approach. Studies were conducted to measure the time-gated light signals caused by radiation exposure to scintillators that were placed on tissue. The purpose was to assess efficacy in conditions common for TBI, such as the large source to surface distance (SSD) commonly used, the reduced dose rate, the inclusion of a plexiglass spoiler, angle of incidence and effects of peripheral patient support structures. Dose validation work was performed on phantoms that mimicked human tissue optical properties and body geometry. For this work, 1.5 cm diameter scintillating disks were developed and affixed to phantoms under various conditions. A time-gated camera synchronized to the linac pulses was used for imaging. Scintillation intensity was quantified in post processing and the values verified with simultaneous thermolumiescent dosimeter (TLD) measurements. Mean scintillation values in each region were compared to TLD measurements to produce dose response curves, and scatter effects from the spoiler and patient bed were quantified.Main results.The dose determined by scintillators placed in TBI conditions agreed with TLD dose determinations to within 2.7%, and did so repeatedly within 1.0% standard deviation variance. A linear fit between scintillator signal and TLD dose was achieved with anR2= 0.996 across several body sites. Scatter from the patient bed resulted in a maximum increase of 19% in dose.Significance.This work suggests that non-contact scintillator imaging dosimetry could be used to verify dose in real time to patients undergoing TBI at the prescribed long SSD and low dose rate. It also has shown that patient transport stretchers can significantly influence surface dose by increasing scatter.

PETRA: A pencil beam trimming algorithm for analytical proton therapy dose calculations with the dynamic collimation system

BACKGROUND

The Dynamic Collimation System (DCS) has been shown to produce superior treatment plans to uncollimated pencil beam scanning (PBS) proton therapy using an in-house treatment planning system (TPS) designed for research. Clinical implementation of the DCS requires the development and benchmarking of a rigorous dose calculation algorithm that accounts for pencil beam trimming, performs monitor unit calculations to produce deliverable plans at all beam energies, and is ideally implemented with a commercially available TPS.

PURPOSE

To present an analytical Pencil bEam TRimming Algorithm (PETRA) for the DCS, with and without its range shifter, implemented in the Astroid TPS (.decimal, Sanford, Florida, USA).

MATERIALS

PETRA was derived by generalizing an existing pencil beam dose calculation model to account for the DCS-specific effects of lateral penumbra blurring due to the nickel trimmers in two different planes, integral depth dose variation due to the trimming process, and the presence and absence of the range shifter. Tuning parameters were introduced to enable agreement between PETRA and a measurement-validated Dynamic Collimation Monte Carlo (DCMC) model of the Miami Cancer Institute's IBA Proteus Plus system equipped with the DCS. Trimmer position, spot position, beam energy, and the presence or absence of a range shifter were all used as variables for the characterization of the model. The model was calibrated for pencil beam monitor unit calculations using procedures specified by International Atomic Energy Agency Technical Report Series 398 (IAEA TRS-398).

RESULTS

The integral depth dose curves (IDDs) for energies between 70 MeV and 160 MeV among all simulated trimmer combinations, with and without the ranger shifter, agreed between PETRA and DCMC at the 1%/1 mm 1-D gamma criteria for 99.99% of points. For lateral dose profiles, the median 2-D gamma pass rate for all profiles at 1.5%/1.5 mm was 99.99% at the water phantom surface, plateau, and Bragg peak depths without the range shifter and at the surface and Bragg peak depths with the range shifter. The minimum 1.5%/1.5 mm gamma pass rates for the 2-D profiles at the water phantom surface without and with the range shifter were 98.02% and 97.91%, respectively, and, at the Bragg peak, the minimum pass rates were 97.80% and 97.5%, respectively.

CONCLUSION

The PETRA model for DCS dose calculations was successfully defined and benchmarked for use in a commercially available TPS.

Integration and dosimetric validation of a dynamic collimation system for pencil beam scanning proton therapy

Objective.To integrate a Dynamic Collimation System (DCS) into a pencil beam scanning (PBS) proton therapy system and validate its dosimetric impact.Approach.Uncollimated and collimated treatment fields were developed for clinically relevant targets using an in-house treatment plan optimizer and an experimentally validated Monte Carlo model of the DCS and IBA dedicated nozzle (DN) system. The dose reduction induced by the DCS was quantified by calculating the mean dose in 10- and 30-mm two-dimensional rinds surrounding the target. A select number of plans were then used to experimentally validate the mechanical integration of the DCS and beam scanning controller system through measurements with the MatriXX-PT ionization chamber array and EBT3 film. Absolute doses were verified at the central axis at various depths using the IBA MatriXX-PT and PPC05 ionization chamber.Main results.Simulations demonstrated a maximum mean dose reduction of 12% for the 10 mm rind region and 45% for the 30 mm rind region when utilizing the DCS. Excellent agreement was observed between Monte Carlo simulations, EBT3 film, and MatriXX-PT measurements, with gamma pass rates exceeding 94.9% for all tested plans at the 3%/2 mm criterion. Absolute central axis doses showed an average verification difference of 1.4% between Monte Carlo and MatriXX-PT/PPC05 measurements.Significance.We have successfully dosimetrically validated the delivery of dynamically collimated proton therapy for clinically relevant delivery patterns and dose distributions with the DCS. Monte Carlo simulations were employed to assess dose reductions and treatment planning considerations associated with the DCS.

Simultaneous photon counting and charge integrating for pulse pile-up correction in paralyzable photon counting detectors

Objective.In photon counting detectors (PCDs), electric pulses induced by two or more x-ray photons can pile up and result in count losses when their temporal separation is less than the detector dead time. The correction of pulse pile-up-induced count loss is particularly difficult for paralyzable PCDs since a given value of recorded counts can correspond to two different values of true photon interactions. In contrast, charge (energy) integrating detectors work by integrating collected electric charge induced by x-rays over time and do not suffer from pile-up losses. This work introduces an inexpensive readout circuit element to the circuits of PCDs to simultaneously collect time-integrated charge to correct pile-up-induced count losses.Approach.Prototype electronics were constructed to collect time-integrated charges simultaneously with photon counts. A splitter was used to feed the electric signal in parallel to both a digital counter and a charge integrator. After recording PCD counts and integrating collected charge, a lookup table can be generated to map raw counts in the total- and high-energy bins and total charge to estimate pile-up-free true counts. Proof-of-concept imaging experiments were performed with a CdTe-based PCD array to test this method.Main results.The proposed electronics successfully recorded photon counts and time-integrated charge simultaneously, and whereas photon counts exhibited paralyzable pulse pile-up, time-integrated charge using the same electric signal as the counts measurement was linear with x-ray flux. With the proposed correction, paralyzable PCD counts became linear with input flux for both total- and high-energy bins. At high flux levels, uncorrected post-log measurements of PMMA objects severely overestimated radiological path lengths for both energy bins. After the proposed correction, the non-monotonic measurements again became linear with flux and accurately represented the true radiological path lengths. No impact on the spatial resolution was observed after the proposed correction in images of a line-pair test pattern.Significance.Time-integrated charge can be used to correct for pulse pile-up in paralyzable PCDs where analytical solutions may be difficult to use, and integrated charge can be collected simultaneously with counts using inexpensive electronics.

Creation of waterproof, TLD probes for dose measurements to validate image-based radiopharmaceutical therapy dosimetry workflow

Voxel-level dosimetry based on nuclear medicine images offers patients the most personalized form of radiopharmaceutical therapy (RPT) treatments. Clinical evidence is emerging demonstrating improvements in treatment precision in patients when voxel-level dosimetry is used. Voxel-level dosimetry requires absolute quantification of activity concentrations in the patient, but images from SPECT/CT scanners are not quantitative and require calibration using nuclear medicine phantoms. While phantom studies can validate a scanner's ability to recover activity concentrations, this is only a surrogate for the true metric of interest: absorbed doses. Measurements using thermoluminescent dosimeters (TLDs) are a versatile and accurate method of measuring absorbed dose. In this work, a TLD probe was manufactured that can fit into currently available nuclear medicine phantoms for the measurement of absorbed dose of RPT agents. Next, 748 MBq of I-131 was administered to a 16 ml hollow source sphere placed in a 6.4 L Jaszczak phantom in addition to six TLD probes, each holding 4 TLD-100 1 x 1 x 1 mm TLD-100 (LiF:Mg,Ti) microcubes. The phantom then underwent a SPECT/CT scan in accordance with a standard SPECT/CT imaging protocol for I-131. The SPECT/CT images were then input into a Monte Carlo based RPT dosimetry platform named RAPID and a three dimensional dose distribution in the phantom was estimated. Additionally, a GEANT4 benchmarking scenario (denoted 'idealized') was created using a stylized representation of the phantom. There was good agreement for all six probes, the differences between measurement and RAPID ranged between -5.5% and 0.9%. The difference between the measured and the idealized GEANT4 scenario was calculated and ranged from -4.3% and -20.5%. This work demonstrates good agreement between TLD measurements and RAPID. In addition, it introduces a novel TLD probe that can be easily introduced into clinical nuclear medicine workflows to provide QA of image-based dosimetry for RPT treatments.

IMRT QA result prediction via MLC transmission decomposition

BACKGROUND

Quality assurance measurement of IMRT/VMAT treatment plans is resource intensive, and other more efficient methods to achieve the same confidence are desirable.

PURPOSE

We aimed to analyze treatment plans in the context of the treatment planning systems that created them, in order to predict which ones will fail a standard quality assurance measurement. To do so, we sought to create a tool external to the treatment planning system that could analyze a set of MLC positions and provide information that could be used to calculate various evaluation metrics.

METHODS

The tool was created in Python to read in DICOM plan files and determine the beam fluence fraction incident on each of seven different zones, each classified based on the RayStation MLC model. The fractions, termed grid point fractions, were validated by analyzing simple test plans. The average grid point fractions, over all control points for 46 plans were then computed. These values were then compared with gamma analysis pass percentages and median dose differences to determine if any significant correlations existed.

RESULTS

Significant correlation was found between the grid point fraction metrics and median dose differences, but not with gamma analysis pass percentages. Correlations were positive or negative, suggesting differing model parameter value sensitivities, as well as potential insight into the treatment planning system dose model.

CONCLUSIONS

By decomposing MLC control points into different transmission zones, it is possible to create a metric that predicts whether the analyzed plan will pass a quality assurance measurement from a dose calculation accuracy standpoint. The tool and metrics developed in this work have potential applications in comparing clinical beam models or identifying their weak points. Implementing the tool within a treatment planning system would also provide more potential plan optimization parameters.

Validation of Monte Carlo simulated absorbed-dose-to-water inside a custom SPECT/CT phantom using active and passive dosimeters: a feasibility study using99mTc

OBJECTIVE

This project aims to provide a novel method for performing dosimetry measurements on TRT radionuclides using a custom-made SPECT/CT compatible phantom, common active and passive detectors, and Monte Carlo simulations. In this work we present a feasibility study using 99mTc for a novel approach to obtaining reproducible measurements of absorbed dose to water from radionuclide solutions using active and passive detectors in a custom phantom for the purpose of benchmarking Monte Carlo-based absorbed dose to water estimates. 
APPROACH: A cylindrical, acrylic SPECT/CT compatible phantom capable of housing an IBA EFD diode, SNC600c Farmer type ion chamber, and TLD-100 microcubes was designed and built for the purpose of assessing internal absorbed-dose-to-water at various points within a solution of 99mTc. The phantom is equipped with removable inserts that allow for numerous detector configurations and is designed to be used for verification of SPECT/CT-based absorbed-dose estimates with traceable detector measurements at multiple locations. Three experiments were conducted with exposure times ranging from 11 to 21 h with starting activities of approximately 10-16 GBq. Measurement data was compared to Monte Carlo simulations using the egs_chamber user code in EGSnrc 2019. 
MAIN RESULTS: In general, the ionization chamber measurements agreed with the Monte Carlo simulations within k=1 uncertainty values (±4% and ±7%, respectively). Measurements from the TLDs yielded results within k=1 agreement of the MC prediction (±6% and ±5%, respectively). Agreement within k=1 uncertainty (±6% and ±7%, respectively) was obtained for the diode for one of three conducted experiments. 
SIGNIFICANCE: While relatively large uncertainties remain, the agreement between measured and simulated doses provides proof of principal that dosimetry of radionuclide solutions with active detectors may be performed using this type of phantom with potential modifications for beta emitting radionuclides to be introduced in future work.&#xD.

Dosimetric delivery validation of dynamically collimated pencil beam scanning proton therapy

Objective. Pencil beam scanning (PBS) proton therapy target dose conformity can be improved with energy layer-specific collimation. One such collimator is the dynamic collimation system (DCS), which consists of four nickel trimmer blades that intercept the scanning beam as it approaches the lateral extent of the target. While the dosimetric benefits of the DCS have been demonstrated through computational treatment planning studies, there has yet to be experimental verification of these benefits for composite multi-energy layer fields. The objective of this work is to dosimetrically characterize and experimentally validate the delivery of dynamically collimated proton therapy with the DCS equipped to a clinical PBS system.Approach. Optimized single field, uniform dose treatment plans for 3 × 3 × 3 cm³target volumes were generated using Monte Carlo dose calculations with depths ranging from 5 to 15 cm, trimmer-to-surface distances ranging from 5 to 18.15 cm, with and without a 4 cm thick polyethylene range shifter. Treatment plans were then delivered to a water phantom using a prototype DCS and an IBA dedicated nozzle system and measured with a Zebra multilayer ionization chamber, a MatriXX PT ionization chamber array, and Gafchromic™ EBT3 film.Main results. For measurements made within the SOBPs, average 2D gamma pass rates exceeded 98.5% for the MatriXX PT and 96.5% for film at the 2%/2 mm criterion across all measured uncollimated and collimated plans, respectively. For verification of the penumbra width reduction with collimation, film agreed with Monte Carlo with differences within 0.3 mm on average compared to 0.9 mm for the MatriXX PT.Significance. We have experimentally verified the delivery of DCS-collimated fields using a clinical PBS system and commonly available dosimeters and have also identified potential weaknesses for dosimeters subject to steep dose gradients.

AAPM medical physics practice guideline 13.a: HDR brachytherapy, part A

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines (MPPGs) will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines: (1) Must and must not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. (2) Should and should not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances. Approved by AAPM's Executive Committee April 28, 2022.

Evaluating on-board kVCT- and MVCT-based dose calculation accuracy using a thorax phantom for helical tomotherapy treatments

Purpose.To evaluate the impact of CT number calibration and imaging parameter selection on dose calculation accuracy relative to the CT planning process in thoracic treatments for on-board helical CT imaging systems used in helical tomotherapy.Methods and Materials.Direct CT number calibrations were performed with appropriate protocols for each imaging system using an electron density phantom. Large volume and SBRT treatment plans were simulated and optimized for planning CT scans of an anthropomorphic thorax phantom and transferred to registered kVCT and MVCT scans of the phantom as appropriate. Relevant DVH metrics and dose-difference maps were used to evaluate and compare dose calculation accuracy relative to the planning CT based on a variation in imaging parameters applied for the on-board systems.Results.For helical kVCT scans of the thorax phantom, median differences in DVH parameters for the large volume treatment plan were less than ±1% with dose to the target volume either over- or underestimated depending on the imaging parameters utilized for CT number calibration and thorax phantom acquisition. For the lung SBRT plan calculated on helical kVCT scans, median dose differences were up to -2.7% with a more noticeable dependence on parameter selection. For MVCT scans, median dose differences for the large volume plan were within +2% with dose to the target overestimated regardless of the imaging protocol.Conclusion.Accurate dose calculations (median errors of <±1%) using a thorax phantom simulating realistic patient geometry and scatter conditions can be achieved with images acquired with a helical kVCT system on a helical tomotherapy unit. This accuracy is considerably improved relative to that achieved with the MV-based approach. In a clinical setting, careful consideration should be made when selecting appropriate kVCT imaging parameters for this process as dose calculation accuracy was observed to vary with both parameter selection and treatment type.

On the perturbation effect and LET dependence of beam quality correction factors in carbon ion beams

BACKGROUND

In a recent study, we reported beam quality correction factors, f_Q , in carbon ion beams using Monte Carlo (MC) methods for a cylindrical and a parallel-plate ionization chamber (IC). A non-negligible perturbation effect was observed; however, the magnitude of the perturbation correction due to the specific IC subcomponents was not included. Furthermore, the stopping power data presented in the International Commission on Radiation Units and Measurements (ICRU) report 73 were used, whereas the latest stopping power data have been reported in the ICRU report 90.

PURPOSE

The aim of this study was to extend our previous work by computing f_Q correction factors using the ICRU 90 stopping power data and by reporting IC-specific perturbation correction factors. Possible energy or linear energy transfer (LET) dependence of the f_Q correction factor was investigated by simulating both pristine beams and spread-out Bragg peaks (SOBPs).

METHODS

The TOol for PArticle Simulation (TOPAS)/GEANT4 MC code was used in this study. A 30 × 30 × 50 cm³ water phantom was simulated with a uniform 10 × 10 cm² parallel beam incident on the surface. A Farmer-type cylindrical IC (Exradin A12) and two parallel-plate ICs (Exradin P11 and A11) were simulated in TOPAS using the manufacturer-provided geometrical drawings. The f_Q correction factor was calculated in pristine carbon ion beams in the 150-450 MeV/u energy range at 2 cm depth and in the middle of the flat region of four SOBPs. The k_Q correction factor was calculated by simulating the f_Qo correction factor in a ⁶⁰ Co beam at 5 cm depth. The perturbation correction factors due to the presence of the individual IC subcomponents, such as the displacement effect in the air cavity, collecting electrode, chamber wall, and chamber stem, were calculated at 2 cm depth for monoenergetic beams only. Additionally, the mean dose-averaged and track-averaged LET was calculated at the depths at which the f_Q was calculated.

RESULTS

The ICRU 90 f_Q correction factors were reported. The p_dis correction factor was found to be significant for the cylindrical IC with magnitudes up to 1.70%. The individual perturbation corrections for the parallel-plate ICs were <1.0% except for the A11 p_cel correction at the lowest energy. The f_Q correction for the P11 IC exhibited an energy dependence of >1.00% and displayed differences up to 0.87% between pristine beams and SOBPs. Conversely, the f_Q for A11 and A12 displayed a minimal energy dependence of <0.50%. The energy dependence was found to manifest in the LET dependence for the P11 IC. A statistically significant LET dependence was found only for the P11 IC in pristine beams only with a magnitude of <1.10%.

CONCLUSIONS

The perturbation and k_Q correction factor should be calculated for the specific IC to be used in carbon ion beam reference dosimetry as a function of beam quality.

The Impact of Parameter Selection and Setup Conditions on Image Quality of an On-Board Helical Kilovoltage Computed Tomography System

Purpose

To evaluate the imaging performance of an on-board helical kilovoltage computed tomography (kVCT) system mounted on a helical tomotherapy unit for various imaging parameters and setup conditions.

Methods

Images of a commonly used computed tomography (CT) image quality phantom were acquired while varying the selection of available parameters (anatomy, mode, body size) as well as phantom positioning and size. Image quality metrics (IQM) including noise, uniformity, contrast, CT number constancy, and spatial resolution were compared for parameter and setup variations. 

Results

The use of fine mode improved noise and contrast metrics by 20-30% compared to normal mode and by nearly a factor of two compared to the coarse mode for otherwise identical protocols. Uniformity, CT number constancy, and spatial resolution were also improved for fine mode. Thorax and pelvis anatomy protocols improved noise, uniformity, and contrast metrics by 10-20% compared to images acquired with head protocols. No significant differences in CT number constancy or spatial resolution were observed regardless of anatomy choice. Increasing body size (milliampere second (mAs)/rotation) improved each image quality metric. Vertical and lateral phantom shifts of up to ±6 cm degraded noise and contrast metrics by up to 30% relative to the isocenter while also worsening uniformity and CT number constancy. IQM were also degraded substantially with the use of annuli to increase the phantom diameter (32 cm vs. 20 cm). Despite variations in image characteristics among the investigated changes, most metrics were within manufacturer specifications when applicable.

Conclusion

This work demonstrates the dependence of image quality on parameter selection and setup conditions for a helical kVCT system utilized in image-guided and adaptive helical tomotherapy treatments. While the overall image quality is robust to variations in imaging parameters, care should be taken when selecting parameters as patient size increases or positioning moves from the isocenter to ensure adequate image quality is still achieved.

Performance evaluation of image reconstruction algorithms for a megavoltage computed tomography system on a helical tomotherapy unit

Objective. To evaluate the impact of image reconstruction algorithm selection, as well as imaging mode and the reconstruction interval, on image quality metrics for megavoltage computed tomography (MVCT) image acquisition for use in image-guided (IGRT) and adaptive radiotherapy (ART) on a next-generation helical tomotherapy system.Approach. A CT image quality phantom was scanned across all available acquisition modes for filtered back projection (FBP) and both iterative reconstruction (IR) algorithms available on the system. Image quality metrics including noise, uniformity, contrast, spatial resolution, and mean CT number were compared. Analysis of DICOM data was performed using ImageJ software and Python code. ANOVA single factor and Tukey's honestly significant difference post-hoc tests were utilized for statistical analysis.Main Results. Application of both IR algorithms noticeably improved noise and image contrast when compared to the FBP algorithm available on all previous-generation helical tomotherapy systems. Use of the FBP algorithm improved image uniformity and spatial resolution in the axial plane, though values for the IR algorithms were well within tolerances recommended for IGRT and/or MVCT-based ART implementation by the American Association of Physicists in Medicine (AAPM). Additionally, longitudinal resolution showed little dependence on the reconstruction algorithm, while a negligible variation in mean CT number was observed regardless of the reconstruction algorithm or acquisition parameters. Statistical analysis confirmed the significance of these results.Significance. An overall improvement in image quality for metrics most important to IGRT and ART-mainly image noise and contrast-was evident in the application of IR when compared to FBP. Furthermore, since other imaging parameters remain identical regardless of the reconstruction algorithm, this improved image quality does not come at the expense of additional patient dose or an increased scan acquisition time for otherwise identical parameters. These improvements are expected to enhance fidelity in IGRT and ART implementation.

Characterization of imaging performance of a novel helical kVCT for use in image‐guided and adaptive radiotherapy

ClearRT helical kVCT imaging for the Radixact helical tomotherapy system recently received FDA approval and is available for clinical use. The system is intended to enhance image fidelity in radiation therapy treatment planning and delivery compared to the prior MV‐based onboard imaging approach. The purpose of this work was to characterize the imaging performance of this system and compare this performance with that of clinical systems used in image‐guided and/or adaptive radiotherapy (ART) or computed tomography (CT) simulation, including Radixact MVCT, TomoTherapy MVCT, Varian TrueBeam kV OBI CBCT, and the Siemens SOMATOM Definition Edge kVCT. A CT image quality phantom was scanned across clinically relevant acquisition modes for each system to evaluate image quality metrics, including noise, uniformity, contrast, spatial resolution, and CT number linearity. Similar noise levels were observed for ClearRT and Siemens Edge, whereas noise for the other systems was ∼1.5–5 times higher. Uniformity was best for Siemens Edge, whereas most scans for ClearRT exhibited a slight “cupping” or “capping” artifact. The ClearRT and Siemens Edge performed best for contrast metrics, which included low‐contrast visibility and contrast‐to‐noise ratio evaluations. Spatial resolution was best for TrueBeam and Siemens Edge, whereas the three kVCT systems exhibited similar CT number linearity. Overall, these results provide an initial indication that ClearRT image quality is adequate for image guidance in radiotherapy and sufficient for delineating anatomic structures, thus enabling its use for ART. ClearRT also showed significant improvement over MVCT, which was previously the only onboard imaging modality available on Radixact. Although the acquisition of these scans does come at the cost of additional patient dose, reported CTDI values indicate a similar or generally reduced machine output for ClearRT compared to the other systems while maintaining comparable or improved image quality overall.

Using 4D dose accumulation to calculate organ-at-risk dose deviations from motion-synchronized liver and lung tomotherapy treatments

Tracking systems such as Radixact Synchrony change the planned delivery of radiation during treatment to follow the target. This is typically achieved without considering the location changes of organs at risk (OARs). The goal of this work was to develop a novel 4D dose accumulation framework to quantify OAR dose deviations due to the motion and tracked treatment. The framework obtains deformation information and the target motion pattern from a four-dimensional computed tomography dataset. The helical tomotherapy treatment plan is split into 10 plans and motion correction is applied separately to the jaw pattern and multi-leaf collimator (MLC) sinogram for each phase based on the location of the target in each phase. Deformable image registration (DIR) is calculated from each phase to the references phase using a commercial algorithm, and doses are accumulated according to the DIR. The effect of motion synchronization on OAR dose was analyzed for five lung and five liver subjects by comparing planned versus synchrony-accumulated dose. The motion was compensated by an average of 1.6 cm of jaw sway and by an average of 5.7% of leaf openings modified, indicating that most of the motion compensation was from jaw sway and not MLC changes. OAR dose deviations as large as 19 Gy were observed, and for all 10 cases, dose deviations greater than 7 Gy were observed. Target dose remained relatively constant (D95% within 3 Gy), confirming that motion-synchronization achieved the goal of maintaining target dose. Dose deviations provided by the framework can be leveraged during the treatment planning process by identifying cases where OAR doses may change significantly from their planned values with respect to the critical constraints. The framework is specific to synchronized helical tomotherapy treatments, but the OAR dose deviations apply to any real-time tracking technique that does not consider location changes of OARs.

Tracking target/chest relationship changes during motion‐synchronized tomotherapy treatments

Background

Radixact Synchrony® is an intrafraction motion tracking system for helical tomotherapy treatments that uses kV radiographs of the target and LEDs on the patient's chest to synchronize the movement of the radiation beam with the respiratory motion of the target. Several works have demonstrated Synchrony's ability to track target motion when the chest and target motions are perfectly correlated.

Purpose

The purpose of this work was to determine Synchrony's ability to accurately adapt to scenarios with a changing target/chest correlation.

Methods

A custom ion chamber mimicking plug with embedded fiducials was placed inside a Delta4 Phantom+ and used as the tracking object. A separate motion stage was programmed to mimic chest motion. The target and chest surrogate phantom were programmed to move sinusoidally and two types of target/chest relationship changes were introduced: rigid shifts and linear drifts of the target position but not surrogate position. Tracking analysis was performed by comparing programmed phantom motion to log files of the Synchrony‐modeled motion. No dosimetry was performed in this work.

Results

At the fastest imaging rate of 2 s/img, Synchrony accurately adapted for gradual drifts in the target location (up to 5 mm/min) with minor increases in tracking errors and adapted for an abrupt 5 mm shift after about 30 s (with an auto‐pause threshold at 60 s). When the imaging period was longer (> 4 s/img), larger tracking errors (> 5 mm) were observed, and the treatment would be paused. The measured delta (MD) parameter (2D target localization error on the most recent image) was found to be a more responsive indicator of tracking errors than the potential difference (PD) parameter (3D estimator of tracking error based on all images in the model). Lastly, the effect of a recent update to the tracking algorithm was found to improve the ability of Synchrony to track target/chest relationship changes.

Conclusions

This work demonstrated that Synchrony can adapt to gradual changes (drifts) in the target/chest relationship, but it takes a finite amount of time to adapt to abrupt shifts. Ability to adapt to these changes increases with increasing imaging frequency. Larger tracking errors were observed in this work than others have reported in the literature due to the introduction of target/chest correlation changes in this work. Future work needs to be performed investigating what type and magnitude of target/chest miscorrelations occur in patients. Lastly, users should ensure they are using the most recent software (3.0.1 or newer) to improve the ability of Synchrony to track these movements.

The Effect of Mouse Size on Dose from an X-Rad320 Irradiator

Irradiation protocols for murine experiments often use standardized dose rate estimates for calculating dose delivered, regardless of physical variations between mouse subjects. This work sought to determine the significance of mouse size on absorbed dose. Five mouse-like phantoms of various sizes based on the mouse whole-body (MOBY) model were 3D printed. The phantoms were placed in an X-Rad320 biological irradiator and a standard irradiation protocol was used to deliver dose. Dose was measured using thermoluminescent dosimeter (TLD) microcubes inside each phantom, and the relative readings were used to calculate output factors (OFs), normalized to the phantom of median volume. Additionally, the OF for each mouse was simulated in Monte Carlo N-Particle (MCNP) code. For both the TLD measurements and MCNP simulations, the OF for each mouse was determined by both experiments and calculations to be unity within the relative standard uncertainties (k = 1). This work supports comparing results across various studies using the X-Rad320 irradiator without need for corrections based on mouse size.

Investigating aperture-based approximations to model a focused dynamic collimation system for pencil beam scanning proton therapy

Purpose. The Dynamic Collimation System (DCS) is an energy layer-specific collimation device designed to reduce the lateral penumbra in pencil beam scanning proton therapy. The DCS consists of two pairs of nickel trimmers that rapidly and independently move and rotate to intercept the scanning proton beam and an integrated range shifter to treat targets less than 4 cm deep. This work examines the validity of a single aperture approximation to model the DCS, a commonly used approximation in commercial treatment planning systems, as well as higher-order aperture-based approximations for modeling DCS-collimated dose distributions.Methods. An experimentally validated TOPAS/Geant4-based Monte Carlo model of the DCS integrated with a beam model of the IBA pencil beam scanning dedicated nozzle was used to simulate DCS- and aperture-collimated 100 MeV beamlets and composite treatment plans. The DCS was represented by three different aperture approximations: a single aperture placed halfway between the upper and lower trimmer planes, two apertures located at the upper and lower trimmer planes, and four apertures, located at both the upstream and downstream faces of each pair of trimmers. Line profiles and three-dimensional regions of interest were used to evaluate the validity and limitations of the aperture approximations investigated.Results. For pencil beams without a range shifter, minimal differences were observed between the DCS and single aperture approximation. For range shifted beamlets, the single aperture approximation yielded wider penumbra widths (up to 18%) in the X-direction and sharper widths (up to 9.4%) in the Y-direction. For the example treatment plan, the root-mean-square errors (RMSEs) in an overall three-dimensional region of interest were 1.7%, 1.3%, and 1.7% for the single aperture, two aperture, and four aperture models, respectively. If the region of interest only encompasses the lateral edges outside of the target, the resulting RMSEs were 1.7%, 1.1%, and 0.5% single aperture, two aperture, and four aperture models, respectively.Conclusions. Monte Carlo simulations of the DCS demonstrated that a single aperture approximation is sufficient for modeling pristine fields at the Bragg depth while range shifted fields require a higher-order aperture approximation. For the treatment plan considered, the double aperture model performed the best overall, however, the four-aperture model most accurately modeled the lateral field edges at the expense of increased dose differences proximal to and within the target.

Fiducial visibility on planar images during motion-synchronized tomotherapy treatments

Objective. Synchrony^®is a motion management system on the Radixact^®that uses planar kV radiographs to locate the target during treatment. The purpose of this work is to quantify the visibility of fiducials on these radiographs.Approach. A custom acrylic slab was machined to hold 8 gold fiducials of various lengths, diameters, and orientations with respect to the imaging axis. The slab was placed on the couch at the imaging isocenter and planar radiographs were acquired perpendicular to the custom slab with varying thicknesses of acrylic on each side. Fiducial signal to noise ratio (SNR) and detected fiducial position error in millimeters were quantified.Main Results. The minimum output protocol (100 kVp, 0.8 mAs) was sufficient to detect all fiducials on both Radixact configurations when the thickness of the phantom was 20 cm. However, no fiducials for any protocol were detected when the phantom was 50 cm thick. The algorithm accurately detected fiducials on the image when the SNR was larger than 4. The MV beam was observed to cause RFI artifacts on the kV images and to decrease SNR by an average of 10%.Significance. This work provides the first data on fiducial visibility on kV radiographs from Radixact Synchrony treatments. The Synchrony fiducial detection algorithm was determined to be very accurate when sufficient SNR is achieved. However, a higher output protocol may need to be added for use with larger patients. This work provided groundwork for investigating visibility of fiducial-free solid targets in future studies and provided a direct comparison of fiducial visibility on the two Radixact configurations, which will allow for intercomparison of results between configurations.

Technical note: On the impact of the kV imaging configuration on doses from planar images during motion-synchronized treatments on Radixact®

Kilovoltage radiographs are acquired during motion‐synchronized treatments on Radixact to localize the tumor during the treatment. Several previous publications have provided estimates of patient dose from these planar radiographs. However, a recent hardware update changed several aspects of the kV imaging system, including a new X‐ray tube, an extended source‐to‐axis distance (SAD), and a larger field size. This is denoted the extended configuration. The purpose of this work was to assess the impact of the configuration change on patient dose from these procedures. Point doses in water were measured using the TG‐61 protocol for tube potentials between 100 and 140 kVp for both the standard and extended configurations under the same water tank setup. Comparisons were made for equal mAs since the same protocols (kVp, mAs) will be used for both configurations. In comparison to the standard configuration, doses per mAs from the extended configuration were found to be ~66% less and falloff less steep due to the increased SAD. However, a larger volume of tissue is irradiated due to the larger field size. Beam quality for a given tube potential was the same as determined by half‐value layer measurements. Both kV configurations are available from the vendor, therefore, the values in this work can be used to compare values previously published in the literature for the standard configuration or to intercompare doses from these two system configurations.

Development and validation of the Dynamic Collimation Monte Carlo simulation package for pencil beam scanning proton therapy

PURPOSE

The aim of this work was to develop and experimentally validate a Dynamic Collimation Monte Carlo (DCMC) simulation package specifically designed for the simulation of collimators in pencil beam scanning proton therapy (PBS-PT). The DCMC package was developed using the TOPAS Monte Carlo platform and consists of a generalized PBS source model and collimator component extensions.

METHODS

A divergent point-source model of the IBA dedicated nozzle (DN) at the Miami Cancer Institute (MCI) was created and validated against on-axis commissioning measurements taken at MCI. The beamline optics were mathematically incorporated into the source to model beamlet deflections in the X and Y directions at the respective magnet planes. Off-axis measurements taken at multiple planes in air were used to validate both the off-axis spot size and divergence of the source model. The DCS trimmers were modeled and incorporated as TOPAS geometry extensions that linearly translate and rotate about the bending magnets. To validate the collimator model, a series of integral depth dose (IDD) and lateral profile measurements were acquired at MCI and used to benchmark the DCMC performance for modeling both pristine and range shifted beamlets. The water equivalent thickness (WET) of the range shifter was determined by quantifying the shift in the depth of the 80% dose point distal to the Bragg peak between the range shifted and pristine uncollimated beams.

RESULTS

A source model of the IBA DN system was successfully commissioned against on- and off-axis IDD and lateral profile measurements performed at MCI. The divergence of the source model was matched through an optimization of the source-to-axis distance and comparison against in-air spot profiles. The DCS model was then benchmarked against collimated IDD and in-air and in-phantom lateral profile measurements. Gamma analysis was used to evaluate the agreement between measured and simulated lateral profiles and IDDs with 1%/1 mm criteria and a 1% dose threshold. For the pristine collimated beams, the average 1%/1 mm gamma pass rates across all collimator configurations investigated were 99.8% for IDDs and 97.6% and 95.2% for in-air and in-phantom lateral profiles. All range shifted collimated IDDs passed at 100% while in-air and in-phantom lateral profiles had average pass rates of 99.1% and 99.8%, respectively. The measured and simulated WET of the polyethylene range shifter was determined to be 40.9 and 41.0 mm, respectively.

CONCLUSIONS

We have developed a TOPAS-based Monte Carlo package for modeling collimators in PBS-PT. This package was then commissioned to model the IBA DN system and DCS located at MCI using both uncollimated and collimated measurements. Validation results demonstrate that the DCMC package can be used to accurately model other aspects of a DCS implementation via simulation.

Effects of variable-width jaw motion on beam characteristics for Radixact Synchrony®

PURPOSE

Radixact Synchrony corrects for target motion during treatment by adjusting the jaw and MLC positions in real time. As the jaws move off axis, Synchrony attempts to adjust for a loss in output due to the un-flattened 6 MV beam by increasing the jaw aperture width. The purpose of this work was to assess the impact of the variable-width aperture on delivered dose using measurements and simulations.

METHODS

Longitudinal beam profile measurements were acquired using an Edge diode with static gantry. Jaw-offset peak, width, and integral factors were calculated for profiles with the jaws in the extreme positions using both variable-width (Synchrony) and fixed-width apertures. Treatment plans with target motion and compensation were compared to planned doses to study the impact of the variable aperture on volumetric dose.

RESULTS

The jaw offset peak factor (JOPF) for the Synchrony jaw settings were 0.964 and 0.983 for the 1.0- and 2.5-cm jaw settings, respectively. These values decreased to 0.925 and 0.982 for the fixed-width settings, indicating that the peak value of the profile would decrease by 7.5% compared to centered if the aperture width was held constant. The IMRT dose distributions reveal similar results, where gamma pass rates are above tolerance for the Synchrony jaw settings but fall significantly for the fixed-width 1-cm jaws.

CONCLUSIONS

The variable-width behavior of Synchrony jaws provides a larger output correction for the 1-cm jaw setting. Without the variable-aperture correction, plans with the 1-cm jaw setting would underdose the target if the jaws spend a significant amount of time in the extreme positions. This work investigated the change in delivered dose with jaws in the extreme positions, therefore overall changes in dose due to offset jaws are expected to be less for composite treatment deliveries.

Experimental and Monte Carlo characterization of a dynamic collimation system prototype for pencil beam scanning proton therapy

PURPOSE

There has been a growing interest in the development of energy-specific collimators for low-energy pencil beam scanning (PBS) to reduce the lateral penumbra. One particular device that has been the focus of several recent published works is the dynamic collimation system (DCS), which provides energy-specific collimation by intercepting the scanned proton beam as it nears to target edge with a set of orthogonal trimmer blades. While several computational studies have shown that this dynamic collimator can provide additional healthy tissue sparing, there has not been any rigorous experimental work to benchmark the theoretical models used in these initial studies. Therefore, it was the purpose of this work to demonstrate an experimental method that could integrate an experimental prototype with a clinical PBS system and benchmark the Monte Carlo methods that have been used to model the DCS.

METHODS

An experimental DCS prototype was designed and built in house to actively collimate individual proton beamlets during PBS within a well-characterized experimental setup. Monte Carlo methods were initially used to assess construction tolerances and later benchmarked against measurements, including integral depth dose and lateral asymmetric beamlet profiles. The experimental apparatus and measurement geometry were modeled using MCNP6 benchmarked from measurements performed at the Northwestern Chicago Proton Center.

RESULTS

Gamma analysis tests were used to evaluate the agreement between the measured and simulated profiles with a strict 1 mm/1% criteria and 5% dose threshold. Excellent agreement was observed between the simulated and measured profiles, which included 1 mm/1% gamma analysis pass rates of at least 100% and 95% for the integral depth dose (IDD) profiles and lateral profiles, respectively. Differences in the relative profile shape were observed experimentally between beamlets collimated on- and off-axis, which was attributed to the partial transmission of the beam through an unfocused collimator. Exposure rates resulting from the activation of the device were monitored with survey meter measurements and were found to agree with Monte Carlo estimates of the exposure rate to within 20%.

CONCLUSION

A DCS prototype was constructed and integrated into a clinical dose delivery system. While the results of this work are not exhaustive, they demonstrate the effects of beam source divergence, device activation, and beamlet deflection during scanning, which were found to be successfully modeled using Monte Carlo methods and experimentally benchmarked. Excellent agreement was achieved between the simulated and measured lateral spot profiles of collimated beamlets delivered on- and off-axis in PBS. The Monte Carlo models adequately predicted the measured elevated plateau region in the integral depth-dose profiles from the low-energy scatter off the collimators.

Technical Note: Patient dose from kilovoltage radiographs during motion-synchronized treatments on Radixact®

PURPOSE

Synchrony is a motion management system available on the Radixact linear accelerator that utilizes kilovoltage (kV) radiographs to track target motion and synchronize the delivery of radiation with the motion. Proper management of this imaging dose requires accurate quantification. The purpose of this work was to use Monte Carlo (MC) simulations to quantify organ-specific patient doses from these images for various patient anatomies.

METHODS

Point doses in water were measured per TG-61 for three beam qualities commonly used on the Radixact. The point doses were used to benchmark a model of the imaging system built using the Monte Carlo N-Particle (MCNP) transport code. Patient computed tomography (CT) datasets were obtained for 5 patients and 100 planar images were simulated for each patient. Patient dose was calculated using energy deposition mesh tallies.

RESULTS

The MCNP model was able to accurately reproduce the measured point doses, with a median dose difference of less than 1%. The median dose (D_50% ) to soft tissue from 100 radiographs among the 5 patient cases ranged from 2.0 to 4.6 mGy. The max dose (D_1% ) to soft tissue ranged from 6.2 to 31.0 mGy and the max dose to bony structures ranged from 20.2 to 71.7 mGy. These doses can be scaled to estimate total patient dose throughout many fractions.

CONCLUSIONS

Patient dose is largely dependent on imaging protocol, patient size, and treatment parameters such as fractionation and gantry period. Organ doses from 100 radiographs (an approximate number for one fraction) on the Radixact are slightly less than the doses from Tomo MVCT setup images. Careful selection of clinical protocols and planning parameters can be used to minimize risk from these images.

Evaluation of radixact motion synchrony for 3D respiratory motion: Modeling accuracy and dosimetric fidelity

The Radixact® linear accelerator contains the motion Synchrony system, which tracks and compensates for intrafraction patient motion. For respiratory motion, the system models the motion of the target and synchronizes the delivery of radiation with this motion using the jaws and multi-leaf collimators (MLCs). It was the purpose of this work to determine the ability of the Synchrony system to track and compensate for different phantom motions using a delivery quality assurance (DQA) workflow. Thirteen helical plans were created on static datasets from liver, lung, and pancreas subjects. Dose distributions were measured using a Delta4® Phantom+ mounted on a Hexamotion® stage for the following three case scenarios for each plan: (a) no phantom motion and no Synchrony (M0S0), (b) phantom motion and no Synchrony (M1S0), and (c) phantom motion with Synchrony (M1S1). The LEDs were placed on the Phantom+ for the 13 patient cases and were placed on a separate one-dimensional surrogate stage for additional studies to investigate the effect of separate target and surrogate motion. The root-mean-square (RMS) error between the Synchrony-modeled positions and the programmed phantom positions was <1.5 mm for all Synchrony deliveries with the LEDs on the Phantom+. The tracking errors increased slightly when the LEDs were placed on the surrogate stage but were similar to tracking errors observed for other motion tracking systems such as CyberKnife Synchrony. One-dimensional profiles indicate the effects of motion interplay and dose blurring present in several of the M1S0 plans that are not present in the M1S1 plans. All 13 of the M1S1 measured doses had gamma pass rates (3%/2 mm/10%T) compared to the planned dose > 90%. Only two of the M1S0 measured doses had gamma pass rates > 90%. Motion Synchrony offers a potential alternative to the current, ITV-based motion management strategy for helical tomotherapy deliveries.

An investigation into the robustness of dynamically collimated proton therapy treatments

PURPOSE

To investigate the dosimetric robustness of dynamically collimated proton therapy (DCPT) treatment plans delivered using a dynamic collimation system (DCS) with respect to random uncertainties in beam spot and collimator position as well as systematic offsets in the DCS mounting alignment. This work also demonstrates a technique that can increase plan robustness while preserving target conformity.

METHODS

Variability in beam spot and collimator positioning can result in changes to a beamlet's dose distribution and incident fluence. The robustness of the DCPT treatment plans was evaluated for three intracranial treatment sites by modeling treatment variability as normally distributed random variables with standard deviations reflecting a clinical system. The simulated treatment plans were then recalculated and compared against their nominal, idealized dose distribution among several trials. It was hypothesized that a plan's robustness to these delivery variables could be reduced by restricting a trimmer's placement toward a beamlet's central axis during collimation.

RESULTS

By introducing a minimum trimmer offset of 1.5 mm, the variation of the planning target volume (PTV) D_95% coverage was reduced to within 2% of the prescribed dose. The treatment plans with trimmers that were placed within 0.5 mm of a collimated beamlet's central axis resulted in the greatest healthy tissue sparing but deviations as high as 11.4% to the PTV D_95% were observed. The nominal conformity of these treatment plans utilizing the 1.5 mm trimmer offset was also well maintained. For each treatment plan studied, the 90% conformity index remained within 6.25% of the conformity index achieved without a minimum trimmer offset, and the D_50% of surrounding healthy tissue increased by no more than 3.1 Gy relative to a plan without a trimmer offset.

CONCLUSIONS

While DCPT can offer a significant reduction in healthy tissue irradiation, the results from this work indicate that special care must be taken to ensure proper PTV coverage amid uncertainties associated with this new treatment modality. A simple approach utilizing a minimum trimmer offset was able to preserve the majority of the target conformity and healthy tissue sparing the DCS technology affords while minimizing the uncertainties in this treatment approach.

Trimmer sequencing time minimization during dynamically collimated proton therapy using a colony of cooperating agents

The dynamic collimation system (DCS) can be combined with pencil beam scanning proton therapy to deliver highly conformal treatment plans with unique collimation at each energy layer. This energy layer-specific collimation is accomplished through the synchronized motion of four trimmer blades that intercept the proton beam near the target boundary in the beam's eye view. However, the corresponding treatment deliveries come at the cost of additional treatment time since the translational speed of the trimmer is slower than the scanning speed of the proton pencil beam. In an attempt to minimize the additional trimmer sequencing time of each field while still maintaining a high degree of conformity, a novel process utilizing ant colony optimization (ACO) methods was created to determine the most efficient route of trimmer sequencing and beamlet scanning patterns for a collective set of collimated proton beamlets. The ACO process was integrated within an in-house treatment planning system optimizer to determine the beam scanning and DCS trimmer sequencing patterns and compared against an analytical approximation of the trimmer sequencing time should a contour-like scanning approach be assumed instead. Due to the stochastic nature of ACO, parameters where determined so that they could ensure good convergence and an efficient optimization of trimmer sequencing that was faster than an analytical contour-like trimmer sequencing. The optimization process was tested using a set of three intracranial treatment plans which were planned using a custom research treatment planning system and were successfully optimized to reduce the additional trimmer sequencing time to approximately 60 s per treatment field while maintaining a high degree of target conformity. Thus, the novel use of ACO techniques within a treatment planning algorithm has been demonstrated to effectively determine collimation sequencing patterns for a DCS in order to minimize the additional treatment time required for trimmer movement during treatment.

LET response variability of Gafchromic TM EBT3 film from a 60 Co calibration in clinical proton beam qualities

PURPOSE

To establish a method of accurate dosimetry required to quantify the expected linear energy transfer (LET) quenching effect of EBT3 film used to benchmark the dose distribution for a given treatment field and specified measurement depth. In order to facilitate this technique, a full analysis of film calibration which considers LET variability at the plane of measurement and as a function of proton beam quality is demonstrated. Additionally, the corresponding uncertainty from the process was quantified for several measurement scenarios.

MATERIALS AND METHODS

The net change in optical density (OD) from a single version of Gafchromic TM EBT3 film was measured using an Epson flatbed scanner and NIST-traceable OD filters. Film OD response was characterized with respect to the known dose to water at the point of measurement for both a NIST-traceable 60 Co beam at the UWADCL and several clinical single-energy and spread-out Bragg peak (SOBP) proton beam qualities at the Northwestern Medicine Chicago Proton Center. Increasing proton LET environments were acquired by placing film at increasing depths of Gammex HE Solid Water® whose water-equivalent thickness was characterized prior to measurement.

RESULTS

A strong LET dependence was observed near the Bragg peak (BP) consistent with previous studies performed with earlier versions of EBT3 film. The influence of range straggling on the film's LET response appears to have a uniform effect toward the BP regardless of the nominal beam energy. Proximal to this depth, the film's response decreased with decreasing energy at the same dose-average LET. The opposite trend was observed for depths past the BP. Changes in the SOBP energy modulation showed a linear relationship between the film's relative response and dose-averaged LET. Relative effectiveness factors (RE) were observed to range between 2%-7% depending on the width of the SOBP and depth of the film. Using the field-specific calibration technique, a total k = 1 uncertainty in the absorbed dose to water was estimated to range from 4.68%-5.21%.

CONCLUSION

While EBT3 film's strong LET dependence is a common problem in proton beam dosimetry, this work has shown that the LET dependence can be taken into account by carefully considering the depth and energy modulation across the film using field-specific corrections. RE factors were determined with a combined k = 1 uncertainty of 3.57% for SOBP environments and between 3.17%-4.69% for uniform, monoenergetic fields proximal to the distal 80% of the BP.

A convex windowless extrapolation chamber to measure surface dose rate from 106 Ru/106 Rh episcleral plaques

PURPOSE

A convex windowless extrapolation chamber was developed as a primary measurement device to determine surface dose rate from curved ¹⁰⁶ Ru/¹⁰⁶ Rh episcleral plaques.

METHODS

A convex extrapolation chamber without an entrance window was constructed for this work, and surface dose rate measurements were performed with two curved CCB-type ¹⁰⁶ Ru/¹⁰⁶ Rh plaques (S/N 2545 and 2596) manufactured by Eckert & Ziegler BEBIG. FARO ® Gage measurements were performed to verify the radius of curvature for the convex electrode and the concave plaque surface. Furthermore, the collecting electrode area was verified through capacitance measurements. Chamber correction factors for divergence and backscatter were generated using the EGSnrc cavity user code. For each source, surface dose rate was measured with the convex extrapolation chamber and compared with on-contact measurements made with curved un-laminated EBT3 film strips. A Monte Carlo correction was generated for radiochromic film measurements to account for volume averaging within the active layer and effects of phantom scatter. Additionally, extrapolation chamber results for each plaque were compared with scintillation detector measurements performed by the manufacturer. For the second source (S/N 2596), a comparison was also made with the Monte Carlo-corrected surface dose rate measured at the National Physical Laboratory (NPL) using cylindrical alanine pellets. Finally, source measurements were performed using conventional ionization chambers (Exradin A26, A1SL, and A20) within a custom fixture to investigate the transfer of extrapolation chamber surface dose rate to clinics.

RESULTS

For the first ¹⁰⁶ Ru/¹⁰⁶ Rh plaque (S/N 2545), average surface dose rate from the convex windowless extrapolation chamber was found to be 1.5% higher than the corresponding value from curved un-laminated EBT3 film measurements and 5.6% lower than the manufacturer value. For the second source (S/N 2596), the extrapolation chamber surface dose rate was 2.5% higher than the un-laminated EBT3 film result, 4.5% lower than the manufacturer value, and 3.9% higher compared to corrected alanine measurements made at NPL. Total uncertainty in the extrapolation chamber measurement was estimated to be approximately  ± 7.0% (k = 2). For the plaque measurements made using conventional ionization chambers with a custom fixture, surface dose rate from the transfer technique was found to agree within 3.8% with the expected convex extrapolation chamber result for S/N 2596.

CONCLUSIONS

A convex windowless extrapolation chamber was developed as a primary measurement device for ¹⁰⁶ Ru/¹⁰⁶ Rh plaques. Through comparison with the extrapolation chamber, the accuracy of surface dose rate measurements from current dosimetry techniques was assessed and agreement was seen within 5.6%. Finally, it was found that conventional ionization chambers could be calibrated with a reference ¹⁰⁶ Ru/¹⁰⁶ Rh plaque in order to transfer the extrapolation chamber result for surface dose rate to clinics.

Technical Note: Optimization of spot and trimmer position during dynamically collimated proton therapy

PURPOSE

To demonstrate a novel theoretical optimization design which considers beam spot and trimmer positioning in addition to beamlet weighting for dynamically collimated proton therapy (DCPT) treatments. Prior to this, the previous methods of plan optimization used to study this emerging technology relied upon an intuitive selection criterion to fix the trimmers blades for a uniform grid of beam spots before determining the individual beamlet weights. To evaluate the potential benefit from this new optimization design, a treatment planning optimization study was performed in order to compare the algorithm's functionality against the existing methods of plan optimization.

MATERIALS AND METHODS

A direct parameter optimization (DPO) method was developed to determine beam spot and trimmer positions cohesively with beamlet weighting for DCPT treatment plans. Gradients were numerically determined from applying small adjustments to the aforementioned parameters and quantifying the resulting impact on an objective function. This technique was compared to the conventional trimmer selection algorithm (TSA) which does not optimize spot position concurrently with trimmer position. Both planning methods were used to optimize a set of brain treatment plans, and the resulting dose distributions were compared with dose-volume histogram quantities in addition to target coverage, homogeneity, and conformity metrics.

RESULTS

An overall improvement to the target conformity and healthy tissue sparing was achieved with DPO over TSA while maintaining an equivalent planning target volume (PTV) coverage index for the three brain patients evaluated in this study. On average, the conformity index improved by 5.5% when utilizing DPO. A similar improvement in reducing the dose to several organs at risk was also noted.

CONCLUSION

Both the TSA and DPO planning methods can achieve highly conformal treatments with the dynamic collimation system (DCS) technology. However, an improvement in the target conformity and healthy tissue sparing was achieved by simultaneously optimizing beam spot position, trimmer location, and beamlet weights using DPO in comparison to the TSA technique.

Technical Note: Characterization of clinical linear accelerator triggering latency for motion management system development

PURPOSE

Latencies for motion management systems have previously been presented as guidelines for system development and implementation. These guidelines consider the overall system latency, including data acquisition, algorithm processing, and linac triggering time. However, during system development, the triggering latency of the clinical linear accelerator is often considered fixed. This paper presents a method to decouple the linac-only triggering latency from the total system latency such that latency can be considered in terms of only the linac-independent aspects of the system.

METHODS

The linac-only latency was investigated by considering the time at which a linac response was observed relative to the time at which a beam-on/off triggering signal was sent to the linac. The relative time between the two signals was analyzed using a multichannel oscilloscope with input signals from a custom gating box to manually trigger the beam state as well as a diode positioned at beam isocenter to monitor the linac response. The beam-on/off latency was measured at multiple energies (6/18 MV) and repetition rates (100-600 MU/min) to investigate beam setting dependencies.

RESULTS

The measured latency was observed to be dependent on the accelerator settings for repetition rate and energy, with beam-on latencies decreasing with increasing repetition rate and decreasing energy. In contrast, the opposite trend was present for the observed beam-off latency. At 600 MU/min, beam-on/off latencies were observed to be 3.37/1.45 ms for a 6 MV beam and 6.02/0.73 ms for an 18 MV beam. Negative latencies were possible for beam-off measurements due to the mechanical latency being less than the pulse separation at given repetition rates.

CONCLUSIONS

The linac latency associated with triggering the beam-on/off was determined to have a minor contribution to the total allowable system latency; thus, the majority of the total system latency can be attributed to linac-independent factors.

Secondary Neutron Dose From a Dynamic Collimation System During Intracranial Pencil Beam Scanning Proton Therapy: A Monte Carlo Investigation

PURPOSE

Patients receiving pencil beam scanning (PBS) proton therapy with the addition of a dynamic collimation system (DCS) are potentially subject to an additional neutron dose from interactions between the incident proton beam and the trimmer blades. This study investigates the secondary neutron dose rates for both single-field uniform dose (SFUD) and intensity modulated proton therapy treatments.

METHODS AND MATERIALS

Secondary neutron dose distributions were calculated for both a dynamically collimated and an uncollimated, dual-field chordoma treatment plan and compared with previously published neutron dose rates from other contemporary scanning treatment modalities. Monte Carlo N-Particle transport code was used to track all primary and secondary particles generated from nuclear reactions within the DCS during treatment through a model of the patient geometry acquired from the computed tomography planning data set. Secondary neutron ambient dose equivalent distributions were calculated throughout the patient using a meshgrid with a tally resolution equivalent to that of the treatment planning computed tomography.

RESULTS

The median healthy-brain neutron ambient dose equivalent for a dynamically collimated intracranial chordoma treatment plan using a DCS was found to be 0.97 mSv/Gy for the right lateral SFUD field, 1.37 mSv/Gy for the apex SFUD field, and 1.24 mSv/Gy for the composite intensity modulated proton therapy distribution from 2 fields.

CONCLUSIONS

These results were at least 55% lower than what has been reported for uniform scanning modalities with brass apertures. However, they still reflect an increase in the excess relative risk of secondary cancer incidence compared with an uncollimated PBS treatment using only a graphite range shifter. Regardless, the secondary neutron dose expected from the DCS for these PBS proton therapy treatments appears to be on the order of, or below, what is expected for alternative collimated proton therapy techniques.

Dosimetric characterization of a new directional low-dose rate brachytherapy source

PURPOSE

CivaTech Oncology Inc. (Durham, NC) has developed a novel low-dose rate (LDR) brachytherapy source called the CivaSheet.TM The source is a planar array of discrete elements ("CivaDots") which are directional in nature. The CivaDot geometry and design are considerably different than conventional LDR cylindrically symmetric sources. Thus, a thorough investigation is required to ascertain the dosimetric characteristics of the source. This work investigates the repeatability and reproducibility of a primary source strength standard for the CivaDot and characterizes the CivaDot dose distribution by performing in-phantom measurements and Monte Carlo (MC) simulations. Existing dosimetric formalisms were adapted to accommodate a directional source, and other distinguishing characteristics including the presence of gold shield x-ray fluorescence were addressed in this investigation.

METHODS

Primary air-kerma strength (SK ) measurements of the CivaDots were performed using two free-air chambers namely, the Variable-Aperture Free-Air Chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center (UWMRRC) and the National Institute of Standards and Technology (NIST) Wide-Angle Free-Air Chamber (WAFAC). An intercomparison of the two free-air chamber measurements was performed along with a comparison of the different assumed CivaDot energy spectra and associated correction factors. Dose distribution measurements of the source were performed in a custom polymethylmethacrylate (PMMA) phantom using GafchromicTM EBT3 film and thermoluminescent dosimeter (TLD) microcubes. Monte Carlo simulations of the source and the measurement setup were performed using MCNP6 radiation transport code.

RESULTS

The CivaDot SK was determined using the two free-air chambers for eight sources with an agreement of better than 1.1% for all sources. The NIST measured CivaDot energy spectrum intensity peaks were within 1.8% of the MC-predicted spectrum intensity peaks. The difference in the net source-specific correction factor determined for the CivaDot free-air chamber measurements for the NIST WAFAC and UW VAFAC was 0.7%. The dose-rate constant analog was determined to be 0.555 cGy h-1 U-1 . The average difference observed in the estimated CivaDot dose-rate constant analog using measurements and MCNP6-predicted value (0.558 cGy h-1 U-1 ) was 0.6% ± 2.3% for eight CivaDot sources using EBT3 film, and -2.6% ± 1.7% using TLD microcube measurements. The CivaDot two-dimensional dose-to-water distribution measured in phantom was compared to the corresponding MC predictions at six depths. The observed difference using a pixel-by-pixel subtraction map of the measured and the predicted dose-to-water distribution was generally within 2-3%, with maximum differences up to 5% of the dose prescribed at the depth of 1 cm.

CONCLUSION

Primary SK measurements of the CivaDot demonstrated good repeatability and reproducibility of the free-air chamber measurements. Measurements of the CivaDot dose distribution using the EBT3 film stack phantom and its subsequent comparison to Monte Carlo-predicted dose distributions were encouraging, given the overall uncertainties. This work will aid in the eventual realization of a clinically viable dosimetric framework for the CivaSheet based on the CivaDot dose distribution.