Characterisation of mixed radiation field produced in medical linear accelerators using foil activation technique

Abstract

The photon spectrum produced in medical linear accelerators and used for tumour therapy was measured using foil activation techniques in this work. The machine employed is the linear medical accelerator SL-25, Philips, installed at the Walsgrave Hospital Radiotherapy Centre in Coventry, U.K. A number of foil sets, with different energy thresholds were irradiated at different points inside a 400 mm by 400 mm treatment field at a nominal dose rate of 400 MU (∼4 Gy/min), and photon energy of 25 MV at the machine's isocentre. The induced activity of each foil was measured using a NaI(Tl) detector and a PC-based multichannel analyzer. The spectrum of the photons was unfolded using the computer code LOUHI82. The relative changes in the spectrum across the treatment field, were also measured using foils placed at 2.5°, 5°, 10° and 13° on both sides of the central axis of the treatment field. In order to estimate the extra dose received by the patient due to the neutron component, the neutron flux distribution at different points across the treatment field was measured using gold foils. The results and implications are discussed.