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Doses to patients from photoneutrons emitted in a medical linear accelerator.

This study of doses to patients from emitted photoneutrons in a medical linear accelerator (Varian 2100C) was carried out. Dose calculation was performed using Monte Carlo Geant4 code. The model was used to calculate the neutron fluence, as a function of the neutron energy inside the treatment room to estimate the effective dose to patients. The ambient dose equivalent versus field size for patients is reported in this study. The ambient dose equivalent using 1 x 1 cm(2) field size, at isocentre and X-ray modes of 20, 18, 15 and 10 MV, was found to be 1.85, 1.79, 0.61 and 0.06 mSv Gy(-1), respectively. The mean energies of emitted photoneutrons were 0.48, 0.44, 0.40 and 0.16 MeV at X-ray modes of 20, 18, 15 and 10 MV, respectively. The results of ambient dose equivalent from emitted photoneutrons cannot be ignored and can represent a risk for healthy tissues. This study emphasised that Geant4 Monte Carlo code is an appropriate choice for studying photoneutron production and transport.