Parametrization and application of scatter kernels for modelling scanned proton beam collimator scatter dose

Abstract

Collimators are routinely used in proton radiotherapy to laterally confine the field and improve the penumbra. Collimator scatter contributes up to 15% of the local dose and is therefore important to include in treatment planning dose calculation. We present a method for reconstruction of the collimator scatter phase space based on the parametrization of pre-calculated scatter kernels. Collimator scatter distributions, generated by the Monte Carlo (MC) package GEANT4.8.2, were scored differential in direction and energy. The distributions were then parametrized so as to enable a fast reconstruction by sampling. MC calculated dose distributions in water based on the parametrized phase space were compared to full MC simulations that included the collimator in the simulation geometry, as well as to experimental data. The experiments were performed at the scanned proton beam line at the The Svedberg Laboratory (TSL) in Uppsala, Sweden. Dose calculations using the parametrization of this work and the full MC for isolated typical cases of collimator scatter were compared by means of the gamma index. The result showed that in total 96.7% (99.3%) of the voxels fulfilled the gamma 2.0%/2.0 mm (3.0%/3.0 mm) criterion. The dose distribution for a collimated field was calculated based on the phase space created by the collimator scatter model incorporated into the generation of the phase space of a scanned proton beam. Comparing these dose distributions to full MC simulations, including particle transport in the MLC, yielded that in total for 18 different collimated fields, 99.1% of the voxels satisfied the gamma 1.0%/1.0 mm criterion and no voxel exceeded the gamma 2.6%/2.6 mm criterion. The dose contribution of collimator scatter along the central axis as predicted by the model showed good agreement with experimental data.

Field size dependence of the output factor in passively scattered proton therapy: Influence of range, modulation, air gap, and machine settings

“…van Luijk et al 6 and Gottschalk 7 previously described Monte Carlo simulations of collimator-scattered particles. Kimstrand et al 8 presented a parametrization of collimator scatter for accurate and fast dose calculation.…”

Commissioning a passive‐scattering proton therapy nozzle for accurate SOBP delivery

“…Field specific apertures influence the dose on the central axis, and thereby the output, by means of collimator scatter, see, e.g., Kimstrand et al 9 and Titt et al 10 We have performed extensive measurements to accurately determine the effect of the patient-specific aperture size, and its distance from isocenter, on the output. These will be published in a separate paper together with data for our other beam lines.…”

Patient Dose Computation

“…The scatter foils used in passively scattered electron and proton beams act as extended sources (Schaffner, 2008) and can hence influence the output due to the viewable area seen upstream through the collimation system, although the distinct angular distributions for charged particles complicate matters. Collimators, if exposed to charged particles, will cause more dose due to scatter than in photon beams, which can be handled with line integrals around the exposed collimating rim (Kimstrand et al, 2008).…”