Secondary dose exposures during 200 MeV proton therapy

Prior to the first scheduled treatments at a new 200 MeV proton therapy facility a study was commissioned to ascertain the nature of extraneous radiations produced by ancillary components in the beam line. Estimates of the dose and dose equivalent were obtained in the vicinity of the isocentre using a tissue-equivalent proportional counter. The character of the scattered radiation was deduced from the measured single-event distributions and varied with lateral displacement from central axis of the primary beam. A fast neutron dose component was identified at the patient position that decreased progressively with lateral displacement and radial distance from the final collimator. Extending beyond the periphery of the patient collimator a forward peaked cone of scattered high energy protons was evident. Dose equivalents determined at the patient position and attributable to scattering from the beam delivery system varied between 33 x 10(-3) and 80 X 10(-3) Sv per treatment gray.