RaySearch Americas Inc. (NY) has introduced a commercial Monte Carlo dose algorithm (RS-MC) for routine clinical use in proton spot scanning. In this report, we provide a validation of this algorithm against phantom measurements and simulations in the GATE software package. We also compared the performance of the RayStation analytical algorithm (RS-PBA) against the RS-MC algorithm. A beam model (G-MC) for a spot scanning gantry at our proton center was implemented in the GATE software package. The model was validated against measurements in a water phantom and was used for benchmarking the RS-MC. Validation of the RS-MC was performed in a water phantom by measuring depth doses and profiles for three spread-out Bragg peak (SOBP) beams with normal incidence, an SOBP with oblique incidence, and an SOBP with a range shifter and large air gap. The RS-MC was also validated against measurements and simulations in heterogeneous phantoms created by placing lung or bone slabs in a water phantom. Lateral dose profiles near the distal end of the beam were measured with a microDiamond detector and compared to the G-MC simulations, RS-MC and RS-PBA. Finally, the RS-MC and RS-PBA were validated against measured dose distributions in an Alderson-Rando (AR) phantom. Measurements were made using Gafchromic film in the AR phantom and compared to doses using the RS-PBA and RS-MC algorithms. For SOBP depth doses in a water phantom, all three algorithms matched the measurements to within  ±3% at all points and a range within 1 mm. The RS-PBA algorithm showed up to a 10% difference in dose at the entrance for the beam with a range shifter and >30 cm air gap, while the RS-MC and G-MC were always within 3% of the measurement. For an oblique beam incident at 45°, the RS-PBA algorithm showed up to 6% local dose differences and broadening of distal fall-off by 5 mm. Both the RS-MC and G-MC accurately predicted the depth dose to within  ±3% and distal fall-off to within 2 mm. In an anthropomorphic phantom, the gamma index (dose tolerance  =  3%, distance-to-agreement  =  3 mm) was greater than 90% for six out of seven planes using the RS-MC, and three out seven for the RS-PBA. The RS-MC algorithm demonstrated improved dosimetric accuracy over the RS-PBA in the presence of homogenous, heterogeneous and anthropomorphic phantoms. The computation performance of the RS-MC was similar to the RS-PBA algorithm. For complex disease sites like breast, head and neck, and lung cancer, the RS-MC algorithm will provide significantly more accurate treatment planning.

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We performed 858 two-dimensional (2D) patient-specific intensity modulated radiotherapy verifications over a period of 18 months. Multifield, composite treatment plans were measured in phantom using calibrated Kodak EDR2 film and compared with the calculated dose extracted from two treatment planning systems. This research summarizes our findings using the normalized agreement test (NAT) index and the percent of pixels failing the gamma index as metrics to represent the agreement between measured and computed dose distributions. An in-house dose comparison software package was used to register and compare all verifications. We found it was important to use an automatic positioning algorithm to achieve maximum registration accuracy, and that our automatic algorithm agreed well with anticipated results from known phantom geometries. We also measured absolute dose for each case using an ion chamber. Because the computed distributions agreed with ion chamber measurements better than the EDR2 film doses, we normalized EDR2 data to the computed distributions. The distributions of both the NAT indices and the percentage of pixels failing the gamma index were found to be exponential distributions. We continue to use both the NAT index and percent of pixels failing gamma with 5%/3 mm criteria to evaluate future verifications, as these two metrics were found to be complementary. Our data showed that using 2%/2 mm or 3%/3 mm criteria produces results similar to those using 5%/3 mm criteria. Normalized comparisons that have a NAT index greater than 45 and/or more than 20% of the pixels failing gamma for 5%/3 mm criteria represent outliers from our clinical data set and require further analysis. Because our QA verification results were exponentially distributed, rather than a tight grouping of similar results, we continue to perform patient-specific QA in order to identify and correct outliers in our verifications. The data from this work could be useful as a reference for other clinics to indicate anticipated trends in 2D verifications under various conditions.

Recently, a commercial treatment planning system (TPS) has implemented aperture collimators for PBS dose calculations which can serve to reduce lateral penumbra. This study characterized the variation in magnitude of lateral penumbra for collimated and un-collimated PBS fields versus the parameters of air gap, depth, and range shifter thickness. Comparisons were performed in a homogenous geometry between measured data and calculations made by a commercial TPS. Beam-specific target volumes were generated for collimated and un-collimated PBS fields and optimized for various range shifter thicknesses and air gaps. Lateral penumbra (80%-20% distance) was measured across each target volume to characterize penumbra variation with depth and air gap. An analytic equation was introduced to predict the reduction in lateral penumbra between un-collimated and collimated PBS treatments. Calculated penumbra values increased with depth across all combinations of range shifters for a constant air gap. At 2 cm depth, the reductions in penumbra due to the aperture were 2.7 mm, 3.7 mm and 4.2 mm when using range shifter thicknesses of 0 cm, 4.0 cm and 7.5 cm, respectively. At a depth of approximately 20 cm and air gap of 5 cm, differences between penumbras of collimated and un-collimated beams were less than 1 mm. Penumbra reductions for the collimated beams were largest at small air gaps. All TPScalculated penumbra values derived in this study were within 1 mm of film measurement values. Finally, the analytic equation was tested using a clinical CT scan, and we found good dosimetric agreement between the model predictions and the result calculated by the TPS. In conclusion, application of collimators to PBS fields can sharpen penumbra by several mm and are most beneficial for shallow targets. Furthermore, measurements indicate that the dose calculation accuracy in the penumbra region of PBS-collimated fields is adequate for clinical use. AbstractRecently, a commercial treatment planning system (TPS) has implemented aperture collimators for PBS dose calculations which can serve to reduce lateral penumbra. This study characterized the variation in magnitude of lateral penumbra for collimated and uncollimated PBS fields versus the parameters of air gap, depth, and range shifter thickness. Comparisons were performed in a homogenous geometry between measured data and calculations made by a commercial TPS. Beam-specific target volumes were generated for collimated and un-collimated PBS fields and optimized for various range shifter thicknesses and air gaps. Lateral penumbra (80%-20% distance) was measured across each target volume to characterize penumbra variation with depth and air gap. An analytic equation was introduced to predict the reduction in lateral penumbra between uncollimated and collimated PBS treatments. Calculated penumbra values increased with depth across all combinations of range shifters for a constant air gap. At 2 cm depth, the reductions in penumbra due to the aperture were 2.7 mm, 3.7 mm and 4.2 mm when using ran...