A Pilot Study of Simulation-Free Hippocampal-Avoidance Whole Brain Radiotherapy Using Diagnostic MRI-Based and Online Adaptive Planning

PURPOSE

We aimed to demonstrate the clinical feasibility and safety of simulation-free hippocampal avoidance whole brain radiation therapy (HA-WBRT) in a pilot study (NCTXXX).

MATERIALS/METHODS

Ten HA-WBRT candidates were enrolled for treatment on a commercially available computed tomography (CT)-guided linear accelerator with online adaptive capabilities. Planning structures were contoured on patient-specific diagnostic MRIs, which were registered to a CT of similar head shape, obtained from an atlas-based database (AB-CT). These patient-specific diagnostic MRI and AB-CT datasets were used for pre-plan calculation, using NRG-CC001 constraints. At first fraction, AB-CTs were used as primary datasets and deformed to patient-specific cone-beam CTs (CBCT) to give patient-matched density information. Brain, ventricle, and brainstem contours were matched through rigid translation and rotation to the corresponding anatomy on CBCT. Lens, optic nerve, and brain contours were manually edited based on CBCT visualization. Pre-plans were then re-optimized through online adaptation to create final, simulation-free plans, which were utilized if they met all objectives. Workflow tasks were timed. In addition, patients underwent CT-simulation to create immobilization devices and for prospective dosimetric comparison of simulation-free and simulation-based plans.

RESULTS

Median time from MRI importation to completion of "pre-plan" was one week-day (range: 1-4). Median on-table workflow duration was 41 minutes (range: 34-70). NRG-CC001 constraints were achieved by 90% of the simulation-free plans. One patient's simulation-free plan failed a planning target volume (PTV) coverage objective (89% instead of 90% coverage); this was deemed acceptable for first-fraction delivery, with an offline replan used for subsequent fractions. Both simulation-free and simulation-CT-based plans otherwise met constraints, without clinically meaningful differences.

CONCLUSION

Simulation-free HA-WBRT using online ART is feasible, safe, and results in dosimetrically comparable treatment plans to simulation-CT-based workflows while providing convenience and time-savings for patients.

In silico trial of simulation-free hippocampal-avoidance whole brain adaptive radiotherapy

Background and Purpose

Hippocampal-avoidance whole brain radiotherapy (HA-WBRT) can be a time-consuming process compared to conventional whole brain techniques, thus potentially limiting widespread utilization. Therefore, we evaluated the in silico clinical feasibility, via dose-volume metrics and timing, by leveraging a computed tomography (CT)-based commercial adaptive radiotherapy (ART) platform and workflow in order to create and deliver patient-specific, simulation-free HA-WBRT.

Materials and methods

Ten patients previously treated for central nervous system cancers with cone-beam computed tomography (CBCT) imaging were included in this study. The CBCT was the adaptive image-of-the-day to simulate first fraction on-board imaging. Initial contours defined on the MRI were rigidly matched to the CBCT. Online ART was used to create treatment plans at first fraction. Dose-volume metrics of these simulation-free plans were compared to standard-workflow HA-WBRT plans on each patient CT simulation dataset. Timing data for the adaptive planning sessions were recorded.

Results

For all ten patients, simulation-free HA-WBRT plans were successfully created utilizing the online ART workflow and met all constraints. The median hippocampi D100% was 7.8 Gy (6.6-8.8 Gy) in the adaptive plan vs 8.1 Gy (7.7-8.4 Gy) in the standard workflow plan. All plans required adaptation at first fraction due to both a failing hippocampal constraint (6/10 adaptive fractions) and sub-optimal target coverage (6/10 adaptive fractions). Median time for the adaptive session was 45.2 min (34.0-53.8 min).

Conclusions

Simulation-free HA-WBRT, with commercially available systems, was clinically feasible via plan-quality metrics and timing, in silico.

Clinical application of a template-guided automated planning routine

PURPOSE

Determine the dosimetric quality and the planning time reduction when utilizing a template-based automated planning application.

METHODS

A software application integrated through the treatment planning system application programing interface, QuickPlan, was developed to facilitate automated planning using configurable templates for contouring, knowledge-based planning structure matching, field design, and algorithm settings. Validations are performed at various levels of the planning procedure and assist in the evaluation of readiness of the CT image, structure set, and plan layout for automated planning. QuickPlan is evaluated dosimetrically against 22 hippocampal-avoidance whole brain radiotherapy patients. The required times to treatment plan generation are compared for the validations set as well as 10 prospective patients whose plans have been automated by QuickPlan.

RESULTS

The generations of 22 automated treatment plans are compared against a manual replanning using an identical process, resulting in dosimetric differences of minor clinical significance. The target dose to 2% volume and homogeneity index result in significantly decreased values for automated plans, whereas other dose metric evaluations are nonsignificant. The time to generate the treatment plans is reduced for all automated plans with a median difference of 9' 50″ ± 4' 33″.

CONCLUSIONS

Template-based automated planning allows for reduced treatment planning time with consistent optimization structure creation, treatment field creation, plan optimization, and dose calculation with similar dosimetric quality. This process has potential expansion to numerous disease sites.

Commissioning a secondary dose calculation software for a 0.35 T MR-linac

Secondary external dose calculations for a 0.35 T magnetic resonance image-guided radiation therapy (MRgRT) are needed within the radiation oncology community to follow safety standards set forth within the field. We evaluate the commercially available software, RadCalc, in its ability to accurately perform monitor unit dose calculations within a magnetic field. We also evaluate the potential effects of a 0.35 T magnetic field upon point dose calculations. Monitor unit calculations were evaluated with (wMag) and without (noMag) a magnetic field considerations in RadCalc for the ViewRay MRIdian. The magnetic field is indirectly accounted for by using asymmetric profiles for calculation. The introduction of double-stacked multi-leaf collimator leaves was also included in the monitor unit calculations and a single transmission value was determined. A suite of simple and complex geometries with a variety field arrangements were calculated for each method to demonstrate the effect of the 0.35 T magnetic field on monitor unit calculations. Finally, 25 patient-specific treatment plans were calculated using each method for comparison. All simple geometries calculated in RadCalc were within 2% of treatment planning system (TPS) values for both methods, except for a single noMag off-axis comparison. All complex muilt-leaf collimator (MLC) pattern calculations were within 5%. All complex phantom geometry calculations were within 5% except for a single field within a lung phantom at a distal point. For the patient calculations, the noMag method average percentage difference was 0.09 ± 2.5% and the wMag average percentage difference was 0.08 ± 2.5%. All results were within 5% for the wMag method. We performed monitor unit calculations for a 0.35 T MRgRT system using a commercially available secondary monitor unit dose calculation software and demonstrated minimal impact of the 0.35 T magnetic field on monitor unit dose calculations. This is the first investigation demonstrating successful calculations of dose using RadCalc in the low-field 0.35 T ViewRay MRIdian system.

Development and Implementation of an Open Source Template Interpretation Class Library for Automated Treatment Planning

PURPOSE

Widespread implementation of automated treatment planning in radiation therapy remains elusive due to variability in clinic and physician preferences making it difficult to ensure consistent plan parameters. We have developed an open-source class library with the aim to improve efficiency and consistency for automated treatment planning in radiation therapy.

METHODS AND MATERIALS

An open source class library has been developed that interprets clinical templates within a commercial treatment planning system into a treatment plan for automated planning. This code was leveraged for the automated planning of 39 patients and retrospectively compared to the 78 clinically approved manual plans.

RESULTS

From the initial 39 patients, 74 of 78 plans were successfully generated without manual intervention. Target dose was more homogenous for automated plans, with an average homogeneity index of 3.30 vs 3.11 for manual and automated plans, respectively (p = 0.107). Generalized equivalent uniform dose decreased in the femurs and rectum for automated plans, with mean gEUD of 3746 cGy vs 3338 cGy (p ≤ 0.001) and 5761 cGy vs 5634 cGy (p ≤ 0.001) for femurs and rectum, respectively. Dose metrics for bladder and rectum (V6500 cGy and V4000 cGy) show recognizable but insignificant improvements. All automated plans delivered for quality assurance passed a gamma analysis (>95%) with an average composite pass rate of 99.3% and 98.8% for pelvis and prostate plans, respectively. Deliverability parameters such as total monitor units and aperture complexity indicate deliverable plans.

CONCLUSIONS

Prostate cancer and pelvic node radiotherapy can be automated using VMAT planning and clinical templates based on a standardized clinical workflow. The class library developed in this study conveniently interfaces between the plan template and the treatment planning system to automatically generate high quality plans on customizable templates.

Application programming interface guided QA plan generation and analysis automation

Purpose

Linear accelerator quality assurance (QA) in radiation therapy is a time consuming but fundamental part of ensuring the performance characteristics of radiation delivering machines. The goal of this work is to develop an automated and standardized QA plan generation and analysis system in the Oncology Information System (OIS) to streamline the QA process.

Methods

Automating the QA process includes two software components: the AutoQA Builder to generate daily, monthly, quarterly, and miscellaneous periodic linear accelerator QA plans within the Treatment Planning System (TPS) and the AutoQA Analysis to analyze images collected on the Electronic Portal Imaging Device (EPID) allowing for a rapid analysis of the acquired QA images. To verify the results of the automated QA analysis, results were compared to the current standard for QA assessment for the jaw junction, light‐radiation coincidence, picket fence, and volumetric modulated arc therapy (VMAT) QA plans across three linacs and over a 6‐month period.

Results

The AutoQA Builder application has been utilized clinically 322 times to create QA patients, construct phantom images, and deploy common periodic QA tests across multiple institutions, linear accelerators, and physicists. Comparing the AutoQA Analysis results with our current institutional QA standard the mean difference of the ratio of intensity values within the field‐matched junction and ball‐bearing position detection was 0.012 ± 0.053 (P = 0.159) and is 0.011 ± 0.224 mm (P = 0.355), respectively. Analysis of VMAT QA plans resulted in a maximum percentage difference of 0.3%.

Conclusion

The automated creation and analysis of quality assurance plans using multiple APIs can be of immediate benefit to linear accelerator quality assurance efficiency and standardization. QA plan creation can be done without following tedious procedures through API assistance, and analysis can be performed inside of the clinical OIS in an automated fashion.

Internal dose escalation associated with increased local control for melanoma brain metastases treated with stereotactic radiosurgery

OBJECTIVE

The internal high-dose volume varies widely for a given prescribed dose during stereotactic radiosurgery (SRS) to treat brain metastases (BMs). This may be altered during treatment planning, and the authors have previously shown that this improves local control (LC) for non-small cell lung cancer BMs without increasing toxicity. Here, they seek to identify potentially actionable dosimetric predictors of LC after SRS for melanoma BM.

METHODS

The records of patients with unresected melanoma BM treated with single-fraction Gamma Knife RS between 2006 and 2017 were reviewed. LC was assessed on a per-lesion basis, defined as stability or a decrease in lesion size. Outcome-oriented approaches were utilized to determine optimal dichotomization for dosimetric variables relative to LC. Univariable and multivariable Cox regression analysis was implemented to evaluate the impact of collected parameters on LC.

RESULTS

Two hundred eighty-seven melanoma BMs in 79 patients were identified. The median age was 56 years (range 31-86 years). The median follow-up was 7.6 months (range 0.5-81.6 months), and the median survival was 9.3 months (range 1.3-81.6 months). Lesions were optimally stratified by volume receiving at least 30 Gy (V30) greater than or equal to versus less than 25%. V30 was ≥ and < 25% in 147 and 140 lesions, respectively. For all patients, 1-year LC was 83% versus 66% for V30 ≥ and < 25%, respectively (p = 0.001). Stratifying by volume, lesions 2 cm or less (n = 215) had 1-year LC of 82% versus 70% (p = 0.013) for V30 ≥ and < 25%, respectively. Lesions > 2 to 3 cm (n = 32) had 1-year LC of 100% versus 43% (p = 0.214) for V30 ≥ and < 25%, respectively. V30 was still predictive of LC even after controlling for the use of immunotherapy and targeted therapy. Radionecrosis occurred in 2.8% of lesions and was not significantly associated with V30.

CONCLUSIONS

For a given prescription dose, an increased internal high-dose volume, as indicated by measures such as V30 ≥ 25%, is associated with improved LC but not increased toxicity in single-fraction SRS for melanoma BM. Internal dose escalation is an independent predictor of improved LC even in patients receiving immunotherapy and/or targeted therapy. This represents a dosimetric parameter that is actionable at the time of treatment planning and warrants further evaluation.

Filmless quality assurance of a Leksell Gamma Knife® Icon™

PURPOSE

The annual quality assurance (QA) of Leksell Gamma Knife^® (LGK) systems are typically performed using films. Film is a good candidate for small field dosimetry due to its high spatial resolution and availability. However, there are multiple challenges with using film; film does not provide real-time measurement and requires batch-specific calibration. Our findings show that active detector-based QA can simplify the procedure and save time without loss of accuracy.

METHODS

Annual QA tests for a LGK Icon™ system were performed using both film-based and filmless techniques. Output calibration, relative output factors (ROF), radiation profiles, sector uniformity/source counting, and verification of the unit center point (UCP) and radiation focal point (RFP) coincidence tests were performed. Radiochromic films, two ionization chambers, and a synthetic diamond detector were used for the measurements. Results were compared and verified with the treatment planning system (TPS).

RESULTS

The measured dose rate of the LGK Icon was within 0.4% of the TPS value set at the time of commissioning using an ionization chamber. ROF for the 8 and 4-mm collimators were found to be 0.3% and 1.8% different from TPS values using the MicroDiamond detector and 2.6% and 1.9% different for film, respectively. Excellent agreement was found between TPS and measured dose profiles using the MicroDiamond detector which was within 1%/1 mm vs 2%/1 mm for film. Sector uniformity was found to be within 1% for all eight sectors measured using an ionization chamber. Verification of UCP and RFP coincidence using the MicroDiamond detector and pinprick film test was within 0.3 mm at isocenter for both.

CONCLUSION

The annual QA of a LGK Icon was successfully performed by employing filmless techniques. Comparable results were obtained using radiochromic films. Utilizing active detectors instead of films simplifies the QA process and saves time without loss of accuracy.

Portal dosimetry scripting application programming interface (PDSAPI) for Winston-Lutz test employing ceramic balls

PURPOSE

Stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) treatments require a high degree of accuracy. Mechanical, imaging, and radiation isocenter coincidence is especially important. As a common method, the Winston-Lutz (WL) test plays an important role. However, weekly or daily WL test can be very time consuming. We developed novel methods using Portal Dosimetry Scripting Application Programming Interface (PDSAPI) to facilitate the test as well as documentation.

METHODS

Winston-Lutz PDSAPI was developed and tested on our routine weekly WL imaging. The results were compared against two commercially available software RIT (Radiological Imaging Technology, Colorado Springs, CO) and DoseLab (Varian Medical Systems, Inc. Palo Alto, CA). Two manual methods that served as ground truth were used to verify PDSAPI results. Twenty WL test image data sets (10 fields per tests, and 200 images in total) were analyzed by these five methods in this report.

RESULTS

More than 99.5% of WL PDSAPI 1D shifts agreed with each of four other methods within ±0.33 mm, which is roughly the pixel width of a-Si 1200 portal imager when source to imager distance (SID) is at 100 cm. 1D shifts agreement for ±0.22 mm and 0.11 mm were 96% and 63%, respectively. Same trend was observed for 2D displacement.

CONCLUSIONS

Winston-Lutz PDSAPI delivers similar accuracy as two commercial applications for WL test. This new application can save time spent transferring data and has the potential to implement daily WL test with reasonable test time. It also provides the data storage capability, and enables easy access to imaging and shift data.

Automated and robust beam data validation of a preconfigured ring gantry linear accelerator using a 1D tank with synchronized beam delivery and couch motions

PURPOSE

To develop an efficient and automated methodology for beam data validation for a preconfigured ring gantry linear accelerator using scripting and a one-dimensional (1D) tank with automated couch motions.

MATERIALS AND METHODS

Using an application programming interface, a program was developed to allow the user to choose a set of beam data to validate with measurement. Once selected the program generates a set of instructions for radiation delivery with synchronized couch motions for the linear accelerator in the form of an extensible markup language (XML) file to be delivered on the ring gantry linear accelerator. The user then delivers these beams while measuring with the 1D tank and data logging electrometer. The program also automatically calculates this set of beams on the measurement geometry within the treatment planning system (TPS) and extracts the corresponding calculated dosimetric data for comparison to measurement. Once completed the program then returns a comparison of the measurement to the predicted result from the TPS to the user and prints a report. In this work lateral, longitudinal, and diagonal profiles were taken for fields sizes of 6 × 6, 8 × 8, 10 × 10, 20 × 20, and 28 × 28 cm² at depths of 1.3, 5, 10, 20, and 30 cm. Depth dose profiles were taken for all field sizes.

RESULTS

Using this methodology, the TPS was validated to agree with measurement. All compared points yielded a gamma value less than 1 for a 1.5%/1.5 mm criteria (100% passing rate). Off axis profiles had >98.5% of data points producing a gamma value <1 with a 1%/1 mm criteria. All depth profiles produced 100% of data points with a gamma value <1 with a 1%/1 mm criteria. All data points measured were within 1.5% or 2 mm distance to agreement.

CONCLUSIONS

This methodology allows for an increase in automation in the beam data validation process. Leveraging the application program interface allows the user to use a single system to create the measurement files, predict the result, and then compare to actual measurement increasing efficiency and reducing the chance for user input errors.

Implementing a Novel Remote Physician Treatment Coverage Practice for Adaptive Radiation Therapy During the Coronavirus Pandemic

PURPOSE

The 2019 coronavirus disease pandemic has placed an increased importance on physical distancing to minimize the risk of transmission in radiation oncology departments. The pandemic has also increased the use of hypofractionated treatment schedules where magnetic resonance-guided online adaptive radiation therapy (ART) can aid in dose escalation. This specialized technique requires increased staffing in close proximity, and thus the need for novel coverage practices to increase physical distancing while still providing specialty care.

METHODS AND MATERIALS

A remote-physician ART coverage practice was developed and described using commercially available software products. Our remote-physician coverage practice provided control to the physician to contour and review of the images and plans. The time from completion of image registration to the beginning of treatment was recorded for 20 fractions before remote-physician ART coverage and 14 fractions after implementation of remote-physician ART coverage. Visual quality was calculated using cross-correlation between the treatment delivery and remote-physician computer screens.

RESULTS

For the 14 fractions after implementation, the average time from image registration to the beginning of treatment was 24.9 ± 6.1 minutes. In comparison, the 20 fractions analyzed without remote coverage had an average time of 29.2 ± 9.8 minutes. The correlation between the console and remote-physician screens was R = .95.

CONCLUSIONS

Our novel remote-physician ART coverage practice is secure, interactive, timely, and of high visual quality. When using remote physicians for ART, our department was able to increase physical distancing to lower the risk of virus transmission while providing specialty care to patients in need.

Implementation of a Novel Remote Physician Stereotactic Body Radiation Therapy Coverage Process during the Coronavirus Pandemic

PURPOSE

During the coronavirus 2019 disease (COVID-19) pandemic, alternative methods of care are needed to reduce the relative risk of transmission in departments. Also needed is the ability to provide vital radiation oncological care if radiation oncologists (RO) are reallocated to other departments. We implemented a novel remote RO stereotactic body radiation therapy (SBRT) coverage practice, requiring it to be reliable, of high audio and visual quality, timely, and the same level of specialty care as our current in-person treatment coverage practice.

METHODS AND MATERIALS

All observed failure modes were recorded during implementation over the first 15 sequential fractions. The time from cone beam computed tomography to treatment was calculated before and after implementation to determine timeliness of remote coverage. Image quality metrics were calculated between the imaging console screen and the RO's shared screen. Comfort levels with audio and visual communication as well as overall comfort in comparison to in-person RO coverage was evaluated using Likert scale surveys after treatment.

RESULTS

Remote RO SBRT coverage was successfully implemented in 14 of 15 fractions with 3 observed process failures that were all corrected before treatment. Average times of pretreatment coverage before and after implementation were 8.74 and 8.51 minutes, respectively. The cross correlation between the imaging console screen and RO's shared screen was r = 0.96 and lag was 0.05 seconds. The average value for all survey questions was more than 4.5, approaching in-person RO coverage comfort levels.

CONCLUSION

Our novel method of remote RO SBRT coverage permits reduced personnel and patient interactions surrounding radiation therapy procedures. This may help to reduce transmission of COVID-19 in our department and provides a means for SBRT coverage if ROs are reallocated to other areas of the hospital for COVID-19 support.

Evaluation of a new secondary dose calculation software for Gamma Knife radiosurgery

Current available secondary dose calculation software for Gamma Knife radiosurgery falls short in situations where the target is shallow in depth or when the patient is positioned with a gamma angle other than 90°. In this work, we evaluate a new secondary calculation software which utilizes an innovative method to handle nonstandard gamma angles and image thresholding to render the skull for dose calculation. 800 treatment targets previously treated with our GammaKnife Icon system were imported from our treatment planning system (GammaPlan 11.0.3) and a secondary dose calculation was conducted. The agreement between the new calculations and the TPS were recorded and compared to the original secondary dose calculation agreement with the TPS using a Wilcoxon Signed Rank Test. Further comparisons using a Mann‐Whitney test were made for targets treated at a 90° gamma angle against those treated with either a 70 or 110 gamma angle for both the new and commercial secondary dose calculation systems. Correlations between dose deviations from the treatment planning system against average target depth were evaluated using a Kendall’s Tau correlation test for both programs. The Wilcoxon Signed Rank Test indicated a significant difference in the agreement between the two secondary calculations and the TPS, with a P‐value < 0.0001. With respect to patients treated at nonstandard gamma angles, the new software was largely independent of patient setup, while the commercial software showed a significant dependence (P‐value < 0.0001). The new secondary dose calculation software showed a moderate correlation with calculation depth, while the commercial software showed a weak correlation (Tau = −.322 and Tau = −.217 respectively). Overall, the new secondary software has better agreement with the TPS than the commercially available secondary calculation software over a range of diverse treatment geometries.

Lateral head flexion as a noncoplanar solution for ring gantry stereotactic radiosurgery

PURPOSE

Ring gantry radiotherapy devices are often limited to deliver beams in the axial plane, severely limiting beam entrance angles and rendering noncoplanar beam delivery impossible. However, a ring gantry geometry greatly simplifies delivery machines and increases the efficiency of treatment with the potential to decrease the overall costs of radiotherapy. This study explores the use of lateral head flexion in order to increase beam entrance angles and extend the available solid angle space for a ring gantry stereotactic radiosurgery (SRS) application.

MATERIALS AND METHODS

A 1.5 T magnetic resonance imaging scanner was used to scan seven healthy volunteers at three different head positions: a neutral position, a left lateral flexion position and a right lateral flexion position. The lateral flexion scans were co-registered to the neutral head position scan using rigid registration and extracting the rotational transformation. The head pitch, roll, and yaw were computed for each registration to evaluate the natural range of motion for all volunteers. A ring gantry plan geometry was used to generate two sets of single fraction SRS plans (21 Gy): one coplanar set for head neutral scans, and a three-arc plan set using the head neutral and lateral head flexion scans. The conformity index (CI), intermediate dose fall-off (R50), low dose spillage (R10), and gradient measure (GM) were used to evaluate both sets of plans. The treatment plans were generated for a ring-gantry linear accelerator (linac) (Varian Halcyon 2.0) as well as radiosurgery linac (Varian Edge) for comparison.

RESULTS

The average pitch, yaw, and roll for the lateral head flexion scans were 4.1° ± 4.7°, 16.9° ± 3.7°, and 2.5° ± 4.9° for the right flexion and 4.9° ± 4.3°, 14.0° ± 3.7° and 2.8° ± 5.4° for left flexion. When comparing the head flexion technique with a fully coplanar geometry, the ring gantry plans showed an average improvement in CI of 7.3% (1.46 ± 0.25 vs 1.36 ± 0.28), a decrease of 13% in R50 (5.46 ± 1.14 vs 4.78 ± 1.12), a decrease of 32% in R10 (85.7 ± 20.3 vs 58.2 ± 15.1), and a decrease of 7.8% in GM (0.53 ± 0.05 vs 0.49 ± 0.04). The Edge plans showed an average improvement in CI of 3.0% (1.49 ± 0.26 vs 1.45 ± 0.25), a decrease of 6.8% in R50 (5.19 ± 1.03 vs 4.82 ± 0.83), a decrease of 29% in R10 (84.1 ± 16.3 vs 59.9 ± 12.5), and a decrease of 5.0% in GM (0.50 ± 0.04 vs 0.47 ± 0.03).

CONCLUSION

Lateral head flexion was shown to increase beam entrance angles considerably improving plan conformity and normal tissue sparing in this pilot study of seven sets of plans. Rigid registrations demonstrated each lateral flexion to be analogous to a 15° couch kick. The head flexion technique outlined here was shown to be a feasible solution for SRS treatments being delivered on ring gantry devices.

Characterization and validation of an intra-fraction motion management system for masked-based radiosurgery

Purpose

Characterize the intra‐fraction motion management (IFMM) system found on the Gamma Knife Icon (GKI), including spatial accuracy, latency, temporal performance, and overall effect on delivered dose.

Methods

A phantom was constructed, consisting of a three‐axis translation mount, a remote motorized flipper, and a thermoplastic sphere surrounding a radiation detector. An infrared marker was placed on the translation mount secured to the flipper. The spatial accuracy of the IFMM was measured via the translation mount in all Cartesian planes. The detector was centered at the radiation focal point. A remote signal was used to move the marker out of the IFMM tolerance and pause the beam. A two‐channel electrometer was used to record the signals from the detector and the flipper when motion was signaled. These signals determined the latency and temporal performance of the GKI.

Results

The spatial accuracy of the IFMM was found to be <0.1 mm. The measured latency was <200 ms. The dose difference with five interruptions was <0.5%.

Conclusion

This work provides a quantitative characterization of the GKI IFMM system as required by the Nuclear Regulatory Commission. This provides a methodology for GKI users to satisfy these requirements using common laboratory equipment in lieu of a commercial solution.

Equivalency of beam scan data collection using a 1D tank and automated couch movements to traditional 3D tank measurements

This work shows the feasibility of collecting linear accelerator beam data using just a 1‐D water tank and automated couch movements with the goal to maximize the cost effectiveness in resource‐limited clinical settings. Two commissioning datasets were acquired: (a) using a standard of practice 3D water tank scanning system (3DS) and (b) using a novel technique to translate a commercial TG‐51 complaint 1D water tank via automated couch movements (1DS). The Extensible Markup Language (XML) was used to dynamically move the linear accelerator couch position (and thus the 1D tank) during radiation delivery for the acquisition of inline, crossline, and diagonal profiles. Both the 1DS and 3DS datasets were used to generate beam models (BM_1DS and BM_3DS) in a commercial treatment planning system (TPS). 98.7% of 1DS measured points had a gamma value (2%/2 mm) < 1 when compared with the 3DS. Static jaw defined field and dynamic MLC field dose distribution comparisons for the TPS beam models BM_1DS and BM_3DS had 3D gamma values (2%/2 mm) < 1 for all 24,900,000 data points tested and >99.5% pass rate with gamma value (1%/1 mm) < 1. In conclusion, automated couch motions and a 1D scanning tank were used to collect commissioning beam data with accuracy comparable to traditionally acquired data using a 3D scanning system. TPS beam models generated directly from 1DS measured data were clinically equivalent to a model derived from 3DS data.

Evaluating the accuracy of a three-term pencil beam algorithm in heterogeneous media

The goal of this work was to evaluate the accuracy of our in-house analytical dose calculation code against MCNPX data in heterogeneous phantoms. The analytical model utilizes a pencil beam model based on Fermi-Eyges theory to account for multiple Coulomb scattering and a least-squares fit to Monte Carlo data to account for nonelastic nuclear interactions as well as any remaining, uncharacterized scatter (the 'nuclear halo'). The model characterized dose accurately (up to 1% of maximum dose in broad fields (4  ×  4 cm² and 10  ×  10 cm²) and up to 0.01% in a narrow field (0.1  ×  0.1 cm²) fit to MCNPX data). The accuracy of the model was benchmarked in three types of stylized phantoms: (1) homogeneous, (2) laterally infinite slab heterogeneities, and (3) laterally finite slab heterogeneities. Results from homogeneous phantoms and laterally infinite slab heterogeneities showed high levels of accuracy (>98% of points within 2% or 0.1 cm distance-to-agreement (DTA)). However, because range straggling and secondary particle production were not included in our model, central-axis dose differences of 2-4% were observed in laterally infinite slab heterogeneities when compared to Monte Carlo dose. In the presence of laterally finite slab heterogeneities, the analytical model resulted in lower pass rates (>96% of points within 2% or 0.1 cm DTA), which was attributed to the use of the central-axis approximation.