Browsing by Author "Adamson, Justus"

Purpose: The multi-leaf collimator (MLC) is a critical component in intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). The MLC discrepancies between planned and actual position directly affects the quality of treatment. This study analyzed MLC positional discrepancies and gantry angle discrepancies via trajectory log files from Varian TrueBeam linear accelerators to determine the consistency of machine performance accuracy over the course of treatment. The mechanical parameters that affect accuracy were examined and evaluated to build a machine learning algorithm to predict moving components’ discrepancies. A tool was developed to predict the treatment delivery discrepancies on a Varian TrueBeam linear accelerator for any given plan, which can simulate radiotherapy treatment delivery without actual delivery.

Methods: Trajectory log files of 116 IMRT plans and 125 VMAT plans from nine Varian TrueBeam linear accelerators were collected and analyzed. Data was binned by treatment site and machine type to determine their relationship with MLC and gantry angle discrepancies. Trajectory log files were used to evaluate whether MLC positional accuracy was consistent between patient-specific quality assurance (QA) and the course of treatment. Mechanical parameters including MLC velocity, MLC acceleration, gantry angle, gantry velocity, gantry acceleration, collimator angle, control point, dose rate, and gravity vector were analyzed to evaluate correlations with delivery discrepancies. A regression model was used to develop a machine learning algorithm to predict delivery discrepancies based on mechanical parameters.

Results: MLC discrepancies at pre-treatment patient-specific QA differed from the course of treatment by a small (mean = 0.0031 ± 0.0036 mm, p = 0.0089 for IMRT; mean = 0.0014 ± 0.0016 mm, p = 0.0003 for VMAT) but statistically significant amount, likely due to setting the gantry angle to zero for QA. Mechanical parameters showed significant correlation with MLC discrepancies, especially MLC velocity, which had an approximately linear relationship (β = -0.0027, R2 = 0.79). Incorporating other mechanical parameters, the final generalized model trained by data from all linear accelerators can predict MLC errors to a high degree of accuracy with high correlation (R2 = 0.86) between predicted and actual errors. The same prediction model performed well across different treatment sites and linear accelerators; however, a significant difference was found in the predictions made by models trained using different treatment techniques (IMRT vs VMAT) (mean difference of RMSE = 0.0153 ± 0.0040 mm).

Conclusion: We have developed a machine learning model using prior trajectory log files to predict the MLC discrepancies on TrueBeam linear accelerators. This model has been a released as a research tool in which a DICOM-RT with predicted MLC positions can be generated using the original DICOM-RT file as input. This tool can be used to simulate radiotherapy treatment delivery and may be useful for studies evaluating plan robustness and dosimetric uncertainties from treatment delivery.

Abstract

Purpose: Updated recommendations for pre-treatment QA of patient-specific intensity modulated radiation therapy (IMRT) and Volumetric modulated arc therapy (VMAT) quality assurance (QA) were recently published by the AAPM task group TG-218. While the traditionally most common QA analysis is to use a Gamma index with dose & spatial analysis criteria of 3% & 3mm, respectively, TG-218 recommends a tighter spatial tolerance of 2mm for standard IMRT QA, and that even tighter tolerances should be considered for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Our purpose is to report our experience with applying the TG-218 recommendations to a large clinical SRS and SBRT program. In addition, a new SRS technique was recently developed at Duke, called Conformal Arc Informed Volumetric Modulated Arc Therapy (CAVMAT), which is designed to be less sensitive to configuration and delivery errors. We measured the agreement of CAVMAT for pre-treatment QA and compared it to the current standard (VMAT) to evaluate whether CAVMAT is more robust to delivery errors than VMAT.

Methods: We re-analyzed the pre-treatment QA with respect to the TG-218 recommendations. For Portal Dosimetry (Varian Medical Systems, Palo Alto, CA), this included IMRT brain (n=25) and SBRT / hypofractionated image guided radiotherapy (HIGRT) cases that utilize flattened photon beams (n=18). For Delta4 (ScandiDos, Madison, WI) this included single target SRS (n=24), multiple target SRS (n=25), SBRT cases using VMAT (n=74), and SBRT cases using IMRT with FFF photons (n=23). For ArcCHECK (Sun Nuclear, Melbourne, FL)), we take 25 single target VMAT SRS cases and 25 multiple target VMAT SRS cases. For SRS MapCHECK(Sun Nuclear, Melboume, FL), we analyze 10 multiple target VMAT SRS cases with 16 targets. A Gamma analysis was performed with 6 spatial/dose criteria combinations: 3%/3mm, 3%/2mm, 3%/1mm, 2%/1mm, 4%/1mm, 5%/1mm. We then calculated the TG-218 action limit and tolerance limit per plan type and compared to the “universal” TG-218 action limit of 90% having a Gamma <1.

To compare CAVMAT and VMAT, log file analysis and pre-treatment QA was performed for 10 patients with 20 plans (10 VMAT, 10 CAVMAT) with 46 targets in total. 10 VMAT plans were re-planned using CAVMAT, and the dosimetric effect due to treatment delivery errors was quantified for V6Gy, V12Gy, and V16Gy of healthy brain along with the maximum, average and minimum doses of each target. Gamma analysis of VMAT and CAVMAT plans was performed using Delta4 and SRS MapCHECK with 3% / 1mm, 2% / 1mm, 1% / 1mm criteria to assess the agreement during patient specific quality assurance.

Result: For Portal Dosimetry QA of IMRT brain and SBRT/HIGRT using a 3%/1mm criteria, the TG-218 action limit was 99.68, and 90.14, respectively; with 3.68% and 3.68% of cases failing the universal 90% criteria. For Delta4 QA of single target SRS, multiple target SRS, and SBRT IMRT with FFF using a 3%/1mm criteria, the TG-218 action limit was 93.64, 97.12, and 92.01, respectively; with 0%, 0%, and 0% of cases failing the universal 90% criteria. For Delta4 QA of SBRT VMAT using a 4%/1mm criteria, the TG-218 action limit was 94.47, with 100% passing. For ArcCHECK QA of single target and multiple target SRS VMAT using a 3%/2mm criteria, the TG-218 action limit was 98.06 and 96.59 respectively, with 100% passing. For SRS MapCHECK QA of multiple target SRS VMAT cases using 3%1mm criteria, the TG-218 action limit was 99.24 with 100% passing.

The average increase in V6Gy, V12Gy, V16Gy due to treatment delivery errors as quantified using the trajectory logfile was 0.94 ± 1.43, 0.90 ± 1.38%, and 1.23 ± 1.54% respectively for VMAT, and 0.035 ± 0.14%, 0.14 ± 0.18%, and 0.28 ± 0.24% for CAVMAT. The average change to target maximum, average, and minimum dose due to delivery errors was 0.53 ± 0.46%, 0.52 ± 0.46%, and 0.53 ± 0.56%, for VMAT, and 0.16  0.18%, 0.11  0.08%, and 0.03  0.24% for CAVMAT. There was no significant difference in magnitude of MLC discrepancies during delivery for VMAT and CAVMAT. For Gamma analysis with strict 1% / 1mm criteria, the average passing rate of VMAT gamma analysis is 94.53 ± 4.42%, while that of CAVMAT is 99.28 ± 1.74%.

Conclusion: For most QA devices, spatial tolerance of pre-treatment QA for SRS/SBRT can be tightened to 1mm while still maintaining an in-control QA process. The gamma criteria to 3%/1mm for all SRS cases and SBRT with IMRT and transitioning to a 4%1mm criteria for SBRT with VMAT have a spatial tolerance that is appropriate for the radiotherapy technique while not resulting in an excessive false positive failure rate. The CAVMAT treatment planning technique resulted in superior gamma analysis passing rate for each gamma analysis criteria.

Purpose: Traditional pre-treatment patient specific QA is known for its high workload for physicist, ineffectiveness at identifying clinically relevant dosimetric uncertainties of treatment plans, and incompatibility with on-line adaptive radiotherapy. Our purpose is to develop a trajectory file based PSQA procedure that allows for a virtual pre-treatment QA that can effectively evaluate the performance and robustness of a treatment plan via a DVH based analysis and can be carried out with online adaptive radiotherapy. For this purpose, we have developed a machine learning model that can predict discrepancy in machine parameters between delivery and treatment plan on a Varian TrueBeam linear accelerator.

Methods: Trajectory log files and DICOM-RT plan files of 30 IMRT plans and 75 VMAT plans from four Varian TrueBeam linear accelerators were collected for analysis. The discrepancy in machine parameters is divided into “conversion error” (from converting DICOM-RT to deliverable machine trajectory) and “delivery error” (difference in machine parameters recorded in trajectory files). Correlation matrices were obtained to determine the linear correlation between actual discrepancy and mechanical parameters, such as MLC velocity, MLC acceleration, control point, dose rate, gravity vector, gantry velocity, and gantry acceleration. Multiple regression algorithms were used to develop machine learning models to predict the total discrepancy in machine parameters and its components based on mechanical parameters. The fully trained models were validated with an independent validation dataset and treatment plans constructed with varying degrees of complexity approaching the limitations of the linear accelerator.

Results: For both IMRT and VMAT, the RMS of conversion error (0.1528 mm) was 4 times greater than the RMS of delivery error (0.0367 mm). A high correlation existed between MLC velocity and both components of discrepancies for IMRT (R2 ∈ [0.61, 0.75]) and VMAT [0.75, 0.85]). Final models trained by data from all linear accelerators can predict MLC delivery errors, conversion errors, and combined errors with a high degree of accuracy and correlation between predicted and actual errors for IMRT (R2 = 0.99, 0.86, 0.98) and VMAT (R2 = 0.84, 0.86, 0.87).

Conclusion: We developed an AI model that can predict total MLC discrepancy on Varian TrueBeam linear accelerator with high accuracy using mechanical parameters from trajectory log files and DICOM-RT plans. The software tool from our previous study has been updated to incorporate the discrepancy in planned position into the predictions of total delivery error. We have released the tool for public uses to enable researchers to simulate a treatment delivery without a physical delivery. The tool also has promise in clinical scenarios by allowing for a virtual pre-treatment QA and can be carried out with online adaptive radiotherapy, thereby increasing the effectiveness of pre-treatment patient specific QA.

Purpose: To test the ability of the TG-119 commissioning process and the IROC credentialing to detect errors in the commissioning process for a commercial treatment planning system.

Methods: We introduced commissioning errors into the commissioning process for the Anisotropic Analytical Algorithm (AAA) within the Eclipse Treatment Planning System (TPS). We included errors in MLC Dosimetric Leaf Gap (DLG), electron contamination modeling parameters, incorrect flattening filter material, and beam profile measurement with an inappropriately large farmer chamber (simulated using sliding window smoothing of the input dose profiles). We evaluated the clinical impact of these errors for a variety of clinical intensity modulated radiation therapy (IMRT) plans by looking at PTV D99 and mean and max dose to OARs. The different cases included a head and neck plan, low and intermediate risk prostate plans, a lung plan, and a scalp plan. Finally, for the errors with substantial clinical impact, we determined the sensitivity of the commissioning & credentialing processes with the TG119 C-shape and RPC IMRT head and neck phantoms. This was determined by comparing plans before and after commissioning errors were introduced in the commissioning process using the technique suggested by each respective organization. IROC IMRT credentialing includes film analysis at the midpoint between PTV and OAR using a 4mm distance to agreement metric along with a 7% TLD dose comparison. The TG119 C-shape IMRT phantom looks at 3 separate dose planes and using gamma criteria of 3% 3mm.

Results: The most clinically significant commissioning errors came from large changes in the MLC DLG with a change of 1mm resulting in up to a 5% change in the primary PTV D99. This resulted in a discrepancy in the IROC TLDs in the PTVs and OARs of 7.1% and 13.6% respectively, which would have resulted in detection. While use of incorrect flattening filter caused only subtle errors (<1%) in clicnical plans, the effect was also most pronounced for the IROC TLDs in the OARs (>6%).

Conclusion: The AAA commissioning process within the Eclipse TPS is surprisingly robust to user error. When errors do occur, the IROC and T119 commissioning credentialing criteria are effective at detecting them; however the OAR TLDs are the most sensitive to errors despite the IROC currently excluding them from their analysis.

Purpose: To develop a comprehensive method to measure the radiation uncertainty and coincidence with the kV-CBCT imaging coordinate system using NIPAM kV-CBCT dosimetry.

Methods: An N-isopropylacrylamide (NIPAM) dosimeter is irradiated at eight gantry/couch combinations which enter the dosimeter at unique orientations such that the beams do not overlap except at the isocenter. 1-3 CBCT images are acquired before and immediately after irradiation, radiation profile is detected per beam, and the displacement from the imaging isocenter is quantified. This test has been performed on SRS cone sizes ranging from 4 mm to 15 mm diameter and a 5 mm diameter MLC field, delivering approximately 16 Gy per beam. Matlab code was developed in house to detect each beam’s geometry and to quantify relevant parameters, including radiation isocenter and coincidence with the CBCT origin and the actual gantry and couch angles per beam. The dose profile of each beam was detected in the CBCT using the contrast-to-noise ratio (CNR) of the irradiated high dose regions relative to the surrounding background signal of the dosimeter. Reproducibility was demonstrated by repeating the test on two separate NIPAM dosimeters using the 6 mm cone. To determine the robustness of our test, our results were compared to the results of the traditional Winston-Lutz test, film based “star shots,” and the Varian Machine Performance Check (MPC). The ability of our Matlab code to detect alignment errors was demonstrated by applying a 0.5 mm shift to the MLCs in the direction of leaf travel.

Results: Setup, irradiation, and imaging can be completed in under 40 minutes. The minimum radius to encompass all beams calculated by automated analysis for the MLCs, 4 mm cone, 6 mm cone, 7.5 mm cone, 12.5 mm cone, and 15 mm cone was 0.38 mm, 0.44 mm, 0.53 mm, 0.48 mm, 0.75 mm, 0.5 mm, and 0.57 mm, respectively. When determined manually, these values slightly decreased to 0.28 mm, 0.40 mm, 0.33 mm, 0.41 mm, 0.61 mm, 0.48, and 0.34 mm, respectively. The isocenter verification test was repeated using the 6 mm cone; in both tests, the smallest radius to encompass all beams was found to be 0.53 mm, indicating that the test is reproducible. For comparison, the 3D isocenter radius was 0.24 mm, 0.25 mm, and 0.28 mm for the traditional Winston-Lutz test with MLCs, the Varian MPC, and a “star shot” QA sample. Lastly, when a 0.5 mm shift was applied to the MLCs, the smallest radius to encompass all beams increased from 0.38 mm to 0.90 mm.

Conclusion: The results of this project demonstrate the feasibility of a comprehensive isocenter verification test using NIPAM kV-CBCT dosimetry which incorporates the evaluation of radiation coincidence with the imaging coordinate system, and is capable of producing sub-mm results. This test is applicable to all SRS cone sizes as well as MLCs and can be performed in a typical QA time slot.

Purpose: Linear accelerator-based Stereotactic Radiosurgery (SRS) is often performed using either dynamic conformal arcs or VMAT. For multifocal disease, multifocal conformal arc techniques can struggle to deliver the desired dose with high conformity for all targets simultaneously. While VMAT may improve coverage and conformity, and can offer the planner more flexibility, it can result in highly modulated treatment plans with non-intuitive MLC trajectories. The complex MLC modulation trajectories can often struggle to shield healthy areas between targets, thus leaving open gaps being irradiated between targets. The purpose of this research is to overcome these limitations by developing a technique for SRS of multifocal targets that combines the intuitive MLC trajectories of dynamic conformal arcs with the flexibility of VMAT.

Methods: A Conformal Arc Informed VMAT (CAVMAT) planning technique was developed in which arcs are assigned subgroups of targets, for which the MLCs are able to effectively conform to all targets in the subgroup. Arc weights are optimized to achieve desired dose per target while minimizing the variation in MU per arc. The optimized conformal arc plan then serves as the starting point in a VMAT inverse optimization to fine tune the dose to each target, optimize conformity, and meet any plan specific objectives. To demonstrate feasibility, ten multifocal VMAT cases were re-planned using the CAVMAT technique. The following metrics of plan quality were used to compare VMAT with CAVMAT: volume of healthy brain receiving 6Gy, 12Gy, and 16Gy, conformity index, and total number of monitor units.

Results: The V6Gy of the healthy brain was 10±13% lower in CAVMAT than in VMAT (range 25% lower to 15% higher for CAVMAT plans than VMAT plans). V12Gy of healthy brain tissue showed 5±14% lower in CAVMAT than in VMAT (range 16% lower to 24% higher in CAVMAT plans than VMAT plans). The V16Gy of healthy brain tissue was 3±16% lower in CAVMAT than in VMAT (range 16% to 4% lower and 41% higher in CAVMAT plans than VMAT plans in one case). The MU (Monitor Units) for the CAVMAT plans were 6156.4MU with a standard deviation of 878.41MU compared to 7031.3MU with a deviation of 1788.89MUs for VMAT. The CI (Conformity Index) for CAVMAT are 1.31 with a standard deviation of 0.13, the VMAT plan has a mean conformity of 1.28 with a standard deviation of 0.18. The mean maximum dose of the CAVMAT plan is 2445.37cGy with a standard deviation of 107.22cGy compared to 2309.28cGy with a standard deviation of 114.72cGy for VMAT.

Conclusion: CAVMAT plans succeeded in lessening low dose spill with lower MUs on average compared to VMAT plans. The conformity indexes are comparable to VMAT plans and maximum doses to patients are higher in the CAVMAT plans than in the VMAT plans.

The recently-developed single-isocenter multi-target volumetric modulated arc therapy (VMAT) provides several clinical benefits including significantly decreased treatment time and cost-effectiveness compared to other traditional therapies for patients presenting with brain metastases. Despite its benefits and increasing popularity in clinical use, certain challenges associated with the technique demand investigation. (1) Current spatial accuracy recommendations are not designed explicitly for this technique. Also, many supplemental end-to-end tests specific to this technique have limitations that may be overcome by the use of a novel end-to-end test using 3D-dosimetry. (2) Several prior studies have raised questions regarding the ability of clinical dose calculation algorithms to accurately model the dose to targets with far distances from the isocenter. Discrepancy between planned and measured dose may be due to an inaccurate modeling of the machine’s dosimetric-leaf-gap (DLG), limitations in beam modeling, or other factors. Therefore, a comprehensive dosimetric examination specifically for targets far off-isocenter is warranted. (3) The single-isocenter multi-target VMAT technique has potential applications to preclinical small-animal irradiation. There are treatment planning, treatment geometry, and immobilization considerations that are involved with determining the feasibility of this application. The purpose of this thesis is to examine these aspects in order to enhance and validate the current state of the treatment of intracranial metastases.

In all examinations in this work, the clinical treatment planning system used was Varian External Beam Treatment Planning Software (TPS) version 13.6 (Varian Medical Systems) along with Anisotropic Analytical Algorithm (version 13.6.23) for dose calculation. All treatment plans in this work were modeled on a Varian Truebeam STX linear accelerator with HD-MLCs.

The presented end-to-end test utilized the 3D-dosimetry system using an n-isopropylacrylamide (NIPAM)-based polymer gel that has the ability to be read out using x-ray CT. This dosimeter was also used in the dosimetric evaluation in combination with the PRESAGE®/optical-CT 3D-dosimetry system. Point-dose measurements to supplement the 3D-dosimetry for the dosimetric accuracy evaluation were performed using the commercially-available Stereotactic End-to-End Verification Phantom (STEEV) manufactured by Computerized Imaging Reference Systems, Inc., (Norfolk, VA). All analysis for these examinations was performed using the Eclipse TPS, Matlab, and the 3D Slicer software. Immobilization considerations for the high-throughput small-animal study involved designing a 3D-model using Autodesk Fusion 360 and printing various iterations of designs using an Ultimaker 2 3D-printer.

In this thesis, we present a complete end-to-end test for the single-isocenter multi-target VMAT treatment planning technique using the NIPAM-based dosimeter. We also present this dosimetry system’s capability for remote dosimetry from the developing group, and methods for performing in-house dose calibration. In the dosimetric accuracy evaluation, we report an 8.36% percent difference between the mean dose ratio for the target located at the treatment isocenter compared to the farthest target in our plan (10 cm from isocenter) from 3D-dosimetry. Further, average gamma analysis for the target at isocenter was 87.8% and 97.95% for 1 mm/ 5% and 2 mm/ 3% criteria, respectively. For the most distal target, the average passing rates were 73.93% and 94.2% using the same criteria. While the largest dose discrepancies were noticed for the target farthest from the isocenter, a general trend of decreasing dose coverage for increasing target distance was present in all 3D-dosimetry trials. Our treatment planning examination of the feasibility of using this technique for preclinical small-animal irradiation shows adequate conformity and sparing for half-brain irradiation for up to six mice at a time. In this treatment plan, we report less than 10% variability among the six targets in median dose to the targeted and contralateral hemispheres.

Purpose: Pre-treatment IMRT and VMAT QA techniques are often commissioned without knowledge of their sensitivity to clinically relevant delivery errors. The purpose of this work is to develop a method to quantify the sensitivity and specificity of pre-treatment IMRT and VMAT QA techniques to treatment delivery errors.

Materials and Methods: To evaluate a QA technique, a population of treatment plans and a population of clinically relevant delivery errors are defined. For each delivery error, a threshold magnitude is determined that induces a substantial change in clinically relevant dosimetric indices. Errors at the threshold magnitude are introduced into the plans and QA is performed with and without intentionally introduced errors. The QA technique is treated as a binary classifier to predict error plans using Receiver Operator Characteristic (ROC) analysis. We applied this technique to evaluate portal imager and 2D ion chamber array based QA for VMAT treatment of brain lesions. Delivery errors included discrepancies in MLC positioning (single leaf and leaf bank); lag of MLC trajectory; and discrepancy in dose rate per control point or gantry angle. The threshold magnitude was determined by achieving a 5% change in target conformity index.

Results: The area under the curve (AUC) for the ROC analysis was 0.592 and 0.509 for the ion chamber array and portal imager, respectively, using a gamma index of 3%, 3mm. The AUC increased to 0.632 and 0.777 when 2%, 2mm was used for the ion chamber array and portal imager, respectively. Comparison based on 3% dose agreement resulted in an AUC of 0.557 and 0.693, respectively.

Conclusion: For both portal imager and ion chamber array based QA, stricter tolerance than 3%, 3mm is needed to detect clinically relevant delivery errors. This method can be used to quantitatively compare the sensitivity of various QA techniques to clinically relevant dosimetric errors.

Brain metastases can occur in 20%-40% of cancer patients. Single isocenter multi-target (SIMT) radiosurgery planned with volumetric modulated arc therapy (VMAT) is a method of treating multiple brain metastases simultaneously. These treatments deliver high doses which emphasizes the need for accuracy and avoidance of critical neurological structures to maintaining a patient’s quality of life (QOL). QOL is highly correlated to patient memory, suggesting the hippocampus as a critical structure to avoid in treatment due to its significant role in episodic memory development. Radiosurgery treatments commonly use specialized MLC leaves (HD) that have a narrower width to improve dose conformity around metastases. This work serves to evaluate the feasibility of avoiding the hippocampus in SIMT treatments using VMAT based on MLC leaf width, as well as the effects of hippocampal avoidance on plan quality.40 patients, each previously treated with SIMT planned with VMAT for between 4-10 brain metastases using HD-MLCs, enrolled in an IRB approved protocol. The plans were evaluated for meeting RTOG 0933 recommended dose constraints to the hippocampus. If constraints were not met, then treatments were replanned with optimization objectives added to the hippocampus and/or arc orientation adjustments to meet constraints without compromising target coverage or other organ-at-risk (OAR) dose constraints. Afterwards, the treatments both with and without hippocampal objectives were replanned using standard width MLC leaves (SD-MLCs). All 4 plan types were then evaluated for plan quality using conformity index, V20%[cc], V50%[cc], V75%[cc] and D50%[cGy]. 8 total hippocampi across 7 patient plans exceeded constraints. 4 of these hippocampi could be spared through optimization objective adjustment, and 1 more with an arc orientation adjustment. Biological effective dose (BED) to the 8 hippocampi decreased by 29% ± 23% (p= 0.007) with no significant changes in dose to normal tissue when planned with the addition of hippocampal objectives. SD-MLCs showed similar results, sparing the same 5 of the 8 hippocampi exceeding constraints, but also increased dose to healthy tissue, most substantially in V20%[cc] which increased 59.56% ± 53% (p= 0.015) and 48.22% ± 32.17% (p= 0.0056) in plans with and without hippocampal objectives respectively. Also, in plans both with and without hippocampal optimization objectives, there was no significant change in conformity index when switching MLCs from HD to SD leaves. Meeting recommended hippocampal constraints was possible in 57% of patients that initially exceeded constraints. The ability to spare this structure was independent of MLC width, and correlated to the distance between the hippocampus and the nearest target. From this data set, the smallest distance avoidable was 0.45cm. All un-avoided hippocampi were at least touching a target, if not overlapped. The larger MLC leaves resulted in higher doses to larger volumes of normal tissue, however the planning technique of VMAT was able to meet target coverage without compromising treatment conformity when larger MLC leaves were used.

Introduction: Stereotactic body radiation therapy (SBRT) is a common treatment techniquethat can be used to treat tumors for multiple cancer sites. Density heterogeneity in the target volume and beam path combined with small treatment fields has made dose calculation in lung SBRT difficult. Dose calculation algorithms used historically have difficulty modelling the extreme density heterogeneity present in lung SBRT and have been shown to overestimate the dose delivered to tumors situated in the lung parenchyma. Recently, more advanced algorithms that directly model heterogeneity have been implemented for clinical treatment planning. The limited accuracy of historically utilized dose calculation algorithms has raised questions about their effects on local control due to the possibility of tumor underdosing. The first part of this work establishes a proper dose normalization technique when implementing these advanced algorithms for treatment planning in order to keep consistent radiation beam settings and to quantify the dosimetric effect of various dose normalizations. The second aim is to quantify the effects dosimetric accuracy has on local control in lung SBRT.

Materials/Methods: 87 lung SBRT plans with doses originally calculated with the AnisotropicAnalytical Algorithm (AAA) had their doses recalculated with the new Acuros XB (AXB) algorithm, which is able to directly model the heterogeneity of the lungs and treatment volume. After recalculation, the plan was normalized to the planning target volume (PTV) D95%, internal target volume (ITV) D99%, and to match the original PTV coverage. The percentage change in total monitor units (MU) between the AXB renormalized plans and the original AAA plans were calculated to quantify how the delivered radiation would change when implementing the AXB algorithm for treatment planning. Percentage changes in relevant PTV and ITV dose metrics as well as absolute changes in relevant organ at risk. (OAR) dose metrics were quantified to compare plan dosimetry. OAR doses were also compared to the current institutional planning objectives to investigate the feasibility of meeting the current objectives with the new algorithm. 162 patients previously treated with SBRT were selected from a retrospective protocol comparing the efficacy of SBRT and surgery for treatment of early-stage non-small cell lung cancer. Plans had their doses originally computed with the Pencil Beam Convolution (PBC, n = 8) algorithm or AAA (n = 156). Each plan was recalculated with AXB with identical beam settings. A subset was also recalculated with Monte Carlo to validate the accuracy of the AXB calculations. Percentage changes in relevant PTV and ITV biologically effective doses (BED) were calculated between the original and AXB plans to quantify the magnitude of the dosimetric differences between the old and new algorithm. A multivariable linear regression was performed to investigate which patient and treatment parameters influenced the magnitude of these dosimetric changes. A competing risk analysis was performed to quantify the association between the magnitude of the dosimetric changes and local failure.

Results: Normalizing the AXB plan to the PTV D95% and keeping the original PTVcoverage typically resulted in a total MU increase (average increase of 7.0% and 7.9%, respectively) while normalizing to the ITV D99% resulted in similar total MU (average increase of 0.31%). When normalizing to the PTV D95%, the AXB plans had increased PTV and ITV D1%[Gy] (median increases of 3.4% and 3.2%, respectively) while normalizing to the ITV D99% showed a median 1.9% decrease. Normalizing the AXB plans to the PTV D95% typically resulted in increased OAR dose for all OARs and an inferior ability to meet the OAR planning constraints. Reoptimization of the renormalized plans showed the current OAR objectives to be manageable when using the AXB algorithm. The AXB dose calculations were much more consistent with Monte Carlo than were the original dose calculations. A large range of dosimetric decreases upon recalculation with AXB were observed for both patients who failed locally and those who were controlled. Higher beam energy was found to increase the magnitude of the dosimetric decreases (expected decrease in PTV mean BED of 3.6%, 5.9%, and 9.1% when using 6X, 10X, or 15X, respectively). Total lung volume was also associated with an increased magnitude of dosimetric decrease (expected decerease of 0.8% per 500 cc for the PTV mean BED). The median follow-up time of the cohort was 26 months. 15 patients experienced local failures. Upon univariate analysis, the dosimetric decreases in the PTV and ITV D1% BED were found to be associated with local failure (hazard ratio (HR) of 0.89 (p=0.04) and 0.87 (p=0.02), respectively). Upon multivariate analysis, the dosimetric decrease in the ITV D1% BED remained significant when controlling for PTV volume (HR=0.89 (p=0.04)).

Conclusions: More accurate dose calculation algorithms are beginning to be implementedfor clinical treatment planning. When implementing these new algorithms, issues arise with dose normalization due to the potential for vast differences between the dose distributions calculated with the different algorithms. Normalizing the dose to the PTV D95% in the AXB plan will result in a delivered dose increase relative to a AAA plan while normalizing to the ITV D99% will keep similar delivered doses between the plans. Dose metrics typically increase when normalizing to the PTV D95% (for targets and OARs) while normalizing to the ITV D99% typically decreased the reported dose metrics. The OAR planning objectives are manageable using the AXB algorithm. Many factors are related to the magnitude of the dosimetric decreases observed when recalculating plans with AXB, including but not limited to beam energy and lung volume. Most of the investigated dose metrics were not associated with local failure, but the change in the PTV and ITV D1% BEDs were found to be associated with local failure in the univariate analysis.

Purpose: The aim of this work is to assess the potential of applying NIPAM polymer gel produced at Duke for CT and CBCT on board imaging for radiation therapy. This is after initial progress has been made in establishing the potential of using this gel but produced at a different location to visualize the dose for multitarget radiosurgery using the KV-CBCT system mounted on-board the linear accelerator.

Materials and Methods: A reproducible procedure for manufacturing polymer gel developed by previous researchers in this field was implemented at the 3D Dosimetry and Bio – imaging lab at Duke University. The use of vacuum seal and oil was introduced as an additional means of minimizing oxygen contamination immediately after production. The technique is to apply the gel for CBCT and sheet Dosimetry

CBCT-Dosimetry: The CBCT acquisition and reconstruction parameters were optimized to maximize low contrast resolution. Varian 600C/D linear accelerator whose QA test has been successful and already in clinical use for SRS was used for spot check. Six different angles were made for each of the gantry, collimator and couch rotations. The radiation isocenter was quantified by determining the center of the smallest intersecting circle; the 3D vector coincidence with the imaging coordinate system was also quantified.

Sheet Dosimetry: A Flat shaped gel was prepared in a vacuum sealed bag. Ten different MU values were delivered at different spots of the gel using the Varian 600C/D linear accelerator which was later scanned using the EPSON flatbed scanner and Diagnostic CT. The optical density values were determined in the three color bands; likewise variation of the mean CT number with dose.

Results: CBCT-D: The optimal values for our CBCT setting were full trajectory, smooth reconstruction filter, strong ring suppression and 80KV. For the star shots, the radius of the smallest circle was 0.5mm, 0.3mm, and 0.6mm for the gantry, couch, and collimator, which were within the recommended tolerance limits specified by TG142 and were within 0.2mm of the value obtained from a traditional film star shot. The 2D vector discrepancy from the imaging coordinate system was 0.1mm, 1.3mm and 1.5mm for the gantry, couch, and collimator, respectively.

Sheet: The optical density values increased with increasing MU in the three color channels. The blue channel showed the largest change in optical density (24% difference); while the red color showed the least. The diagnostic scan showed a linear increase of CT number with MU. At lower MU (0 – 200), the increase fluctuated due to background noise and dose scatter. Above 200MU showed a linear increase in polymerization.

Conclusion: All the irradiated spots and spoke polymerized to the degree of radiation dose quantity and easily visible; which supports the feasibility of the NIPAM polymer gel produced at Duke for dose distribution and QA test. Specifically, the calibration potential of the polymer gel when used as sheet for both the optical and diagnostic CT scan has now been established.

Purpose: Cherenkov light is a natural byproduct of MV radiotherapy; recent results demonstrate that it can activate the drug psoralen sufficiently to induce cytotoxicity and increase MHC1 signal in vitro. Here, we investigate Cherenkov radiation from common radiotherapy beams using Monte Carlo, as well as methods to maximize Cherenkov production per unit dose, using filters placed in the beam path.

Methods: GAMOS, a GEANT4-based framework for Monte Carlo simulations, was used to model primary photon beams using spectra from a Varian linear accelerator and mono-energetic electron beams. Cherenkov photon spectra and track length along with dose were scored when irradiating a sphere of water with radius 50cm and SSD=50cm. Further measurements were taken with photon beams irradiating a 17.8cm3 cubic water phantom at 1mm3 detectors with depths of 8 to 9cm; SSD was set to 94cm. Finally, measurements were taken with filters of varying material and thickness placed 15cm below a 10MV FFF beam source.

Results: Simulated Cherenkov spectra were found to have strong overlap with the psoralen absorbance spectrum; dose and Cherenkov photon track length measurements established that higher beam energies had greater Cherenkov production per unit dose, with 18MV providing greater Cherenkov/dose than 6MV by a factor of 4. Simulations with filters suggest that copper and iron filters increase Cherenkov per dose more than aluminum for a given filter thickness, but that aluminum yields a greater boost for a given dose rate.

Conclusion: Initial work has been completed to show that the Cherenkov spectrum produced by radiotherapy beams is well suited for activation of psoralen, and that higher energy photon beams will result in more psoralen activation due to greater Cherenkov radiation per unit dose. We have also demonstrated that significant boosts in Cherenkov/dose can be achieved with the use of filters without overly compromising dose rate. Future work should expand analysis to include optical properties of tissues as well as additional filter materials.

Purpose:

A new development in linac-based intracranial stereotactic radiosurgery (SRS) and extracranial stereotactic body radiation therapy (SBRT) is treatment of multiple targets using single isocenter volumetric modulated arc therapy (VMAT) technique, dramatically reducing treatment time while maintaining high target conformality and steep dose gradients between targets and surrounding organs at risk (OAR). In VMAT, the gantry rotates around the patient while continuously delivering radiation. Throughout the VMAT arc, the beam is modulated based on an inverse optimization algorithm in order to spare organs at risk. Single isocenter multi-target VMAT has already been implemented for intracranial SRS and is increasingly used for extracranial SBRT treatments. Despite the increasing popularity of this technique, certain inherent clinically meaningful challenges warrant further investigation. Specifically, single isocenter, multifocal SRS and SBRT can result in small volumes targets with a large off-axis distance from the treatment isocenter. Consequently, angular errors in the collimator, patient support assembly (PSA), or gantry could have an increased impact on target coverage, warranting a re-evaluation of routine linear accelerator QA tolerance recommendations in TG-142. Also, questions have arisen regarding the ability of clinical dose calculation algorithms to calculate dose accurately for these cases at large off-axis distances. Specifically, it is questionable whether or not the MLC model used is sophisticated enough to accurately model the dose off axis. This is of concern because the MLC leaves have different dimensions outside of the HD region, but the dosimetric-leaf-gap model used is the same for both regions. Applying the single isocenter technique to extracranial SBRT of oligometastases introduces additional unique challenges that must be addressed. These include greater intra and inter-fractional setup uncertainties, and dosimetric interplay since immobilization is more difficult and internal motion is non-negligible. The purpose of this thesis is to explore these specific physics and treatment planning considerations for single isocenter multi-target intracranial radiosurgery and extracranial SBRT.

Materials and Methods:

Intracranial SRS

For single isocenter multifocal stereotactic radiosurgery, we evaluated potential dose deviations from mechanical errors in PSA, collimator, and gantry angle within the tolerance recommended by TG-142 for radiosurgery machines. Systematic errors in PSA, collimator, and gantry angle were introduced at the recommended tolerance levels into both multifocal SRS plans and traditional single target SRS using dynamic conformal arcs, and the resulting dosimetric effect were quantified within the treatment planning system. In addition, we quantified the accuracy of the treatment planning system dose calculation algorithm for targets located at large off-axis distances with 3D Slicer analysis software. The dose distribution from the treatment planning system was compared to the distribution measured using a high-resolution 3D dosimetry system (PRESAGE®/Optical-CT). Comparisons were made using DVH and gamma analysis (criteria: 2mm/3%, 3mm/3%, and 3mm/5%). DVH comparisons included a shell analysis in which we compared the dose from 2mm within the target to the target’s surface, the surface to 2mm outside the target, and 2mm to 4mm outside the target for both the plan and the dosimeter.

Extracranial SBRT

For applying the single isocenter multifocal technique to extracranial oligometastases, we propose a treatment method that addresses intra and inter-fractional motion as well as dosimetric interplay. The developed technique uses a Single Isocenter with Distinct Optimizations (SIDO) in which all Volumetric Modulated Arc Therapy (VMAT) fields share an isocenter but each field treats only one target. When necessary, setup uncertainties from rotations and deformations are mitigated by applying a couch translation between VMAT arcs, and interplay is minimized using dynamic conformal arcs (DCAs) as the starting point for inverse optimization. We evaluated this planning technique using relevant dose indices including conformity index, gradient index and modulation factor.

Results:

Intracranial SRS

Induced errors at TG-142 tolerance levels showed the greatest change in multifocal SRS target coverage for collimator and gantry rotations, while minimal change in coverage was noted for errors in PSA rotation. For single isocenter cases, the largest dose discrepancies were a result of 1° errors in the collimator and gantry angles, specifically with respect to the volume of the PTV receiving the prescription dose. These errors caused up to 33% and 18% deviations, respectively, to the volume of the PTV receiving the prescription dose with mean deviations of 5% and 2%, respectively. When the collimator and gantry errors in single isocenter plans were reduced to 0.5°, the discrepancies in the volume of the PTV receiving the prescription dose were reduced to a max value of <5% for the gantry and collimator with mean values of approximately 1%. For 1° errors in DCA plans, however, deviations to the volume of the PTV receiving the prescription dose did not exceed 5% for the collimator, couch, or gantry and similar results were seen in all other dosimetric indices investigated.

A preliminary analysis of the Eclipse dose calculation algorithm in comparison to actual dose delivered to targets shows agreement with 89.46%, 94.87%, and 96.39% of voxels having a passing gamma index with criteria of 2mm/3%, 3mm/3%, and 3mm/5% respectively (distance from isocenter ranged from 0-10cm). Targets within 8cm of the isocenter showed less than 2% discrepancy between the plan and measurement with respect to the percent of the target receiving the prescription dose. The target at 10cm from the isocenter, however, had a 15% discrepancy between the plan and measurement with respect to the percent of the target receiving the prescription dose and therefore warrants further investigation.

Extracranial SBRT

As the distance between targets increases, the probability for requiring a second translation between treatment arcs also increases. Assuming a margin of 5mm and considering six lung and five liver patient cases, a second translational shift would be required 0%, ~10%, and ~25% of the time for target separations of 5cm, 10cm, and 15cm respectively.

For greater than 3cm separation between targets in extracranial SBRT, SIDO and SIDO with DCA have an average conformity index of 0.862, and 0.864 respectively, which is comparable to the average conformity of traditional multifocal treatment techniques at these target separations of 0.901. When separation between PTVs is less than 3cm, however, traditional single isocenter VMAT has superior conformity with a mean value of 0.875, as opposed to 0.772 and 0.782 for SIDO and SIDO with DCA respectively; and decreasing conformity with decreasing target separation. SIDO with DCA had superior GI over all other planning techniques for almost all cases with a mean value of 7.31 across all target separations. SIDO with DCA even performed better than the DCA technique with a mean GI of 7.43 across all target separations, which was assumed to be the best method for obtaining a desirable GI. SIDO with DCA had a comparable MF to the DCA plans and was closer to 1 than all other planning techniques. The mean MF values across all target separations for SIDO with DCA and DCA were 1.17 and 0.83 respectively.

Conclusions:

Intracranial SRS

Institutions utilizing a single isocenter VMAT technique for multifocal disease should pay careful attention to the angular mechanical tolerances in designing a robust and complete QA program, especially with respect to the collimator and gantry recommended tolerances. We recommend reducing collimator and gantry tolerances from 1.0° to 0.5° to decrease the potential magnitude of deviations between the planning and delivered dose distributions. The PRESAGE®/Optical-CT 3D dosimetry system verified the accuracy of the Eclipse dose calculation algorithm to within 2% for targets located up to 8cm from the isocenter. Further investigation is required for more distal targets, as they did not have sufficient agreement.

Extracranial SBRT

A single isocenter approach for SBRT treatment of extracranial oligometastases may be feasible using the proposed SIDO and SIDO with DCA treatment planning techniques. SIDO for extracranial oligometastases allows flexibility to mitigate spatial uncertainties from rotation and deformation, and has comparable dosimetry to traditional VMAT with low modulation when inverse optimization begins with DCAs. These advantages make SIDO beneficial for target separations of greater than 3cm, however, for target separations less than 3cm a traditional single isocenter technique is more appropriate.

Purpose: Linac based radiosurgery to multiple metastases is commonly planned with VMAT as it effectively achieves high conformality to complex target arrangements. However, as the number of targets increases, VMAT may struggle to block between targets, which may lead to highly modulated and/or nonconformal MLC trajectories. This phenomenon is particularly apparent in multi-target geometries as targets often share the same MLC leaf pair, creating an MLC opening and yielding insufficient inter-target collimation. Given the complex geometries, multi-target SRS necessitates high degrees of dosimetric accuracy. Dosimetric accuracy may be impacted by beam commissioning as a single dosimetric leaf gap (DLG) must be selected and used for all targets, geometries, and MLCs. Conformal Arc Informed VMAT (CAVMAT) aims to reduce healthy tissue dose by utilizing simplified MLC trajectories and by producing more conformal dose distributions. It is hypothesized that the simplified leaf motion and reduced complexity of CAVMAT may reduce sensitivity to commissioning and treatment delivery uncertainty.

Materials & Methods: CAVMAT is a hybrid treatment planning technique which combines the conformal MLC trajectories of dynamic conformal arcs with the MLC modulation and versatility of inverse optimization. CAVMAT has three main steps. First, targets are assigned to subgroups to maximize MLC blocking between targets. Second, arc weights are optimized to achieve the desired target dose, while minimizing MU variation between arcs. Third, the optimized conformal arc plan serves as the starting point for limited inverse optimization to improve dose conformity to each target. Twenty multifocal VMAT cases were re-planned with CAVMAT with 20Gy applied to each target. The total volume receiving 2.5 Gy, 6 Gy, 12 Gy, and 16 Gy, conformity index, treatment delivery time, and the total MU were used to compare the VMAT and CAVMAT plans. Of the 20 VMAT plans, 10 were selected and replanned with CAVMAT, at DLG values of 0.4 mm, 0.8 mm, and 1.2 mm and the change in V6Gy [cc], V12Gy [cc], V16Gy [cc], and target dose was quantified. The 10 VMAT and CAVMAT plans were delivered to a Delta4 QA phantom and dose agreement was quantified using gamma index with 3%/1mm, 2%/1mm, and 1%/1mm criteria. Trajectory log files were collected and analyzed to quantify MLC positioning errors during delivery. 16 targets were selected, with at least one target from each plan, and were delivered to an SRS Mapcheck QA phantom to evaluate dose difference per DLG.

Results: After replanning the 20 VMAT cases, CAVMAT reduced the average V2.5Gy[cc] by 25.25±19.23%, V6Gy[cc] by 13.68±18.97%, V12Gy[cc] by 11.40±19.44%, and V16Gy[cc] by 6.38±19.11%. CAVMAT improved conformity by 3.81±7.57%, while maintaining comparable target dose. MU for the CAVMAT plans increased by 24.35±24.66%, leading

to an increased treatment time of ~2 minutes. For the DLG analysis, the 10 VMAT plans

demonstrated an average sensitivity to variation of V6Gy [cc], V12Gy [cc], V16Gy [cc] of 35.83 ± 9.48%/mm,

34.12 ± 6.61%/mm, and 39.22 ± 8.41%/mm, respectively, compared to 23.18 ± 4.53 %/mm, 22.45± 4.28 %/mm, and 24.88 ± 4.91 %/mm for CAVMAT. VMAT was found to be roughly twice as sensitive as CAVMAT to changes in target doses for a varying DLG. For the plans delivered to the Delta4 CAVMAT demonstrated an improved dose agreement, with the strictest criteria of 1%/1mm resulting in a passing rate of 94.53 ± 4.42% for VMAT compared to 99.28 ± 1.74% for CAVMAT. Log file analysis demonstrated CAVMAT’s improved resistance to treatment delivery uncertainties, though the difference compared to VMAT is not expected to be substantial.

Conclusions: The CAVMAT technique successfully eliminated insufficient MLC blocking between targets prior to the inverse optimization, leading to less complex treatment plans and improved tissue sparing. CAVMAT is more robust to dosimetric and treatment delivery uncertainties and better maintains target dose and healthy tissue sparing. The reduced complexity, inherent tissue sparing, improved conformity, and reduced sensitivity to uncertainties indicates CAVMAT to be a promising method to treat brain metastases.

Purpose: New developments in radiation therapy such as AI based contouring and online adaptive radiotherapy have led to an increase in the number of structures and normal tissues being delineated with less human oversight, requiring review in a shorter timeframe. Addressing this problem, we aim to develop and validate a novel method for QA by streamlining the verification of radiotherapy contours. This method identifies subsections of organs at risk that could result in clinically relevant dose constraints being violated, should the contour be inaccurate. Methods: Structures with planning constraints are evaluated by adding increasing margins and checking against dose constraints. Structures that violate constraints when expanded with margins are flagged for manual review; The smallest margin that violates constraints is prioritized, with the location necessitating manual review presented in a suitable manner, such as the maximum dose point for the contours specified in this study. We applied this method in a retrospective analysis of 92 stereotactic radiosurgery plans, evaluating brainstem, optic nerves, and optic chiasm. Margins of 0, 1, 3, and 5mm were applied to define risk levels (very high, high, medium, low) for manual review. Contours and associated MR images were independently reviewed by 2 physicists with contouring experience using the locations flagged for review by our method to determine whether clinically relevant contouring errors existed. Results: 12 contours (brainstem n=10, optic nerve n=1, chiasm n=1) from 11 plans were flagged for manual review at risk levels of very high (n=1), high (n=4), medium (n=2), and low (n=5). Review by the 2 physicists indicated a mean offset value of 0.45mm, with a mean difference of 0.19mm (SD = 0.57mm) and a correlation value of 0.7 between the two sets of observations. Notably, one case exhibited a mean contouring error of 1.75mm, significantly beyond the standard tolerance for SRS of 1mm, suggesting a critical area of concern. Conclusion: Our results indicate that the method described here has potential to improve both the efficacy and efficiency of the plan review process. When applied to radiosurgery, efficacy improved as a number of previously unidentified contouring errors were identified in critical locations among the 12% of cases flagged for manual review, suggesting potential to reduce medical errors. Potential improvements in efficiency are highlighted by the 88% of cases for which the tool indicated that contouring errors would not have a clinically relevant dosimetric effect, indicating review is not necessary. Further investigation is warranted to explore the application of this method to other treatment sites.

Surface guided radiation therapy (SGRT) is an emerging technology that uses non-ionizing methods for patient positioning and motion tracking during radiotherapy delivery. However, the use of radiotherapy boluses, which are tissue-equivalent materials placed on the skin to increase surface dose, has been shown to interfere with SGRT systems due to reflections from the bolus surface. This thesis presents the development and validation of an opaque and non-glossy radiotherapy bolus called the "Surface Guidance Optimized" (SGO), which is a variation of the previously developed transparent Clearsight bolus.The Surface Guidance Optimized bolus was rendered opaque by adding 0.6% titanium dioxide and given a matte finish using matte release paper. Spectroscopy measurements confirmed optimal opaqueness, while gloss meter readings verified a non-glossy surface. The bolus density was quantified to be 0.853 g/cm3 using water displacement and CT methods. Dosimetric characterization through direct surface dose measurements and Monte Carlo simulations demonstrated the SGO bolus mimics the dose deposition of water-equivalent materials when accounting for density differences. Compatibility testing with the AlignRT SGRT system showed the bolus allowed accurate surface reconstruction and submillimeter tracking (within 0.4 mm) under different lighting conditions. Overall, the SGO bolus mitigates issues of transparency and glossiness that interfered with SGRT systems, while maintaining desirable dosimetric properties for clinical use as a radiotherapy bolus compatible with modern surface guided techniques.

Abstract

Purpose

Single-isocenter multi-target (SIMT) Volumetric Modulated Arc Therapy (VMAT) technique can produce highly conformal dose distributions and short treatment delivery times for the treatment of multiple brain metastases. SIMT radiosurgery using VMAT is primarily limited to linear accelerators utilizing 2.5mm leaf width MLCs. We explore feasibility of applying this technique to linear accelerators utilizing MLCs with leaf width of 5mm to broaden the applicability of SIMT radiosurgery using VMAT to include the greater number of linear accelerators with standard 5mm MLCs.

Methods

Twenty patients with 3-10 intracranial brain metastases originally treated with 2.5 mm leaf width MLCs were re-planned using standard 5mm leaf width MLCs and the same treatment geometry (3-5 VMAT arcs). Conformity index, low (V30%), and moderate (V50%) isodose spill were selected for analysis. V12Gy was also analyzed for single fraction cases. We tested the effects of several strategies to mitigate degradations of dose quality values when 5 mm leaf width MLCs were used; these included duplicating each VMAT arc with altered collimator angles by 10°, 15°, and 90°, and adding 1-2 VMAT arcs, with all arcs equally spaced.

Results

Wider MLCs caused small changes in total MUs (5827±2334 vs 5572±2220, p=.006), and Conformity Index (CI) (2.22%±0.05%, p=.045), but produced more substantial increases in brain V30%[%] and V50%[%] (27.75%±0.16% and 20.04%±0.13% respectively, p < .001 for both), and V12Gy[cc] (16.91%±0.12%, p < .001).

Conclusion

SIMT radiosurgery delivered via VMAT using 5mm leaf width MLCs can achieve similar CI compared to that using 2.5mm leaf width MLCs but with moderately increased isodose spill, which can be only partially mitigated by increasing the number of VMAT arcs.