Assessment of residual error for online cone-beam CT-guided treatment of prostate cancer patients

Management of the baseline shift using a new and simple method for respiratory-gated radiation therapy: detectability and effectiveness of a flexible monitoring system

PURPOSE

In respiratory-gated radiation therapy, a baseline shift decreases the accuracy of target coverage and organs at risk (OAR) sparing. The effectiveness of audio-feedback and audio-visual feedback in correcting the baseline shift in the breathing pattern of the patient has been demonstrated previously. However, the baseline shift derived from the intrafraction motion of the patient's body cannot be corrected by these methods. In the present study, the authors designed and developed a simple and flexible system.

METHODS

The system consisted of a web camera and a computer running our in-house software. The in-house software was adapted to template matching and also to no preimage processing. The system was capable of monitoring the baseline shift in the intrafraction motion of the patient's body. Another marker box was used to monitor the baseline shift due to the flexible setups required of a marker box for gated signals. The system accuracy was evaluated by employing a respiratory motion phantom and was found to be within AAPM Task Group 142 tolerance (positional accuracy <2 mm and temporal accuracy <100 ms) for respiratory-gated radiation therapy. Additionally, the effectiveness of this flexible and independent system in gated treatment was investigated in healthy volunteers, in terms of the results from the differences in the baseline shift detectable between the marker positions, which the authors evaluated statistically.

RESULTS

The movement of the marker on the sternum [1.599 +/- 0.622 mm (1 SD)] was substantially decreased as compared with the abdomen [6.547 +/- 0.962 mm (1 SD)]. Additionally, in all of the volunteers, the baseline shifts for the sternum [-0.136 +/- 0.868 (2 SD)] were in better agreement with the nominal baseline shifts than was the case for the abdomen [-0.722 +/- 1.56 mm (2 SD)]. The baseline shifts could be accurately measured and detected using the monitoring system, which could acquire the movement of the marker on the sternum. The baseline shift-monitoring system with the displacement-based methods for highly accurate respiratory-gated treatments should be used to make most of the displacement-based gating methods.

CONCLUSIONS

The advent of intensity modulated radiation therapy and volumetric modulated radiation therapy facilitates margin reduction for the planning target volumes and the OARs, but highly accurate irradiation is needed to achieve target coverage and OAR sparing with a small margin. The baseline shifts can affect treatment not only with the respiratory gating system but also without the system. Our system can manage the baseline shift and also enables treatment irradiation to be undertaken with high accuracy.

Image-guided positioning and tracking

Radiation therapy aims at maximizing tumor control while minimizing normal tissue complication. The introduction of stereotactic treatment explores the volume effect and achieves dose escalation to tumor target with small margins. The use of ablative irradiation dose and sharp dose gradients requires accurate tumor definition and alignment between patient and treatment geometry. Patient geometry variation during treatment may significantly compromise the conformality of delivered dose and must be managed properly. Setup error and interfraction/intrafraction motion are incorporated in the target definition process by expanding the clinical target volume to planning target volume, whereas the alignment between patient and treatment geometry is obtained with an adaptive control process, by taking immediate actions in response to closely monitored patient geometry. This article focuses on the monitoring and adaptive response aspect of the problem. The term "image" in "image guidance" will be used in a most general sense, to be inclusive of some important point-based monitoring systems that can be considered as degenerate cases of imaging. Image-guided motion adaptive control, as a comprehensive system, involves a hierarchy of decisions, each of which balances simplicity versus flexibility and accuracy versus robustness. Patient specifics and machine specifics at the treatment facility also need to be incorporated into the decision-making process. Identifying operation bottlenecks from a system perspective and making informed compromises are crucial in the proper selection of image-guidance modality, the motion management mechanism, and the respective operation modes. Not intended as an exhaustive exposition, this article focuses on discussing the major issues and development principles for image-guided motion management systems. We hope these information and methodologies will facilitate conscientious practitioners to adopt image-guided motion management systems accounting for patient and institute specifics and to embrace advances in knowledge and new technologies subsequent to the publication of this article.

Multi-system verification of registrations for image-guided radiotherapy in clinical trials

PURPOSE

To provide quantitative information on the image registration differences from multiple systems for image-guided radiotherapy (IGRT) credentialing and margin reduction in clinical trials.

METHODS AND MATERIALS

Images and IGRT shift results from three different treatment systems (Tomotherapy Hi-Art, Elekta Synergy, Varian Trilogy) have been sent from various institutions to the Image-Guided Therapy QA Center (ITC) for evaluation for the Radiation Therapy Oncology Group (RTOG) trials. Nine patient datasets (five head-and-neck and four prostate) were included in the comparison, with each patient having 1-4 daily individual IGRT studies. In all cases, daily shifts were re-calculated by re-registration of the planning CT with the daily IGRT data using three independent software systems (MIMvista, FocalSim, VelocityAI). Automatic fusion was used in all calculations. The results were compared with those submitted from institutions. Similar regions of interest (ROIs) and same initial positions were used in registrations for inter-system comparison. Different slice spacings for CBCT sampling and different ROIs for registration were used in some cases to observe the variation of registration due to these factors.

RESULTS

For the 54 comparisons with head-and-neck datasets, the absolute values of differences of the registration results between different systems were 2.6±2.1 mm (mean±SD; range 0.1-8.6 mm, left-right [LR]), 1.7±1.3 mm (0.0-4.9 mm, superior-inferior [SI]), and 1.8±1.1 mm (0.1-4.0 mm, anterior-posterior [AP]). For the 66 comparisons in prostate cases, the differences were 1.1±1.0 mm (0.0-4.6 mm, LR), 2.1±1.7 mm (0.0-6.6 mm, SI), and 2.0±1.8 mm (0.1-6.9 mm, AP). The differences caused by the slice spacing variation were relatively small, and the different ROI selections in FocalSim and MIMvista also had limited impact.

CONCLUSION

The extent of differences was reported when different systems were used for image registration. Careful examination and quality assurance of the image registration process are crucial before considering margin reduction using IGRT in clinical trials.

Evaluation of patient residual deviation and its impact on dose distribution for proton radiotherapy

The residual deviations after final patient repositioning based on bony anatomy and the impact of such deviations on the proton dose distributions was investigated. Digitally reconstructed radiographs (DRRs) and kilovoltage (kV) "portal verification" images from 10 patients treated with passively scattered proton radiotherapy was used to estimate the residual deviation. These changes were then applied to the location of isocenter points that, in effect, moved the isocenter relative to the apertures and compensators. A composite verification plan was obtained and compared with the original clinical treatment plan to evaluate any changes in dose distributions. The residual deviations were fitted to a Gaussian distribution with μ = -0.9 ± 0.1 mm and σ = 2.55 ± 0.07 mm. The dose distribution showed under- and overcovered dose spots with complex dose distributions both in the target volumes and in the organs at risk. In some cases, this amounts to 63.5% above the intended clinical plan. Although patient positioning is carefully verified before treatment delivery and setup uncertainties are accounted for by using compensator smearing and aperture margins, a residual shift in a patient's position can considerably affect the dose distribution.

A survey of image-guided radiation therapy use in the United States

BACKGROUND

Image-guided radiation therapy (IGRT) is a novel array of in-room imaging modalities that are used for tumor localization and patient setup in radiation oncology. The prevalence of IGRT use among US radiation oncologists is unknown.

METHODS

A random sample of 1600 radiation oncologists was surveyed by Internet, e-mail and fax regarding the frequency of IGRT use, clinical applications, and future plans for use. The definition of IGRT included imaging technologies that are used for setup verification or tumor localization during treatment.

RESULTS

Of 1089 evaluable respondents, 393 responses (36.1%) were received. The proportion of radiation oncologists using IGRT was 93.5%. When the use of megavoltage (MV) portal imaging was excluded from the definition of IGRT, the proportion using IGRT was 82.3%. The majority used IGRT rarely (in <25% of their patients; 28.9%) or infrequently (in 25%-50% of their patients; 33.1%). The percentages using ultrasound, video, MV-planar, kilovoltage (kV)-planar, and volumetric technologies were 22.3%, 3.2%, 62.7%, 57.7%, and 58.8%, respectively. Among IGRT users, the most common disease sites treated were genitourinary (91.1%), head and neck (74.2%), central nervous system (71.9%), and lung (66.9%). Overall, 59.1% of IGRT users planned to increase use, and 71.4% of nonusers planned to adopt IGRT in the future.

CONCLUSIONS

IGRT is widely used among radiation oncologists. On the basis of prospective plans of responders, its use is expected to increase. Further research will be required to determine the safety, cost efficacy, and optimal applications of these technologies.

Residual translational and rotational errors after kV X-ray image-guided correction of prostate location using implanted fiducials

PURPOSE

To evaluate the residual errors and required safety margins after stereoscopic kilovoltage (kV) X-ray target localization of the prostate in image-guided radiotherapy (IGRT) using internal fiducials.

PATIENTS AND METHODS

Radiopaque fiducial markers (FMs) have been inserted into the prostate in a cohort of 33 patients. The ExacTrac/Novalis Body™ X-ray 6d image acquisition system (BrainLAB AG, Feldkirchen, Germany) was used. Corrections were performed in left-right (LR), anterior-posterior (AP), and superior-inferior (SI) direction. Rotational errors around LR (x-axis), AP (y) and SI (z) have been recorded for the first series of nine patients, and since 2007 for the subsequent 24 patients in addition corrected in each fraction by using the Robotic Tilt Module™ and Varian Exact Couch™. After positioning, a second set of X-ray images was acquired for verification purposes. Residual errors were registered and again corrected.

RESULTS

Standard deviations (SD) of residual translational random errors in LR, AP, and SI coordinates were 1.3, 1.7, and 2.2 mm. Residual random rotation errors were found for lateral (around x, tilt), vertical (around y, table), and longitudinal (around z, roll) and of 3.2°, 1.8°, and 1.5°. Planning target volume (PTV)-clinical target volume (CTV) margins were calculated in LR, AP, and SI direction to 2.3, 3.0, and 3.7 mm. After a second repositioning, the margins could be reduced to 1.8, 2.1, and 1.8 mm.

CONCLUSION

On the basis of the residual setup error measurements, the margin required after one to two online X-ray corrections for the patients enrolled in this study would be at minimum 2 mm. The contribution of intrafractional motion to residual random errors has to be evaluated.

Imaging in radiation oncology: a perspective

This paper reviews the integration of imaging and radiation oncology, and discusses challenges and opportunities for improving the practice of radiation oncology with imaging.

An inherent goal of radiation therapy is to deliver enough dose to the tumor to eradicate all cancer cells or to palliate symptoms, while avoiding normal tissue injury. Imaging for cancer diagnosis, staging, treatment planning, and radiation targeting has been integrated in various ways to improve the chance of this occurring. A large spectrum of imaging strategies and technologies has evolved in parallel to advances in radiation delivery. The types of imaging can be categorized into offline imaging (outside the treatment room) and online imaging (inside the treatment room, conventionally termed image-guided radiation therapy). The direct integration of images in the radiotherapy planning process (physically or computationally) often entails trade-offs in imaging performance. Although such compromises may be acceptable given specific clinical objectives, general requirements for imaging performance are expected to increase as paradigms for radiation delivery evolve to address underlying biology and adapt to radiation responses. This paper reviews the integration of imaging and radiation oncology, and discusses challenges and opportunities for improving the practice of radiation oncology with imaging.

Adaptive radiation therapy for prostate cancer

Adaptive radiotherapy has been introduced to manage an individual's treatment by, including patient-specific treatment variation identified and quantified during the course of radiotherapy in the treatment planning and delivering optimization. Early studies have demonstrated that this technique could significantly improve the therapeutic ratio by safely reducing the large target margin that has to be used in conventional radiotherapy for prostate cancer treatment. Clinical application of off-line image-guided adaptive radiotherapy for prostate cancer has demonstrated encouraging clinical outcome. Long-term clinical follow-up has shown significant improvement in terms of tumor control and low toxicity profile, emphasizing the beneficial effect of image-guidance and adaptive treatment. Continuous development in adaptive radiotherapy has made possible additional increases in target dose by further reducing target margin when using online image-guided adaptive intensity-modulated radiation therapy. However, clinical implementation of new techniques should be explored cautiously and should include a comprehensive management strategy to address uncertainties in target definition and delineation in the preclinical implementation studies.

Planning target volume margins for prostate radiotherapy using daily electronic portal imaging and implanted fiducial markers

Background

Fiducial markers and daily electronic portal imaging (EPI) can reduce the risk of geographic miss in prostate cancer radiotherapy. The purpose of this study was to estimate CTV to PTV margin requirements, without and with the use of this image guidance strategy.

Methods

46 patients underwent placement of 3 radio-opaque fiducial markers prior to prostate RT. Daily pre-treatment EPIs were taken, and isocenter placement errors were corrected if they were ≥ 3 mm along the left-right or superior-inferior axes, and/or ≥ 2 mm along the anterior-posterior axis. During-treatment EPIs were then obtained to estimate intra-fraction motion.

Results

Without image guidance, margins of 0.57 cm, 0.79 cm and 0.77 cm, along the left-right, superior-inferior and anterior-posterior axes respectively, are required to give 95% probability of complete CTV coverage each day. With the above image guidance strategy, these margins can be reduced to 0.36 cm, 0.37 cm and 0.37 cm respectively. Correction of all isocenter placement errors, regardless of size, would permit minimal additional reduction in margins.

Conclusions

Image guidance, using implanted fiducial markers and daily EPI, permits the use of narrower PTV margins without compromising coverage of the target, in the radiotherapy of prostate cancer.

Clinical evaluation of soft tissue organ boundary visualization on cone-beam computed tomographic imaging

PURPOSE

Cone-beam computed tomographic images (CBCTs) are increasingly used for setup correction, soft tissue targeting, and image-guided adaptive radiotherapy. However, CBCT image quality is limited by low contrast and imaging artifacts. This analysis investigates the detectability of soft tissue boundaries in CBCT by performing a multiple-observer segmentation study.

METHODS AND MATERIALS

In four prostate cancer patients prostate, bladder and rectum were repeatedly delineated by five observers on CBCTs and fan-beam CTs (FBCTs). A volumetric analysis of contouring variations was performed by calculating coefficients of variation (COV: standard deviation/average volume). The topographical distribution of contouring variations was analyzed using an average surface mesh-based method.

RESULTS

Observer- and patient-averaged COVs for FBCT/CBCT were 0.09/0.19 for prostate, 0.05/0.08 for bladder, and 0.09/0.08 for rectum. Contouring variations on FBCT were significantly smaller than on CBCT for prostate (p < 0.03) and bladder (p < 0.04), but not for rectum (p < 0.37; intermodality differences). Intraobserver variations from repeated contouring of the same image set were not significant for either FBCT or CBCT (p < 0.05). Average standard deviations of individual observers' contour differences from average surface meshes on FBCT vs. CBCT were 1.5 vs. 2.1 mm for prostate, 0.7 vs. 1.4 mm for bladder, and 1.3 vs. 1.5 mm for rectum. The topographical distribution of contouring variations was similar for FBCT and CBCT.

CONCLUSION

Contouring variations were larger on CBCT than FBCT, except for rectum. Given the well-documented uncertainty in soft tissue contouring in the pelvis, improvement of CBCT image quality and establishment of well-defined soft tissue identification rules are desirable for image-guided radiotherapy.

Impact of anatomical interventions on the localization of post-prostatectomy cancer patients

PURPOSE

Anatomical deformations of prostate-bed, rectum, and bladder can compromise the targeting accuracy in post-prostatectomy cancer patients. In this work, the impact of anatomical interventions on the localization data from post-prostatectomy patients who received image-guided IMRT was analyzed.

METHODS

Patients were localized daily with online kilovoltage cone-beam computed tomography (kV-CBCT). The target and the organs at risk (OARs) positional and volumetric changes were evaluated and couch shifts were applied. For patients with large target or OAR volumetric changes, quantified by either a rectal or bladder wall displacement of >5 mm on the CBCT sagittal images compared to the planning CT, repeated localization CBCT scans were performed following an interventional procedure. The procedure involves insertion of a catheter to deflate the rectum, evacuation of stools, and/or adjustment of bladder filling. The required shifts were then evaluated, and the IMRT treatment was subsequently delivered after proper patient positioning. The pre- and post-intervention shifts were compared in the lateral [left-right (LR)], longitudinal [superior-inferior (SI)], and vertical [anterior-posterior (AP)] directions. The percentage of shifts larger than 5 mm in all directions was also compared. Clinical target volume to planning target volume (CTV-to-PTV) expansion margins were estimated based on the pre- and post-intervention localization data.

RESULTS

Intervention was performed on all patients (n=17) treated between October 2008 and March 2009. The number of interventions ranged from 2 to 12 with a median number of 5, resulting in a total of 96 pairs of pre- and post-intervention shifts. The mean value (sigma) and standard deviation (sigma) of the shifts from pre- versus post-intervention data were LR, 0.0 +/- 3.0 mm vs. 0.5 +/- 2.8 mm; SI, 0.2 +/- 3.1 mm vs. -1.0 +/- 2.1 mm; and AP, -2.6 +/- 5.8 mm vs. 1.7 +/- 3.9 mm. The mean 3D shift distance was 7.0 +/- 3.1 mm vs. 5.0 +/- 2.6 mm. The percentage of pre-intervention shifts larger than 5 mm were 7%, 7%, and 45% in the LR, SI, and AP directions, respectively, compared to 8%, 4%, and 21% for post-intervention. Localization data from pre- and post-intervention procedures suggest that treatments that do not include intervention to correct for rectum/bladder anatomical variations require an additional 3.3 mm CTV-to-PTV margin.

CONCLUSIONS

Anatomical interventions reduced the localization errors arising from large volume and shape changes in the rectum and/or bladder compared to treatments without interventions.

Quantitative evaluation of cone-beam computed tomography in target volume definition for offline image-guided radiation therapy of prostate cancer

PURPOSE

To quantitatively evaluate cone-beam CT (CBCT) in target volume definition in an offline image guidance environment.

METHODS AND MATERIALS

Fifteen patients each with five helical CTs (HCT) and eight CBCTs were included. A single physician manually delineated prostate and seminal vesicles (SVs) on each CT. The clinical target volume (CTV) was prostate for low risk group (G1), plus SVs for intermediate risk group (G2). The internal target volumes (ITVs) on CBCT (ITV(CBCT)) were constructed and compared with ITV(HCT). The following comparisons were performed: CTV and ITV in HCT and CBCT; similarity of ITVs using overlap index (OI); surface differences between ITVs; quality assurance of ITV(CBCT) using CTV from weekly CBCT; and dosimetric evaluations of ITV(HCT) coverage on plans from ITV(CBCT).

RESULTS

There was no statistical significant difference of CTV or ITV. The ITV OIs were 91%/88% for G1/G2 patients. They improved significantly with 1-2mm margins. Therefore, the ITVs were mostly within 2mm. The CTVs from weekly CBCT had >95% overlap with ITV(CBCT). The ITV dose differences (D(95), and D(mean)) were <0.3%.

CONCLUSIONS

It is feasible to use CBCT for target definition in offline image guidance, thereby eliminating the separate helical CT scan process.

Image quality and stability of image-guided radiotherapy (IGRT) devices: A comparative study

INTRODUCTION

Our aim was to implement standards for quality assurance of IGRT devices used in our department and to compare their performances with that of a CT simulator.

MATERIALS AND METHODS

We investigated image quality parameters for three devices over a period of 16months. A multislice CT was used as a benchmark and results related to noise, spatial resolution, low contrast visibility (LCV) and uniformity were compared with a cone beam CT (CBCT) at a linac and simulator.

RESULTS

All devices performed well in terms of LCV and, in fact, exceeded vendor specifications. MTF was comparable between CT and linac CBCT. Integral nonuniformity was, on average, 0.002 for the CT and 0.006 for the linac CBCT. Uniformity, LCV and MTF varied depending on the protocols used for the linac CBCT. Contrast-to-noise ratio was an average of 51% higher for the CT than for the linac and simulator CBCT. No significant time trend was observed and tolerance limits were implemented.

DISCUSSION

Reasonable differences in image quality between CT and CBCT were observed. Further research and development are necessary to increase image quality of commercially available CBCT devices in order for them to serve the needs for adaptive and/or online planning.

Image quality & dosimetric property of an investigational imaging beam line MV-CBCT

To measure and compare the contrast to noise ratio (CNR) as a function of dose for the CBCTs produced by the mega‐voltage (MV) imaging beam line (IBL) and the treatment beam line (TBL), and to compare the dose to target and various critical structures of pediatric patients for the IBL CBCT versus standard TBL orthogonal port films.

Two Siemens Oncor linear accelerators were modified at our institution such that the MV‐CBCT would operate under an investigational IBL rather than the standard 6MV TBL. Prior to the modification, several CBCTs of an electron density phantom were acquired with the TBL at various dose values. After the modification, another set of CBCTs of the electron density phantom were acquired for various doses using the IBL. The contrast to noise ratio (CNR) for each tissue equivalent insert was calculated. In addition, a dosimetric study of pediatric patients was conducted comparing the 1 cGy IBL CBCT and conventional TBL orthogonal pair port films.

The CNR for eight tissue equivalent inserts at five different dose settings for each type of CBCT was measured. The CNR of the muscle insert was 0.8 for a 5 cGy TBL CBCT, 1.1 for a 1.5 cGy IBL CBCT, and 2.8 for a conventional CT. The CNR of the trabecular bone insert was 2.9 for a 5 cGy TBL CBCT, 5.5 for a 1.5 cGy IBL CBCT, and 14.8 for a conventional CT. The IBL CBCT delivered approximately one‐fourth the dose to the target and critical structures of the patients as compared to the TBL orthogonal pair port films.

The IBL CBCT improves image quality while simultaneously reducing the dose to the patient as compared to the TBL CBCT. A 1 cGy IBL CBCT, which is used for bony anatomy localization, delivers one‐fourth the dose as compared to conventional ortho‐pair films.

PACS number: 87.57.Q, 87.57.cj, 87.53.Jw

PET and Radiation Therapy Planning and Delivery for Prostate Cancer

PET imaging has become an integral component of the diagnosis and management of a substantial number of lymphatic and solid malignancies. One of the greatest dilemmas in prostate cancer remains the need for greater personalization of treatment recommendations based on the true extent of disease, so that patients with extraprostatic, micrometastatic disease can be identified early and managed accordingly. These sites currently remain under the level of detection with standard imaging and continue to confound clinicians. Novel PET tracers to complement anatomic data from CT and MR imaging can truly make a difference, and ongoing research holds the greatest promise.

Analysis of daily setup variation with tomotherapy megavoltage computed tomography

The purpose of this study was to evaluate different setup uncertainties for various anatomic sites with TomoTherapy pretreatment megavoltage computed tomography (MVCT) and to provide optimal margin guidelines for these anatomic sites. Ninety-two patients with tumors in head and neck (HN), brain, lung, abdominal, or prostate regions were included in the study. MVCT was used to verify patient position and tumor target localization before each treatment. With the anatomy registration tool, MVCT provided real-time tumor shift coordinates relative to the positions where the simulation CT was performed. Thermoplastic facemasks were used for HN and brain treatments. Vac-Lok cushions were used to immobilize the lower extremities up to the thighs for prostate patients. No respiration suppression was administered for lung and abdomen patients. The interfractional setup variations were recorded and corrected before treatment. The mean interfractional setup error was the smallest for HN among the 5 sites analyzed. The average 3D displacement in lateral, longitudinal, and vertical directions for the 5 sites ranged from 2.2-7.7 mm for HN and lung, respectively. The largest movement in the lung was 2.0 cm in the longitudinal direction, with a mean error of 6.0 mm and standard deviation of 4.8 mm. The mean interfractional rotation variation was small and ranged from 0.2-0.5 degrees, with the standard deviation ranging from 0.7-0.9 degrees. Internal organ displacement was also investigated with a posttreatment MVCT scan for HN, lung, abdomen, and prostate patients. The maximum 3D intrafractional displacement across all sites was less than 4.5 mm. The interfractional systematic errors and random errors were analyzed and the suggested margins for HN, brain, prostate, abdomen, and lung in the lateral, longitudinal, and vertical directions were between 4.2 and 8.2 mm, 5.0 mm and 12.0 mm, and 1.5 mm and 6.8 mm, respectively. We suggest that TomoTherapy pretreatment MVCT can be used to improve the accuracy of patient positioning and reduce tumor margin.

Phantom evaluation of a commercially available three modality image guided radiation therapy system

The authors describe a detailed evaluation of the capabilities of imaging and image registration systems available with Varian linear accelerators for image guided radiation therapy (IGRT). Specifically, they present modulation transfer function curves for megavoltage planar, kilovoltage (kV) planar, and cone beam computed tomography imaging systems and compare these with conventional computed tomography. While kV planar imaging displayed the highest spatial resolution, all IGRT imaging techniques were assessed as adequate for their intended purpose. They have also characterized the image registration software available for use in conjunction with these imaging systems through a comprehensive phantom study involving translations in three orthogonal directions. All combinations of imaging systems and image registration software were found to be accurate, although the planar kV imaging system with automatic registration was generally superior, with both accuracy and precision of the order of 1 mm, under the conditions tested. Based on their phantom study, the attainable accuracy for rigid body translations using any of the features available with Varian equipment will more likely be limited by the resolution of the couch readouts than by inherent limitations in the imaging systems and image registration software. Overall, the accuracy and precision of currently available IGRT technology exceed published experience with the accuracy and precision of contouring for planning.

Dose perturbations from implanted helical gold markers in proton therapy of prostate cancer

Implanted gold fiducial markers are widely used in radiation therapy to improve targeting accuracy. Recent investigations have revealed that metallic fiducial markers can cause severe perturbations in dose distributions for proton therapy, suggesting smaller markers should be considered. The objective of this study was to estimate the dosimetric impact of small gold markers in patients receiving proton therapy for prostate cancer. Small, medium, and large helical wire markers with lengths of 10 mm and helix diameters of 0.35 mm, 0.75 mm, and 1.15 mm, respectively, were implanted in an anthropomorphic phantom. Radiographic visibility was confirmed using a kilovoltage x‐ray imaging system, and dose perturbations were predicted from Monte Carlo simulations and confirmed by measurements. Monte Carlo simulations indicated that size of dose perturbation depended on marker size, orientation, and distance from the beam's end of range. Specifically, the perturbation of proton dose for the lateral‐opposed‐pair treatment technique was 31% for large markers and 23% for medium markers in a typical oblique orientation. Results for perpendicular and parallel orientations were respectively lower and higher. Consequently, these markers are not well suited for use in patients receiving proton therapy for prostate cancer. Dose perturbation was not observed for the small markers, but these markers were deemed too fragile for transrectal implantation in the prostate.

PACS number: 87.53.Pb

Analysis of interfraction prostate motion using megavoltage cone beam computed tomography

PURPOSE

Determine the degree of interfraction prostate motion and its components measured by using daily megavoltage (MV) cone beam computed tomography (CBCT) imaging.

METHODS AND MATERIALS

A total of 984 daily MV CBCT images from 24 patients undergoing definitive intensity-modulated radiotherapy for localized prostate cancer were analyzed retrospectively. Pretreatment couch shifts, based on physician registration of MV CBCT to planning CT data sets, were used as a measure of daily interfraction motion. Off-line bony registration was performed to separate bony misalignment from internal organ motion. Interobserver and intraobserver variation studies were performed on 20 MV CBCT images.

RESULTS

Mean interfraction prostate motion was 6.7 mm, with the greatest single-axis deviation in the anterior-posterior (AP) direction. The largest positional inaccuracy was accounted for by systematic deviations in bony misalignment, whereas random deviations occurred from bony misalignment and internal prostate motion. In the aggregate, AP motion did not correlate with days elapsed since beginning therapy or on average with rectal size at treatment planning. Interobserver variation was greatest in the AP direction, decreased in experienced observers, and further decreased in intraobserver studies. Mean interfraction motion during the first 6 days of therapy, when used as a subsequent offset, reduced acceptable AP planning target volume margins by 50%.

CONCLUSION

The MV CBCT is a practical direct method of daily localization that shows significant interfraction motion with respect to conventional three-dimensional conformal and intensity-modulated radiotherapy margins, similar to that measured in other modalities.

Intrafraction motion of the prostate during an IMRT session: a fiducial-based 3D measurement with Cone-beam CT

BACKGROUND

Image-guidance systems allow accurate interfractional repositioning of IMRT treatments, however, these may require up to 15 minutes. Therefore intrafraction motion might have an impact on treatment precision. 3D geometric data regarding intrafraction prostate motion are rare; we therefore assessed its magnitude with pre- and post-treatment fiducial-based imaging with cone-beam-CT (CBCT).

METHODS

39 IMRT fractions in 5 prostate cancer patients after 125I-seed implantation were evaluated. Patient position was corrected based on the 125I-seeds after pre-treatment CBCT. Immediately after treatment delivery, a second CBCT was performed. Differences in bone- and fiducial position were measured by seed-based grey-value matching.

RESULTS

Fraction time was 13.6 +/- 1.6 minutes. Median overall displacement vector length of 125I-seeds was 3 mm (M = 3 mm, Sigma = 0.9 mm, sigma = 1.7 mm; M: group systematic error, Sigma: SD of systematic error, sigma: SD of random error). Median displacement vector of bony structures was 1.84 mm (M = 2.9 mm, Sigma = 1 mm, sigma = 3.2 mm). Median displacement vector length of the prostate relative to bony structures was 1.9 mm (M = 3 mm, Sigma = 1.3 mm, sigma = 2.6 mm).

CONCLUSION

a) Overall displacement vector length during an IMRT session is < 3 mm.b) Positioning devices reducing intrafraction bony displacements can further reduce overall intrafraction motion.c) Intrafraction prostate motion relative to bony structures is < 2 mm and may be further reduced by institutional protocols and reduction of IMRT duration.

A technique for reducing patient setup uncertainties by aligning and verifying daily positioning of a moving tumor using implanted fiducials

This study aimed to validate and implement a methodology in which fiducials implanted in the periphery of lung tumors can be used to reduce uncertainties in tumor location. Alignment software that matches marker positions on two-dimensional (2D) kilovoltage portal images to positions on three-dimensional (3D) computed tomography data sets was validated using static and moving phantoms. This software also was used to reduce uncertainties in tumor location in a patient with fiducials implanted in the periphery of a lung tumor. Alignment of fiducial locations in orthogonal projection images with corresponding fiducial locations in 3D data sets can position both static and moving phantoms with an accuracy of 1 mm. In a patient, alignment based on fiducial locations reduced systematic errors in the left-right direction by 3 mm and random errors by 2 mm, and random errors in the superior-inferior direction by 3 mm as measured by anterior-posterior cine images. Software that matches fiducial markers on 2D and 3D images is effective for aligning both static and moving fiducials before treatment and can be implemented to reduce patient setup uncertainties.

A quality assurance program for image quality of cone-beam CT guidance in radiation therapy

The clinical introduction of volumetric x-ray image-guided radiotherapy systems necessitates formal commissioning of the hardware and image-guided processes to be used and drafts quality assurance (QA) for both hardware and processes. Satisfying both requirements provides confidence on the system's ability to manage geometric variations in patient setup and internal organ motion. As these systems become a routine clinical modality, the authors present data from their QA program tracking the image quality performance of ten volumetric systems over a period of 3 years. These data are subsequently used to establish evidence-based tolerances for a QA program. The volumetric imaging systems used in this work combines a linear accelerator with conventional x-ray tube and an amorphous silicon flat-panel detector mounted orthogonally from the accelerator central beam axis, in a cone-beam computed tomography (CBCT) configuration. In the spirit of the AAPM Report No. 74, the present work presents the image quality portion of their QA program; the aspects of the QA protocol addressing imaging geometry have been presented elsewhere. Specifically, the authors are presenting data demonstrating the high linearity of CT numbers, the uniformity of axial reconstructions, and the high contrast spatial resolution of ten CBCT systems (1-2 mm) from two commercial vendors. They are also presenting data accumulated over the period of several months demonstrating the long-term stability of the flat-panel detector and of the distances measured on reconstructed volumetric images. Their tests demonstrate that each specific CBCT system has unique performance. In addition, scattered x rays are shown to influence the imaging performance in terms of spatial resolution, axial reconstruction uniformity, and the linearity of CT numbers.

Accuracy of 3D volumetric image registration based on CT, MR and PET/CT phantom experiments

Registration is critical for image‐based treatment planning and image‐guided treatment delivery. Although automatic registration is available, manual, visual‐based image fusion using three orthogonal planar views (3P) is always employed clinically to verify and adjust an automatic registration result. However, the 3P fusion can be time consuming, observer dependent, as well as prone to errors, owing to the incomplete 3‐dimensional (3D) volumetric image representations. It is also limited to single‐pixel precision (the screen resolution). The 3D volumetric image registration (3DVIR) technique was developed to overcome these shortcomings. This technique introduces a 4th dimension in the registration criteria beyond the image volume, offering both visual and quantitative correlation of corresponding anatomic landmarks within the two registration images, facilitating a volumetric image alignment, and minimizing potential registration errors. The 3DVIR combines image classification in real‐time to select and visualize a reliable anatomic landmark, rather than using all voxels for alignment. To determine the detection limit of the visual and quantitative 3DVIR criteria, slightly misaligned images were simulated and presented to eight clinical personnel for interpretation. Both of the criteria produce a detection limit of 0.1 mm and 0.1°. To determine the accuracy of the 3DVIR method, three imaging modalities (CT, MR and PET/CT) were used to acquire multiple phantom images with known spatial shifts. Lateral shifts were applied to these phantoms with displacement intervals of 5.0±0.1mm. The accuracy of the 3DVIR technique was determined by comparing the image shifts determined through registration to the physical shifts made experimentally. The registration accuracy, together with precision, was found to be: 0.02±0.09mm for CT/CT images, 0.03±0.07mm for MR/MR images, and 0.03±0.35mm for PET/CT images. This accuracy is consistent with the detection limit, suggesting an absence of detectable systematic error. This 3DVIR technique provides a superior alternative to the 3P fusion method for clinical applications.

PACS numbers: 87.57.nj, 87.57.nm, 87.57.‐N, 87.57.‐s

Prostate intrafraction motion evaluation using kV fluoroscopy during treatment delivery: a feasibility and accuracy study

Margin reduction for prostate radiotherapy is limited by uncertainty in prostate localization during treatment. We investigated the feasibility and accuracy of measuring prostate intrafraction motion using kV fluoroscopy performed simultaneously with radiotherapy. Three gold coils used for target localization were implanted into the patient's prostate gland before undergoing hypofractionated online image-guided step-and-shoot intensity modulated radiation therapy (IMRT) on an Elekta Synergy linear accelerator. At each fraction, the patient was aligned using a cone-beam computed tomography (CBCT), after which the IMRT treatment delivery and fluoroscopy were performed simultaneously. In addition, a post-treatment CBCT was acquired with the patient still on the table. To measure the intrafraction motion, we developed an algorithm to register the fluoroscopy images to a reference image derived from the post-treatment CBCT, and we estimated coil motion in three-dimensional (3D) space by combining information from registrations at different gantry angles. We also detected the MV beam turning on and off using MV scatter incident in the same fluoroscopy images, and used this information to synchronize our intrafraction evaluation with the treatment delivery. In addition, we assessed the following: the method to synchronize with treatment delivery, the dose from kV imaging, the accuracy of the localization, and the error propagated into the 3D localization from motion between fluoroscopy acquisitions. With 0.16 mAs/frame and a bowtie filter implemented, the coils could be localized with the gantry at both 0 degrees and 270 degrees with the MV beam off, and at 270 degrees with the MV beam on when multiple fluoroscopy frames were averaged. The localization in two-dimensions for phantom and patient measurements was performed with submillimeter accuracy. After backprojection into 3D the patient localization error was (-0.04 +/- 0.30) mm, (0.09 +/- 0.36)mm, and (0.03 +/- 0.68)mm in the right-left (RL), anterior-posterior (AP), and superior-inferior (SI) axes, respectively. Simulations showed that while oscillating (stationary) motion cannot be effectively represented in 3D, linearly drifting (nonstationary) motion is detectable with good accuracy. These results show that measuring prostate intrafraction motion using a single kV imager during radiotherapy is feasible and can be performed with acceptable accuracy.

Image guided IMRT dosimetry using anatomy specific MOSFET configurations

We have investigated the feasibility of using a set of multiple MOSFETs in conjunction with the mobileMOSFET wireless dosimetry system, to perform a comprehensive and efficient quality assurance (QA) of IMRT plans. Anatomy specific MOSFET configurations incorporating 5 MOSFETs have been developed for a specially designed IMRT dosimetry phantom. Kilovoltage cone beam computed tomography (kV CBCT) imaging was used to increase the positional precision and accuracy of the detectors and phantom, and so minimize dosimetric uncertainties in high dose gradient regions. The effectiveness of the MOSFET based dose measurements was evaluated by comparing the corresponding doses measured by an ion chamber. For 20 head and neck IMRT plans the agreement between the MOSFET and ionization chamber dose measurements was found to be within −0.26±0.88% and 0.06±1.94% (1σ) for measurement points in the high dose and low dose respectively. A precision of 1 mm in detector positioning was achieved by using the X‐Ray Volume Imaging (XVI) kV CBCT system available with the Elekta Synergy Linear Accelerator. Using the anatomy specific MOSFET configurations, simultaneous measurements were made at five strategically located points covering high dose and low dose regions. The agreement between measurements and calculated doses by the treatment planning system for head and neck and prostate IMRT plans was found to be within 0.47±2.45%. The results indicate that a cylindrical phantom incorporating multiple MOSFET detectors arranged in an anatomy specific configuration, in conjunction with image guidance, can be utilized to perform a comprehensive and efficient quality assurance of IMRT plans.

PACS number: 87.55.Qr

Proton therapy coverage for prostate cancer treatment

PURPOSE

To determine the impact of prostate motion on dose coverage in proton therapy.

METHODS AND MATERIALS

A total of 120 prostate positions were analyzed on 10 treatment plans for 10 prostate patients treated using our low-risk proton therapy prostate protocol (University of Florida Proton Therapy Institute 001). Computed tomography and magnetic resonance imaging T(2)-weighted turbo spin-echo scans were registered for all cases. The planning target volume included the prostate with a 5-mm axial and 8-mm superoinferior expansion. The prostate was repositioned using 5- and 10-mm one-dimensional vectors and 10-mm multidimensional vectors (Points A-D). The beam was realigned for the 5- and 10-mm displacements. The prescription dose was 78 Gy equivalent (GE).

RESULTS

The mean percentage of rectum receiving 70 Gy (V(70)) was 7.9%, the bladder V(70) was 14.0%, and the femoral head/neck V(50) was 0.1%, and the mean pelvic dose was 4.6 GE. The percentage of prostate receiving 78 Gy (V(78)) with the 5-mm movements changed by -0.2% (range, 0.006-0.5%, p > 0.7). However, the prostate V(78) after a 10-mm displacement changed significantly (p < 0.003) with different movements: 3.4% (superior), -5.6% (inferior), and -10.2% (posterior). The corresponding minimal doses were also reduced: 4.5 GE, -4.7 GE, and -11.7 GE (p < or = 0.003). For displacement points A-D, the clinical target volume V(78) coverage had a large and significant reduction of 17.4% (range, 13.5-17.4%, p < 0.001) in V(78) coverage of the clinical target volume. The minimal prostate dose was reduced 33% (25.8 GE), on average, for Points A-D. The prostate minimal dose improved from 69.3 GE to 78.2 GE (p < 0.001) with realignment for 10-mm movements.

CONCLUSION

The good dose coverage and low normal doses achieved for the initial plan was maintained with movements of < or = 5 mm. Beam realignment improved coverage for 10-mm displacements.

Emerging role of intensity-modulated radiation therapy in anorectal cancer

Although radiation therapy has an established role to play in the management of rectal and anal tumors, there are often treatment-related morbidities that negatively impact on patients. There is a long-standing interest in radiation oncology on maximizing treatment efficacy while minimizing treatment-related toxicities, which can be pronounced in the treatment of pelvic malignancies. Intensity-modulated radiation therapy is a recently introduced technology that has the potential to increase the efficacy:toxicity ratio. It has been implemented in the treatment of prostate and head and neck tumors with success. This article reviews the rationale for its use in treating anorectal tumors and discusses early clinical data supporting its continued investigation.