Neutron production from a 10 MV medical linac

Measurements have been made of the neutron flux density and neutron fluence per photon rad at isocentre at four different locations around the George Washington University Medical Center Varian Clinac-18 accelerator operating in the 10MV photon mode, since there may be hazards to machine operators and other personnel. One of the locations was outside the treatment room door. Two different calibrated neutron detectors were used to cover the large range of neutron fluxes expected at the different locations. Conversion of the neutron fluence results to dose showed that the average whole-body neutron dose (in rads) to the radiotherapy patient is of the order of 0.001% of the isocentre photon dose, and (in rem) 0.01% of the photon dose, assuming a quality factor of 10 for fast neutrons. The measured fluence of 6.3 n cm-2rad-1 outside the treatment door corresponded to 8 x 106 n cm-2yr-1, or 6% of the occupational MPD. Neutrons do