Absorbed dose to technicians due to induced activity in linear accelerators for radiation therapy.

Absorbed dose to the trunk and to the hands of technicians working with accelerators for radiotherapy have been measured with TL dosimeters for seven different accelerators. The contribution from induced activity in the accelerator and from radiation transmitted through the walls of the treatment room have been estimated separately. The total annual absorbed dose to the trunk and to the hands have been estimated to be 2 mGy, of which the induced activity contributes one-third (0.7 mGy). The exposure of the technicians was found to be dominated by radiation penetrating the walls of the treatment room. For one accelerator the absorbed dose rate in the treatment room was measured continuously between 0.5 min and 48 h after end of treatment. Immediately after irradiation with high-energy photons the radiation is dominated by 28Al and 62Cu T1/2 = 2.3 and 9.7 min respectively) and later by radionuclides with longer half-lives, 187W and 57Ni (T1/2 = 24 and 36 h respectively). Due to these radionuclides the radioactivity in the accelerator will build up and the technicians will therefore be irradiated every time they enter the treatment room and not only directly after a treatment with high-energy photons.