Additional Dose Assessment from the Activation of High-Energy Linear Accelerators Used in Radiation Therapy

Abstract

It is well known that medical linear accelerators generate activation products when operated above certain electron (photon) energies. The aim of the present work is to assess the activation behavior of a medium-energy radiotherapy linear accelerator by applying in situ gamma-ray spectrometry and dose measurements, and to estimate the additional dose to radiotherapy staff on the basis of these results. Spectral analysis was performed parallel to dose rate measurements in the isocenter of the linear accelerator, immediately after the termination of irradiation. The following radioisotopes were detected by spectral analysis: (28)Al, (62)Cu, (56)Mn, (64)Cu, (187)W, and (57)Ni. The short-lived isotopes such as (28)Al and (62)Cu are the most important factors of the clinical routine, while the contribution to the radiation dose of medium-lived isotopes such as (56)Mn, (57)Ni, (64)Cu, and (187)W increases during the working day. Measured dose rates at the isocenter ranged from 2.2 mu Sv/h to 10 mu Sv/h in various measuring points of interest for the members of the radiotherapy staff. Within the period of 10 minutes, the dose rate decreased to values of 0.8 mu Sv/h. According to actual workloads in radiotherapy departments, a realistic exposure scenario was set, resulting in a maximal additional annual whole body dose to the radiotherapy staff of about 3.5 mSv.