Purpose: Radiotherapy linacs producing high energy photon beams create unwanted activated by-products through photodisintegration and neutron capture. Activated material may pose a risk to staff and patients.
Methods: Measurements (Varian TrueBeam) were performed with 6 and 10 MV (both regular & FFF), and 15 MV photon beams. Data was collected at isocentre and elsewhere using calibrated survey meters. Because of pulse pile-up, all data collection began 30 s post beam-off.
Results: Activation below 10 MV is negligibly low. A half-life of about 3.5 min, measured over the first 30 min was observed for the 15 MV beam. The initial activation rate of the 15 MV beam is a function of linac dose rate, and beamon time. When the half-life, initial dose rate, and time of irradiation are known, the dose near the linac following beam-off can be estimated. For example, a person entering a linac after a treatment delivery of 500 MU, at 600 MU/min, with a 15 MV beam is subject to an initial exposure rate near isocentre of approximately 5.0 microSv/hr with a half-life of 3.5 min. Staying 2 min in this area would yield a dose of about 0.1 microSv.
Conclusions: Linac activation, dose-rate and half-life can be measured and parameterized to predict ambient dose rate conditions inside the linac bunker. These parameters allow an estimation of the dose to personnel and patients as a function of beam energy, linac dose rate, MU delivered, location and duration of stay in the bunker.