Modelling small block aperture in an in-house developed GPU-accelerated Monte Carlo-based dose engine for pencil beam scanning proton therapy

Purpose: To enhance an in-house graphic-processing-unit (GPU) accelerated virtual particle (VP)-based Monte Carlo (MC) proton dose engine (VPMC) to model aperture blocks in both dose calculation and optimization for pencil beam scanning proton therapy (PBSPT)-based stereotactic radiosurgery (SRS). Methods and Materials: A block aperture module was integrated into VPMC. VPMC was validated by an opensource code, MCsquare, in eight water phantom simulations with 3cm thick brass apertures: four were with aperture openings of 1, 2, 3, and 4cm without a range shifter, while the other four were with same aperture opening configurations with a range shifter of 45mm water equivalent thickness. VPMC was benchmarked with MCsquare and RayStation MC for 10 patients with small targets (average volume 8.4 cc). Finally, 3 patients were selected for robust optimization with aperture blocks using VPMC. Results: In the water phantoms, 3D gamma passing rate (2%/2mm/10%) between VPMC and MCsquare were 99.71$\pm$0.23%. In the patient geometries, 3D gamma passing rates (3%/2mm/10%) between VPMC/MCsquare and RayStation MC were 97.79$\pm$2.21%/97.78$\pm$1.97%, respectively. The calculation time was greatly decreased from 112.45$\pm$114.08 seconds (MCsquare) to 8.20$\pm$6.42 seconds (VPMC), both having statistical uncertainties of about 0.5%. The robustly optimized plans met all the dose-volume-constraints (DVCs) for the targets and OARs per our institutional protocols. The mean calculation time for 13 influence matrices in robust optimization by VPMC was 41.6 seconds. Conclusion: VPMC has been successfully enhanced to model aperture blocks in dose calculation and optimization for the PBSPT-based SRS.Comment: 3 tables, 3 figure

Report on use of a methodology for commissioning and quality assurance of a VMAT system.

INTRODUCTION: Results of use of methodology for VMAT commissioning and quality assurance, utilizing both control point tests and dosimetric measurements are presented. METHODS AND MATERIALS: A generalizable, phantom measurement approach is used to characterize the accuracy of the measurement system. Correction for angular response of the measurement system and inclusion of couch structures are used to characterize the full range gantry angles desirable for clinical plans. A dose based daily QA measurement approach is defined. RESULTS: Agreement in the static vs. VMAT picket fence control point test was better than 0.5 mm. Control point tests varying gantry rotation speed, leaf speed and dose rate, demonstrated agreement with predicted values better than 1%. Angular dependence of the MatriXX array, varied over a range of 0.94-1.06, with respect to the calibration condition. Phantom measurements demonstrated central axis dose accuracy for un-modulated four field box plans was ≥2.5% vs. 1% with and without angular correction respectively with better results for VMAT (0.4%) vs. IMRT (1.6%) plans. Daily QA results demonstrated average agreement all three chambers within 0.4% over 9 month period with no false positives at a 3% threshold. DISCUSSION: The methodology described is simple in design and characterizes both the inherit limitations of the measurement system as well at the dose based measurements that may be directly related to patient plan QA

Mean angular correction value for all chambers in the detector array vs. gantry angle.

<p>Mean angular correction value for all chambers in the detector array vs. gantry angle.</p

Central Axis dose agreement between measurement and calculation.

<p>Central axis dose agreement using the cubic target. The impact of angular correction is demonstrated.</p

Gamma analysis of phantom measurements.

<p>Gamma analysis for the cubic target plans with focus on the high dose (80% threshold) region. The impact of angular correction is demonstrated.</p

Gamma analysis on patient plans of simple targets for full and partial (*) arc plans.

<p>Gamma analysis on patient plans of simple targets for full and partial (*) arc plans.</p

Summary of control point tests on a) dose rate-gantry speed and b) leaf speed-dose rate plotting VMAT (solid) versus open field (dashed).

<p>Fluence agreement was within 0.87% and 0.56% for dose rate – gantry and leaf speed-dose rate, respectively.</p

Phantom measurement geometry utilized a cubic target (10 or 20 cm on a side) treated with a) un-modulated four field box, b) five field IMRT and c) two arc VMAT plans, including the couch as a structure.

<p>Transverse and coronal views of the dose given to the 20 cm target, as calculated by the treatment planning system, are illustrated in d.</p

Patient plans are routinely evaluated in the a) coronal and b) sagittal planes, examining dose profiles through areas corresponding to target and critical normal structures.

<p>Evaluation of γ(4 mm/4%,10%) >1 is used to highlight pixels requiring further attention. A failing pixel due to an out of plane dose gradient that is large compared to the chamber dimension is illustrated in c.</p