Neutron activation in the radiotherapy bunker: strategies for managing radiation safety


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.

In Medical Physics
Logan Montgomery
Logan Montgomery
PhD Student
John Kildea
John Kildea
Assistant Professor of Medical Physics