Dose levels due to neutrons in the vicinity of high-energy medical accelerators

High-energy photons are generated for use in radiation therapy by the deceleration of electrons in metal targets. Fast neutrons are also generated as a result of (γ, n) and (e, e'n) interactions in the target, beam compensator filter, and collimator material. In this work the absorbed dose due to neutrons was measured at the center of a 10 x 10-cm photon beam and 5 cm outside of the beam edge for a number of treatment units (Varian Clinac-18, Allis--Chalmers 25-MeV betatron, and Brown Boveri 45-MeV betatron). Dose levels due to slow and fast neutrons were also established outside of the treatment rooms and a Bonner sphere neutron spectrometer system was employed to determine the neutron energy spectrum due to stray neutron radiation at each accelerator. For the linac it was found that the neutron dose at the beam center was 0.0039% of the photon dose and values of 0.04% and 0.053% were observed for the Allis--Chalmers betatron and the Brown Boveri betatron. Dose-equivalent rates in the range 0.3--22.5 mremh were measured for points outside the treatment rooms when the accelerators were operated at a photon dose rate of 100 rad/min at the treatment position