Defining the role of intensity modulation in electron conformal therapy for the treatment of head and neck cancer.

Bolus electron conformal therapy (BECT) in the treatment of cancers of the head and neck is often limited by an inability to reduce dosimetric hot spots resulting from surface irregularity or tissue heterogeneity. We examined the potential benefits of using intensity modulation for electron therapy (IM-BECT) to reduce hotspots in patients undergoing electron beam therapy for superficial cancers of the head and neck (HN). Twenty patients with HN cancer previously treated with BECT were identified. Each case included the treatment targets and a primary organ at risk (OAR) that were defined by the radiation oncologist. A target +2 cm rind structure was created for analysis of the dose deposition in areas surrounding the target volume as a measure of conformality. Each patient plan was transferred into the novel IM-BECT planning software and each case was recomputed as per the original parameters. Next, each case was replanned with the inclusion of intensity modulation, as well as a new custom conformal bolus that was redesigned for optimized range compensation when paired with an intensity modulator. The plans were then normalized to prescription dose and compared for target coverage/dose and OAR dose. For patients who had a hotspot of 125% or greater, the hotspot was on average reduced by 13.1% with IM-BECT. For IM-BECT, the average primary OAR means dose and target+2cm rind mean dose increased slightly by 10.6% and 6.4%, respectively (primary OAR mean [p = 0.0001], and Target+2cm rind mean [p = 0.0001], paired t-test). IM-BECT is an effective method of reducing hotspots in patients with superficial HN cancer. Improvements came at the expense of slight increases in dose to the underlying tissues. This retrospective planning study represents the first example of IM-BECT to actual HN patient cases. Expanding the role of IM-BECT in other disease sites could potentially compared to conventional BECT.

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

BACKGROUND AND PURPOSE: Radiation dose escalation may improve local control (LC) and overall survival (OS) in select pancreatic ductal adenocarcinoma (PDAC) patients. We prospectively evaluated the safety and efficacy of ablative stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) for borderline resectable (BRPC) and locally advanced pancreas cancer (LAPC). The primary endpoint of acute grade ≥ 3 gastrointestinal (GI) toxicity definitely related to SMART was previously published with median follow-up (FU) 8.8 months from SMART. We now present more mature outcomes including OS and late toxicity. MATERIALS AND METHODS: This prospective, multi-center, single-arm open-label phase 2 trial (NCT03621644) enrolled 136 patients (LAPC 56.6 %; BRPC 43.4 %) after ≥ 3 months of any chemotherapy without distant progression and CA19-9 ≤ 500 U/mL. SMART was delivered on a 0.35 T MR-guided system prescribed to 50 Gy in 5 fractions (biologically effective dose10 [BED10] = 100 Gy). Elective coverage was optional. Surgery and chemotherapy were permitted after SMART. RESULTS: Mean age was 65.7 years (range, 36-85), induction FOLFIRINOX was common (81.7 %), most received elective coverage (57.4 %), and 34.6 % had surgery after SMART. Median FU was 22.9 months from diagnosis and 14.2 months from SMART, respectively. 2-year OS from diagnosis and SMART were 53.6 % and 40.5 %, respectively. Late grade ≥ 3 toxicity definitely, probably, or possibly attributed to SMART were observed in 0 %, 4.6 %, and 11.5 % patients, respectively. CONCLUSIONS: Long-term outcomes from the phase 2 SMART trial demonstrate encouraging OS and limited severe toxicity. Additional prospective evaluation of this novel strategy is warranted.

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

PURPOSE: Magnetic resonance (MR) image guidance may facilitate safe ultrahypofractionated radiation dose escalation for inoperable pancreatic ductal adenocarcinoma. We conducted a prospective study evaluating the safety of 5-fraction Stereotactic MR-guided on-table Adaptive Radiation Therapy (SMART) for locally advanced (LAPC) and borderline resectable pancreatic cancer (BRPC). METHODS AND MATERIALS: Patients with LAPC or BRPC were eligible for this multi-institutional, single-arm, phase 2 trial after ≥3 months of systemic therapy without evidence of distant progression. Fifty gray in 5 fractions was prescribed on a 0.35T MR-guided radiation delivery system. The primary endpoint was acute grade ≥3 gastrointestinal (GI) toxicity definitely attributed to SMART. RESULTS: One hundred thirty-six patients (LAPC 56.6%, BRPC 43.4%) were enrolled between January 2019 and January 2022. Mean age was 65.7 (36-85) years. Head of pancreas lesions were most common (66.9%). Induction chemotherapy mostly consisted of (modified)FOLFIRINOX (65.4%) or gemcitabine/nab-paclitaxel (16.9%). Mean CA19-9 after induction chemotherapy and before SMART was 71.7 U/mL (0-468). On-table adaptive replanning was performed for 93.1% of all delivered fractions. Median follow-up from diagnosis and SMART was 16.4 and 8.8 months, respectively. The incidence of acute grade ≥3 GI toxicity possibly or probably attributed to SMART was 8.8%, including 2 postoperative deaths that were possibly related to SMART in patients who had surgery. There was no acute grade ≥3 GI toxicity definitely related to SMART. One-year overall survival from SMART was 65.0%. CONCLUSIONS: The primary endpoint of this study was met with no acute grade ≥3 GI toxicity definitely attributed to ablative 5-fraction SMART. Although it is unclear whether SMART contributed to postoperative toxicity, we recommend caution when pursuing surgery, especially with vascular resection after SMART. Additional follow-up is ongoing to evaluate late toxicity, quality of life, and long-term efficacy.

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

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

Proposal and Evaluation of a Physician-Free, Real-Time On-Table Adaptive Radiotherapy (PF-ROAR) Workflow for the MRIdian MR-Guided LINAC.

With the implementation of MR-LINACs, real-time adaptive radiotherapy has become a possibility within the clinic. However, the process of adapting a patient's plan is time consuming and often requires input from the entire clinical team, which translates to decreased throughput and limited patient access. In this study, the authors propose and simulate a workflow to address these inefficiencies in staffing and patient throughput. Two physicians, three radiation therapists (RTT), and a research fellow each adapted bladder and bowel contours for 20 fractions from 10 representative patient plans. Contouring ability was compared via calculation of a Dice Similarity Index (DSI). The DSI for bladder and bowel based on each potential physician-therapist pair, as well as an inter-physician comparison, exhibited good overlap amongst all comparisons (p = 0.868). Plan quality was compared through calculation of the conformity index (CI), as well as an evaluation of the plan's dose to a 'gold standard' set of structures. Overall, non-physician plans passed 91.2% of the time. Of the eight non-physician plans that failed their clinical evaluation, six also failed their evaluation against the 'gold standard'. Another two plans that passed their clinical evaluation subsequently failed in their evaluation against the 'gold standard'. Thus, the PF-ROAR process has a success rate of 97.5%, with 78/80 plans correctly adapted to the gold standard or halted at treatment. These findings suggest that a physician-free workflow can be well tolerated provided RTTs continue to develop knowledge of MR anatomy and careful attention is given to understanding the complexity of the plan prior to treatment.

Research productivity of radiation therapy physics faculty in the United States.

PURPOSE: Research productivity metrics are important for decisions regarding hiring, retention, and promotion in academic medicine, and these metrics can vary widely among different disciplines. This article examines productivity metrics for radiation therapy physicists (RTP) in the United States. METHODS AND MATERIALS: Database searches were performed for RTP faculty at US institutions that have RTP residencies accredited by the Commission on Accreditation of Medical Physics Education Programs (CAMPEP). Demographics, academic rank, number of publications, academic career length, Hirsch index (h-index), m-quotient, and history of National Institutes of Health (NIH) funding as a principal investigator (PI) were collected for each RTP. Logistic regression was performed to determine the probability of academic rank as a function of h-index and m-quotient. Statistical tests used included the Wilcoxon ranked sum test and the Pearson χ2 test. RESULTS: A total of 1038 faculty and staff were identified at 78 institutions with CAMPEP-accredited residencies. The average RTP academic career duration is 13.5 years, with 46.7 total publications, h-index of 10.7, and m-quotient of 0.66. Additionally, 10.5% of RTP have a history of NIH funding as a PI. Large disparities were found in academic productivity of doctoral-prepared physicists compared to those with a terminal master's degree. For differences in junior and senior faculty, statistical tests yielded significance in career duration, number of publications, h-index, and m-quotient. Gender disparities were identified in the overall distribution of RTP consistent with the membership of the American Association of Physicists in Medicine. Further gender disparities were found in the number of doctoral-prepared RTP and physicists in senior faculty roles. CONCLUSIONS: This manuscript provides objective benchmark data regarding research productivity of academic RTP. These data may be of interest to faculty preparing for promotion, and also to institutional leadership.

Impact of Radiation Oncology Alternative Payment Model on Community Cancer Centers.

PURPOSE: An episode-based payment model, the Radiation Oncology Alternative Payment Model (RO-APM), is scheduled to go into effect in January 2022. This article investigates the effects of RO-APM on hospital-based and freestanding community centers. METHODS: Historical Medicare data used to generate the RO-APM base rates were reviewed. A sensitivity analysis was performed to show how the RO-APM reimbursements compare with current reimbursements for commonly accepted treatment schedules and with current reimbursements at a large community practice. RESULTS: The RO-APM base rates represent a 2.2% decrease in overall Medicare reimbursement. Freestanding centers have historically billed at higher rates than hospital-based centers, however, and the RO-APM base rates represent a 6% decrease in global reimbursement for freestanding centers. The sensitivity analysis showed that, except for proton therapy, moderately hypofractionated treatment schedules will receive comparable reimbursement under RO-APM. Treatments using higher numbers of fractions of intensity-modulated radiation therapy or protons will see larger decreases in reimbursement. Application of the RO-APM base rates to the 2020 Medicare treatments in our health care network would result in small changes in expected reimbursement, but our sensitivity analysis indicated that Medicare reimbursement reductions could be as large as 23%. CONCLUSION: Compared with historical Medicare reimbursement, RO-APM base rates provide lower reimbursement for many common treatment scenarios, and this will have a larger effect on centers that use complex treatment techniques and longer fractionation schedules or have a large Medicare population.

Whole brain radiotherapy with hippocampal sparing using Varian HyperArc.

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

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

PURPOSE: The purpose of this study was to evaluate the performance of three common deformable image registration (DIR) packages across algorithms and institutions. METHODS AND MATERIALS: The Deformable Image Registration Evaluation Project (DIREP) provides ten virtual phantoms derived from computed tomography (CT) datasets of head-and-neck cancer patients over a single treatment course. Using the DIREP phantoms, DIR results from 35 institutions were submitted using either Velocity, MIM, or Eclipse. Submitted deformation vector fields (DVFs) were compared to ground-truth DVFs to calculate target registration error (TRE) for six regions of interest (ROIs). Statistical analysis was performed to determine the variability between each DIR software package and the variability of users within each algorithm. RESULTS: Overall mean TRE was 2.04 ± 0.35 mm for Velocity, 1.10 ± 0.29 mm for MIM, and 2.35 ± 0.15 mm for Eclipse. The MIM mean TRE was significantly different than both Velocity and Eclipse for all ROIs. Velocity and Eclipse mean TREs were not significantly different except for when evaluating the registration of the cord or mandible. Significant differences between institutions were found for the MIM and Velocity platforms. However, these differences could be explained by variations in Velocity DIR parameters and MIM software versions. CONCLUSIONS: Average TRE was shown to be <3 mm for all three software platforms. However, maximum errors could be larger than 2 cm indicating that care should be exercised when using DIR. While MIM performed statistically better than the other packages, all evaluated algorithms had an average TRE better than the largest voxel dimension. For the phantoms studied here, significant differences between algorithm users were minimal suggesting that the algorithm used may have more impact on DIR accuracy than the particular registration technique employed. A significant difference in TRE was discovered between MIM versions showing that DIR QA should be performed after software upgrades as recommended by TG-132.

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

PURPOSE: An episode-based payment model, the Radiation Oncology Alternative Payment Model (RO-APM), has been proposed for Medicare reimbursement of radiation services provided to oncology patients. RO-APM may have significant impact on reimbursement for specific patient populations. METHODS: This investigation compares historical fee-for-service technical reimbursement estimates at a large hospital-based system to the RO-APM for advanced radiotherapy treatment of specific cancer types. These advanced techniques, stereotactic radiosurgery (SRS), stereotactic body radiotherapy (SBRT), online-adaptive SBRT, and proton therapy, were specifically chosen because they are resource intensive and are correspondingly among the most expensive radiation oncology procedures. A total of 203 Medicare patients were analyzed. RESULTS: RO-APM base-rate reimbursements were similar for SRS and were 38%-47% higher for SBRT. The proposed rates were 1%-31% lower for online-adaptive SBRT, and 48%-71% lower for proton therapy. CONCLUSION: These data suggest that the RO-APM may have the desired effect of encouraging shorter courses of radiotherapy, such as SBRT. However, emerging technologies that require large capital and operating investments may see an overall significant reduction in proposed reimbursement.

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

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

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

PURPOSE: Real-time magnetic resonance guided radiation therapy (MRgRT) uses 2D cine imaging for target tracking. This work evaluates the percent image uniformity (PIU) and spatial integrity of cine images in the presence of multileaf collimator (MLC) and gantry motion in order to simulate sliding window and volumetric modulated arc therapy (VMAT) conditions. METHODS: Percent image uniformity and spatial integrity of cine images were measured (1) during MLC motion, (2) as a function of static gantry position, and (3) during gantry rotation. PIU was calculated according to the ACR MRI Quality Control Manual. Spatial integrity was evaluated by measuring the geometric distortion of 16 measured marker positions (10 cm or 15.225 cm from isocenter). RESULTS: The PIU of cine images did not vary by more than 1% from static linac conditions during MLC motion and did not vary by more than 3% during gantry rotation. Banding artifacts were present during gantry rotation. The geometric distortion in the cine images was less than 0.88 mm for all points measured throughout MLC motion. For all static gantry positions, the geometric distortion was less than 0.88 mm at 10 cm from isocenter and less than 1.4 mm at 15.225 cm from isocenter. During gantry rotation, the geometric distortion remained less than 0.92 mm at 10 cm from isocenter and less than 1.60 mm at 15.225 cm from isocenter. CONCLUSION: During MLC motion, cine images maintained adequate PIU, and the geometric distortion of points within 15.225 cm from isocenter was less than the 1 mm threshold necessary for real-time target tracking and gating. During gantry rotation, PIU was negatively affected by banding artifacts, and spatial integrity was only maintained within 10 cm from isocenter. Future work should investigate the effects imaging artifacts have on real-time target tracking during MRgRT.

Technical Report: Diagnostic Scan-Based Planning (DSBP), A Method to Improve the Speed and Safety of Radiation Therapy for the Treatment of Critically Ill Patients.

PURPOSE: Treating critically ill patients in radiation oncology departments poses multiple safety risks. This study describes a method to improve the speed of radiation treatment for patients in the intensive care unit by eliminating the need for computed tomography (CT) simulation or on-table treatment planning using patients' previously acquired diagnostic CT scans. METHODS AND MATERIALS: Initially, a retrospective planning study was performed to assess the applicability and safety of diagnostic scan-based planning (DSBP) for 3 typical indications for radiation therapy in patients in the intensive care unit: heterotopic ossification (10), spine metastases (cord compression; 10), and obstructive lung lesions (5). After identification of an appropriate diagnostic CT scan, treatment planning was performed using the diagnostic scan data set. These treatment plans were then transferred to the patients' simulation scans, and a dosimetric comparison was performed between the 2 sets of plans. Additionally, a time study of the first 10 patients treated with DSBP in our department was performed. RESULTS: The retrospective analysis demonstrated that DSBP resulted in treatment plans that, when transferred to the CT simulation data sets, provided excellent target coverage, a median D95% of 96% (range, 86%-100%) of the prescription dose with acceptable hot spots, and a median Dmax108% (range, 102%-113%). Subsequently, DSBP has been used for 10 critically ill patients. The patients were treated without CT simulation, and the median time between patient check-in to the department and completion of radiation therapy was 28 minutes (range, 18-47 minutes.) CONCLUSIONS: This study demonstrates that it is possible to safely use DSBP for the treatment of critically ill patients. This method has the potential to simplify the treatment process and improve the speed and safety of treatment.

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

PURPOSE: To describe the commissioning of AIRO mobile CT system (AIRO) for adaptive proton therapy on a compact double scattering proton therapy system. METHODS: A Gammex phantom was scanned with varying plug patterns, table heights, and mAs on a CT simulator (CT Sim) and on the AIRO. AIRO-specific CT-stopping power ratio (SPR) curves were created with a commonly used stoichiometric method using the Gammex phantom. A RANDO anthropomorphic thorax, pelvis, and head phantom, and a CIRS thorax and head phantom were scanned on the CT Sim and AIRO. Clinically realistic treatment plans and nonclinical plans were generated on the CT Sim images and subsequently copied onto the AIRO CT scans for dose recalculation and comparison for various AIRO SPR curves. Gamma analysis was used to evaluate dosimetric deviation between both plans. RESULTS: AIRO CT values skewed toward solid water when plugs were scanned surrounded by other plugs in phantom. Low-density materials demonstrated largest differences. Dose calculated on AIRO CT scans with stoichiometric-based SPR curves produced over-ranged proton beams when large volumes of low-density material were in the path of the beam. To create equivalent dose distributions on both data sets, the AIRO SPR curve's low-density data points were iteratively adjusted to yield better proton beam range agreement based on isodose lines. Comparison of the stoichiometric-based AIRO SPR curve and the "dose-adjusted" SPR curve showed slight improvement on gamma analysis between the treatment plan and the AIRO plan for single-field plans at the 1%, 1 mm level, but did not affect clinical plans indicating that HU number differences between the CT Sim and AIRO did not affect dose calculations for robust clinical beam arrangements. CONCLUSION: Based on this study, we believe the AIRO can be used offline for adaptive proton therapy on a compact double scattering proton therapy system.

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

PURPOSE: To characterize the accuracy of noncoplanar image guided radiation therapy with the Varian Edge radiosurgery system for intracranial stereotactic radiosurgery (SRS) treatments by assessing the accuracy of kV/kV orthogonal pair registration with Optical Surface Monitoring System (OSMS) monitoring relative to cone beam computed tomography (CT). METHODS AND MATERIALS: A Computerized Imaging Reference System head phantom and Encompass SRS Immobilization System were used to determine collision-free space for orthogonal image pairs (kV/kV) for couch rotations (CRs) of 45°, 30°, 15°, 345°, 330°, and 315°. Couch-induced shifts were measured using kV/kV orthogonal image pairs, OSMS, and cone beam CT. The kV/kV image pairs and OSMS localization accuracy was also assessed with respect to cone beam CT. RESULTS: Mean orthogonal image pair differences for 315°, 330°, 345°, 15°, 30°, and 45° CRs were ≤±0.60 mm and ±0.37°. OSMS localization accuracy was ≤±0.25 mm and ±0.20°. Correspondingly, kV/kV localization accuracy was ≤±0.30 mm and ±0.5°. Shift differences for various image pairs at all CRs were ≤±1.10 mm and ±0.7°. Cone beam CT deviation was 0.10 mm and 0.00° without patient motion or CR. CONCLUSION: Based on our study, CR-induced shifts with the Varian Edge radiosurgery system will not produce noticeable dosimetric effects for SRS treatments. Thus, replacing cone beam CT with orthogonal kV/kV pairs coupled with OSMS at the treatment couch angle could reduce the number of cone beam CT scans that are acquired during a standard SRS treatment while providing an accurate and safe treatment with negligible dosimetric effects on the treatment plan.

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

PURPOSE: The purpose of this study was to characterize the Mobius AIRO Mobile CT System for localization and image-guided proton therapy. This is the first known application of the AIRO for proton therapy. METHODS: Five CT images of a Catphan® 504 phantom were acquired on the AIRO Mobile CT System, Varian EDGE radiosurgery system cone beam CT (CBCT), Philips Brilliance Big Bore 16 slice CT simulator, and Siemens SOMATOM Definition AS 20 slice CT simulator. DoseLAB software v.6.6 was utilized for image quality analysis. Modulation transfer function, scaling discrepancy, geometric distortion, spatial resolution, overall uniformity, minimum uniformity, contrast, high CNR, and maximum HU deviation were acquired. Low CNR was acquired manually using the CTP515 module. Localization accuracy and CT Dose Index were measured and compared to reported values on each imaging device. For treatment delivery systems (Edge and Mevion), the localization accuracy of the 3D imaging systems were compared to 2D imaging systems on each system. RESULTS: The AIRO spatial resolution was 0.21 lp mm-1 compared with 0.40 lp mm-1 for the Philips CT Simulator, 0.37 lp mm-1 for the Edge CBCT, and 0.35 lp mm-1 for the Siemens CT Simulator. AIRO/Siemens and AIRO/Philips differences exceeded 100% for scaling discrepancy (191.2% and 145.8%). The AIRO exhibited higher dose (>27 mGy) than the Philips CT Simulator. Localization accuracy (based on the MIMI phantom) was 0.6° and 0.5 mm. Localization accuracy (based on Stereophan) demonstrated maximum AIRO-kV/kV shift differences of 0.1 mm in the x-direction, 0.1 mm in the y-direction, and 0.2 mm in the z-direction. CONCLUSIONS: The localization accuracy of AIRO was determined to be within 0.6° and 0.5 mm despite its slightly lower image quality overall compared to other CT imaging systems at our institution. Based on our study, the Mobile AIRO CT system can be utilized accurately and reliably for image-guided proton therapy.

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

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

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

PURPOSE: Intrafraction motion during step-and-shoot (SNS) IMRT is known to affect the target dosimetry by a combination of dose blurring and interplay effects. These effects are typically managed by adding a margin around the target. A quantitative analysis was performed, assessing the relationship between target motion, margin size, and target dosimetry with the goal of introducing new margin recipes. METHODS: A computational algorithm was used to calculate 1,174 motion-encoded dose distributions and DVHs within the patient's CT dataset. Sinusoidal motion tracks were used simulating intrafraction motion for nine lung tumor patients, each with multiple margin sizes. RESULTS: D95% decreased by less than 3% when the maximum target displacement beyond the margin experienced motion less than 5 mm in the superior-inferior direction and 15 mm in the anterior-posterior direction. For target displacements greater than this, D95% decreased rapidly. CONCLUSIONS: Targets moving in excess of 5 mm outside the margin can cause significant changes to the target. D95% decreased by up to 20% with target motion 10 mm outside the margin, with underdosing primarily limited to the target periphery. Multi-fractionated treatments were found to exacerbate target under-coverage. Margins several millimeters smaller than the maximum target displacement provided acceptable motion protection, while also allowing for reduced normal tissue morbidity.

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

PURPOSE: To describe the first use of the commercially available Calypso 4D Localization System in the lung. METHODS AND MATERIALS: Under an institutional review board-approved protocol and an investigational device exemption from the US Food and Drug Administration, the Calypso system was used with nonclinical methods to acquire real-time 4-dimensional lung tumor tracks for 7 lung cancer patients. The aims of the study were to investigate (1) the potential for bronchoscopic implantation; (2) the stability of smooth-surface beacon transponders (transponders) after implantation; and (3) the ability to acquire tracking information within the lung. Electromagnetic tracking was not used for any clinical decision making and could only be performed before any radiation delivery in a research setting. All motion tracks for each patient were reviewed, and values of the average displacement, amplitude of motion, period, and associated correlation to a sinusoidal model (R(2)) were tabulated for all 42 tracks. RESULTS: For all 7 patients at least 1 transponder was successfully implanted. To assist in securing the transponder at the tumor site, it was necessary to implant a secondary fiducial for most transponders owing to the transponder's smooth surface. For 3 patients, insertion into the lung proved difficult, with only 1 transponder remaining fixed during implantation. One patient developed a pneumothorax after implantation of the secondary fiducial. Once implanted, 13 of 14 transponders remained stable within the lung and were successfully tracked with the tracking system. CONCLUSIONS: Our initial experience with electromagnetic guidance within the lung demonstrates that transponder implantation and tracking is achievable though not clinically available. This research investigation proved that lung tumor motion exhibits large variations from fraction to fraction within a single patient and that improvements to both transponder and tracking system are still necessary to create a clinical daily-use system to assist with actual lung radiation therapy.

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

PURPOSE: To evaluate the impact of 3-dimensional (3D) surface imaging on daily patient setup for breast radiotherapy. MATERIALS AND METHODS: Fifty patients undergoing treatment for whole breast radiotherapy were setup daily using an AlignRT system (VisionRT, London, UK) for 3D surface-based alignment. Daily alignments were performed against a reference surface topogram and shifts from skin marks were recorded daily. This investigation evaluated the following: (1) the performance of the surface-based imaging system for daily breast alignment; (2) the absolute displacements between setup with skin marks and setup with the surface-based imaging system; and (3) the dosimetric effect of daily alignments with skin marks versus surface-based alignments. RESULTS: Displacements from 1258 treatment fractions were analyzed. Sixty percent of those fractions (749) were reviewed against MV portal imaging in order to assess the performance of the AlignRT system. Daily setup errors were given as absolute displacements, comparing setup marks against shifts determined using the surface-based imaging system. Averaged over all patients, the mean displacements were 4.1 ± 2.6 mm, 2.7 ± 1.4 mm, and 2.6 ± 1.2 mm in the anteroposterior (AP), superoinferior (S/I), and left-right (L/R) directions, respectively. Furthermore, the standard deviation of the random error (σ) was 3.2 mm, 2.2 mm, and 2.2 mm in the A/P, S/I, and L/R directions, respectively. CONCLUSIONS: Daily alignment with 3D surface imaging was found to be valuable for reducing setup errors when comparing with patient alignment from skin marks. The result of the surface-based alignments specifically showed that alignment with skin marks was noticeably poor in the anteroposterior directions. The overall dosimetric effect of the interfractional variations was small, but these variations showed a potential for increased dose deposition to both the heart and lung tissues. Although these interfractional variations would not negatively affect the quality of patient care for whole breast radiotherapy, it may require an increase in PTV margin, especially in cases of partial breast irradiation.