Unwanted photon and neutron radiation resulting from collimated photon beams interacting with the body of radiotherapy patients

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Abstract
Monte Carlo calculations were made to determine the energies delivered by photons and neutrons to the human body irradiated by collimated photon beams. The beams were monoenergetic and ranged from 100 keV to 40 MeV. The energy deposition in the body was sorted into 2 regions: inside and outside the irradiated volume. Most of the results obtained were for a beam size of 100 cm2 although some calculations were also made for 600 cm2 beams. The effect of beam size on energy deposition in the 2 regions was investigated for 60Co gamma rays. Graphs are presented which give the integral doses delivered by neutrons and photons to the 2 regions for therapy beams of various energies. These graphs can be used to calculate the integral doses which are delivered inside and outside the treatment volume for photon spectra from most medical accelerators. Calculations of energy deposition were also made for the spectra from 2 particular accelerators. These were done using Monte Carlo as well as by simply folding the spectra into the results for monoenergetic photons. The results obtained by both methods were in good agreement and indicated that the integral doses deposited outside the treatment volume by neutrons were more than 2 orders of magnitude smaller than those deposited by scattered photons.