Additional dose assessment from the activation of high-energy linear accelerators used in radiation therapy

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 gammaray spectrometry and dose measurements, and to estimate the additional dose to radio therapy staff on the basis of these results. Spectral analysis was per formed 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: 28Al, 62Cu, 56Mn, 64Cu, 187W, and 57Ni. The short-lived isotopes such as 28Al and 62Cu are the most important factors of the clini cal routine, while the contribution to the radiation dose of medium-lived isotopes such as 56Mn, 57Ni, 64Cu, and 187W in creases during the working day. Measured dose rates at the isocenter ranged from 2.2 μSv/h to 10 μSv/h in various measuring points of interest for the members of the radio therapy staff. Within the period of 10 minutes, the dose rate decreased to values of 0.8 μSv/h. According to actual work loads in radio therapy departments, a realistic exposure scenario was set, resulting in a maximal additional annual whole body dose to the radio therapy staff of about 3.5 mSv.