Potential Hazard Due to Induced Radioactivity Secondary to Radiotherapy: The Report of AAPM Task Group 136
Fred B. Bateman, Bruce R. Thomadsen, Ravinder Nath, Jonathan Farr, Cal Glisson, Mohammad K. Islam, T. LaFrance, M. E. Moore, X. G. Xu, Mark Yudelev
External-beam radiation therapy mostly uses high-energy photons (x-rays) produced by medical accelerators but many facilities now use proton beams and a few use neutrons. High energy photons offer several advantages over lower energy photons in terms of better dose distributions for deep seated tumors, lower skin dose, less sensitivity to tissue heterogeneities etc. However, for beams operating above 10 MV some of the materials in the accelerator room and the radiotherapy patient become radioactive due primarily to photonuclear reactions and neutron capture, exposing therapy staff and patients to unwanted radiation dose. Recent advances in radiotherapy technology use much longer beam-on times for the same delivered dose and compared to the conventional treatment techniques and fractionation schemes, the activation dose to personnel can be substantially higher. All proton and neutron beams used for radiotherapy activate materials in the treatment room. In this report, we review critically the published literature on radiation exposures from induced radioactivity in radiotherapy. We conclude that the additional exposure to the patient due to induced radioactivity is negligible compared to the overall radiation exposure as a part of the treatment. The additional exposure to the staff due to induced activity from photon beams is small at an estimated level of about 1-2 mSv y-1 (100-200 mrem y-1). This is well below the allowed occupational exposure limits. Therefore, the potential hazard to staff from induced radioactivity in the use of high energy x-rays is considered to be low and no specific actions are considered necessary or mandatory. However, in the spirit of the ALARA program, we recommend some reasonable steps that can be taken to reduce this small exposure to an even lower level. The dose reduction strategies suggested should be followed only if these actions are considered reasonable and practical in the individual clinics.