Technical note: development and validation of a Monte Carlo tool for analysis of patient-generated photon scatter.

Scattered radiation unavoidably generated in the patient will negatively impact both kilovoltage (KV) and megavoltage (MV) imaging applications. Recently, 'hybrid' methods (i.e. combining analytical and Monte Carlo (MC) techniques) are being investigated as a solution to accurately yet quickly calculate the scattered contribution for both KV and MV images. We have developed a customized MC simulation user code for investigating the individual components of patient-scattered photon fluence, which serves as a valuable tool in this area of research. The MC tool is based on the EGSnrc/DOSXYZnrc user code. The IAUSFL flag options associated with subroutine AUSGAB, combined with LATCH tracking, are used to classify the various interactions of particles with the media. Photons are grouped into six different categories: primary, 1st Compton scatter, 1st Rayleigh scatter, multiple scatter, bremsstrahlung, and positron annihilation. We take advantage of the geometric boundary check in DOSXYZnrc, to write exiting photon particle information to a phase-space file. The tool is validated using homogeneous and heterogeneous phantom configurations with monoenergetic and polyenergetic beams under parallel and divergent beam geometry, comparing MC-simulated exit primary fluence and singly-scattered fluence to corresponding analytical calculations. This MC tool has been validated to separately score the primary and scatter fluence components of the KV and MV imaging applications in the field of radiation therapy. The results are acceptable for the various configurations and beam energies tested here. Overall, the mean percentage differences are less than 0.2% and standard deviations less than 1.6%. This will be a critical test instrument for research in photon scatter applications and particularly for the development of hybrid methods, and is freely available from the authors for research purposes..