Evaluation of neutron dose to radiotherapy patients treated with 10 MV X-ray beams based on photoneutron spectrometry

Neutron dose to radiotherapy patients treated with 10 MV x-ray beams was evaluated based on the spectrometry of incident photoneutrons. Neutron energy spectra could be obtained by unfolding measured responses of neutron capture rates of indium activation foils in a polyethylene phantom with response functions calculated by Monte Carlo simulation. Unfolded neutron spectra inside and outside the x-ray beam indicated that photoneutrons distributed in the energy region of less than 1 MeV with the maximum fluence per unit lethargy at around 0.1 MeV. Effective dose to the patient was adopted as a fundamental protection quantity since photoneutrons from the accelerator head were found to enter the whole body of the patient with approximately flat intensity. Outside the X-ray beam, the dose was evaluated to be 59 μSv/photon Gy from the neutron energy fluence to the effective dose conversion-coefficients tabulated by the International Commission of Radiological Protection (ICRP) for anterior-posterior irradiation geometry. Since total effective dose received by the patient during the treatment period corresponding to an integral photon dose of 60 Gy was estimated to be as small as 3.5 mSv, no additional shield would be required for contaminant photoneutrons generated by 10 MV therapeutic x-rays. (author)