Validation of Varian TrueBeam electron phase-spaces for Monte Carlo simulation of MLC-shaped fields

This work evaluates Varian's electron phase-space sources for Monte Carlo simulation of the TrueBeam for modulated electron radiation therapy (MERT) and combined, modulated photon and electron radiation therapy (MPERT) where fields are shaped by the photon multileaf collimator (MLC) and delivered at 70 cm SSD.

Monte Carlo simulations performed with EGSnrc-based BEAMnrc/DOSXYZnrc and penelope-based PRIMO are compared against diode measurements for 5 × 5, 10 × 10, and 20 × 20 cm(2) MLC-shaped fields delivered with 6, 12, and 20 MeV electrons at 70 cm SSD (jaws set to 40 × 40 cm(2)). Depth dose curves and profiles are examined. In addition, EGSnrc-based simulations of relative output as a function of MLC-field size and jaw-position are compared against ion chamber measurements for MLC-shaped fields between 3 × 3 and 25 × 25 cm(2) and jaw positions that range from the MLC-field size to 40 × 40 cm(2).

Percent depth dose curves generated by BEAMnrc/DOSXYZnrc and PRIMO agree with measurement within 2%, 2 mm except for PRIMO's 12 MeV, 20 × 20 cm(2) field where 90% of dose points agree within 2%, 2 mm. Without the distance to agreement, differences between measurement and simulation are as large as 7.3%. Characterization of simulated dose parameters such as FWHM, penumbra width and depths of 90%, 80%, 50%, and 20% dose agree within 2 mm of measurement for all fields except for the FWHM of the 6 MeV, 20 × 20 cm(2) field which falls within 2 mm distance to agreement. Differences between simulation and measurement exist in the profile shoulders and penumbra tails, in particular for 10 × 10 and 20 × 20 cm(2) fields of 20 MeV electrons, where both sets of simulated data fall short of measurement by as much as 3.5%. BEAMnrc/DOSXYZnrc simulated outputs agree with measurement within 2.3% except for 6 MeV MLC-shaped fields. Discrepancies here are as great as 5.5%.

TrueBeam electron phase-spaces available from Varian have been implemented in two distinct Monte Carlo simulation packages to produce dose distributions and outputs that largely reflect measurement. Differences exist in the profile shoulders and penumbra tails for the 20 MeV phase-space off-axis and in the outputs for the 6 MeV phase-space.