Technical note: Evaluation and second check of a commercial Monte Carlo dose engine for small‐field apertures in pencil beam scanning proton therapy

Abstract

Purpose To evaluate the accuracy of the RayStation Monte Carlo dose engine (RayStation MC) in modeling small‐field block apertures in proton pencil beam scanning. Furthermore, we evaluate the suitability of MCsquare as a second check for RayStation MC. Methods We have enhanced MCsquare to model block apertures. To test the accuracy of both RayStation MC and the newly enhanced MCsquare, we compare the dose predictions of each to in‐water dose measurements obtained using diode detectors and radiochromic film. Nine brass apertures with openings of 1, 2, 3, 4, and 5 cm and either 2 cm or 4 cm thickness were used in the irradiation of a water phantom. Two measurement setups were used, one with a range shifter and 119.7 MeV proton beam energy and the other with no range shifter and 147 MeV proton beam energy. To further test the validity of RayStation MC and MCsquare in modeling block apertures and to evaluate MCsquare as a second check tool, 10 small‐field (average target volume 8.3 cm3) patient treatment plans were calculated by each dose engine followed by a statistical comparison. Results Comparing to the absolute dose measurements in water, RayStation MC differed by 1.2% ± 1.0% while MCsquare differed by −1.8% ± 3.7% in the plateau region of a pristine Bragg peak. Compared to the in‐water film measurements, RayStation MC and MCsquare both performed well with an average 2D‐3D gamma passing rate of 99.4% and 99.7% (3%/3 mm), respectively. A t‐test comparing the agreement with the film measurements between RayStation MC and MCsquare suggested that the relative spatial dose distributions calculated by MCsquare and RayStation MC were statistically indistinguishable. Directly comparing the dose calculations between MCsquare and RayStation MC over 10 patients resulted in an average 3D‐3D gamma passing rates of 98.5% (3%/3 mm) and 94.1% (2%/2 mm), respectively. Conclusion The validity of RayStation MC algorithm for use with patient‐specific apertures has been expanded to include small apertures. MCsquare has been enhanced to model apertures and was found to be an adequate second check of RayStation MC in this scenario.

Exploratory study of seed spots analysis to characterize dose and linear‐energy‐transfer effect in adverse event initialization of pencil‐beam‐scanning proton therapy

“…The dose and LET of all PBS plans were calculated by an open‐source fast Monte Carlo (MC) code, MCsquare 67–70 . This dose engine has been fully commissioned and benchmarked with other MC codes 68,71–75 . It has been incorporated in our in‐house treatment planning system, Shiva, 39,55,76–79 and clinically used as our second monitor unit check system 68,71 …”

“…68,[71][72][73][74][75] It has been incorporated in our in-house treatment planning system, Shiva, 39,55,[76][77][78][79] and clinically used as our second monitor unit check system. 68,71 The dose-averaged LET 45,69 was used to calculate the LET at each voxel from multiple fields. The LET at each voxel was calculated using the following equation:…”

“…[67][68][69][70] This dose engine has been fully commissioned and benchmarked with other MC codes. 68,[71][72][73][74][75] It has been incorporated in our in-house treatment planning system, Shiva, 39,55,[76][77][78][79] and clinically used as our second monitor unit check system. 68,71 The dose-averaged LET 45,69 was used to calculate the LET at each voxel from multiple fields.…”

Collimating individual beamlets in pencil beam scanning proton therapy, a dosimetric investigation

“…A key technical milestone of proton therapy has been the advent and proliferation of pencil beam scanning (PBS) (1). PBS has made patient-specific apertures obsolete in many, but not all, scenarios, particularly for cases involving shallow and complex tumors (2,3). The latest generation of proton machines typically have small in-air spot sizes, approximately 6 mm (s at isocenter) at the lowest energies and 2 mm at the highest energies (4-14), yet because clinical accelerators cannot produce protons with energy lower than about 70 MeV, range shifters (2,(15)(16)(17)(18)(19)(20) are required to treat shallow tumors.…”

“…PBS has made patient-specific apertures obsolete in many, but not all, scenarios, particularly for cases involving shallow and complex tumors (2,3). The latest generation of proton machines typically have small in-air spot sizes, approximately 6 mm (s at isocenter) at the lowest energies and 2 mm at the highest energies (4-14), yet because clinical accelerators cannot produce protons with energy lower than about 70 MeV, range shifters (2,(15)(16)(17)(18)(19)(20) are required to treat shallow tumors. Unfortunately, range shifters, which may be 25 cm or more upstream of the patient, significantly increase spot sizes due to multiple Coulomb scattering in the range shifter and the subsequent divergence in space.…”