Monte Carlo estimation of photoneutrons spectra and dose equivalent around an 18 MV medical linear accelerator

Abstract

A fully detailed Monte Carlo geometrical model of an 18MV Varian Clinac 2100C medical linear accelerator, lodged at Blida Anti-Cancer Centre in Algeria, was developed during this study to estimate the photoneutrons spectra and doses at the patient table in a radiotherapy treatment room, for radiation protection purposes.Subsequently, thanks to this model, extensive M.C. calculations of neutron fluences and ambient dose equivalent were carried out at three selected locations on the patient table in the medical facility for a reference field size of 10×10cm2 and a Source to Surface Distance (SDD) of 100cm. At the patient table, the calculated neutron ambient doses equivalent were 3.50, 0.89 and 0.62mSvGy−1 at isocenter, 40cm and 100cm from the isocenter.Thereafter, a sensitivity analysis was performed to investigate the effect of variation of some pertinent parameters (source term model, Multi-Leaf Collimators and the shielding walls of the treatment room) on the production of the photoneutrons doses at the patient table.Finally, a full comparative study has been performed, in order, to estimate the consistency of the author's results. Therefore, the obtained M.C. results were compared, firstly, to a few measurements acquired, for the same irradiation facility using CR-39 Nuclear Track Detectors (NTDs), at the isocenter assuming different field sizes, and secondly, to both theoretical and experimental results available in the literature for the same medical facility and for different irradiation conditions. From these comparisons, a mean difference (for all the locations and field sizes considered) of about 15% was found.Despite, the possible differences in the accelerator modeling, including variations in the accelerator head and room geometries and differences in the computational tools and photoneutrons cross-sections data used in the literature, the results obtained through this investigation are encouraging, highlighting thereby the consistency of the geometrical M.C. model assumed in this work.