Application of the phase-space distribution approach of Monte Carlo for radiation contamination dose estimation from the (n,γ), (γ,n) nuclear reactions and linac leakage photons in the megavoltage radiotherapy facility.

Abstract

The aim of this study was to characterize the radiation contamination inside and outside the megavoltage radiotherapy room. Radiation contamination components in the 18MV linac room are the secondary neutron, prompt gamma ray, electron and linac leakage radiation. An 18MV linac modeled in a typical bunker employing the MCNPX code of Monte Carlo. For fast calculation, phase-space distribution (PSD) file modeling was applied and the calculations were conducted for the radiation contamination components dose and spectra at 6 locations inside and outside the bunker. The results showed that the difference of measured and calculated percent depth-dose (PDD) and photo beam-profile (PBP) datasets were lower than acceptable values. At isocenter, the obtained photon dose and neutron fluence were 2.4×10-14 Gy/initial e° and 2.22×10-8 n°/cm2, respectively. Then, neutron apparent source strength (QN) value was found as 1.34×1012 n°/Gy X at isocenter and the model verified to photon and neutron calculations. A surface at 2cm below the flattening filter was modeled as phase-space (PS) file for PDD and PBP calculations. Then by use of a spherical cell in the center of the linac target as a PS surface, contaminant radiations dose, fluence and spectra were estimated at 6 locations in a considerably short time, using the registered history of all particles and photons in the 13GB PSD file as primary source in the second step. Designing the PSD file in MC modeling helps user to solve the problems with complex geometry and physics precisely in a shorter run-time.