Linac output as a function of field sizes has a phantom and a head scatter component. This last term can be measured in-air with appropriate build-up ensuring a complete electron equilibrium and the absence of the contaminant electrons. Equilibrium conditions could be achieved using a build-up cap or a mini-phantom. Monte Carlo simulations in a virtual phantom mimicking a mini-phantom were analysed with the aim of better understanding the setup conditions for measuring the collimator scatter factor that is the head scatter component of the linac output factors.
Beams of 6 and 15 MV from a TrueBeam, with size from 4 × 4 to 40 × 40 cm2 were simulated in cylindrical acrylic phantoms 20 cm long, of different diameters, from 0.5 to 4 cm, with the cylinder axis coincident with the beam central axis. The PRIMO package, based on PENELOPE Monte Carlo code, was used. The phase-space files for a Varian TrueBeam linac, provided by the linac vendor, were used for the linac head simulation. Depth dose curves were analysed, and collimator scatter factors estimated at different depth in the different phantom conditions. Additionally, in-air measurements using acyrilic and brass build-up caps, as well as acrylic mini-phantom were acquired for 6 and 18 MV beams from a Varian Clinac DHX.
The depth dose curves along the cylinders were compared, showing, in each phantom, very similar curves for all analysed field sizes, proving the correctness in estimating the collimator scatter factor in the mini-phantom, provided to position the detector to a sufficient depth to exclude electron contamination. The results were confirmed by the measurements, where the acrylic build-up cap showed to be inadequate to properly estimate the collimator scatter factors, while the mini-phantom and the brass caps gave reasonable measurements.
A better understanding of the beam characteristics inside a virtual mini-phantom through the analysis of depth dose curves, showed the critical points of using the acrylic build-up cap, and suggested the use of the mini-phantom for the collimator scatter factor measurements in the medium-large field size range.