[Assessment of potential exposure risk for radiotherapy staff working with lineal accelerators]

Radiation exposure to the radiotherapy staff operating with linear accelerators comes from both normal exposure, which can be easily quantified by direct measurement, and potential exposure, whose evaluation is made difficult by its stochastic character. International guidelines recommend that risk be of the same order of magnitude for both types of exposure. We evaluated the health risk associated with potential exposure following the fault-tree approach suggested by the International Commission on Radiological Protection (IRCP) in its Publication 76.

Considering a typical radiotherapy installation we identified four possible staff irradiation scenarios, namely: 1) entry into the treatment room after a high-energy photon beam treatment, when induced radioactivity from photonuclear reactions has not decayed; 2) unintentional entry into the treatment room when the radiation beam is on; 3) beam failing to turn off at the end of treatment, and subsequent entry into the treatment room; 4) treatment room door inadvertently left ajar, and subsequent entry when the radiation beam is on. Each scenario depends on a particular set of parameters which are related to failure probabilities and workload. Average absorbed dose, exposure probability and related risk have been evaluated for each scenario.

Under standard parameter set-up, the overall risk did not exceed the IRCP threshold (i.e., .0002) by more than four orders of magnitude. Two main sources of potential exposure have been identified, that is early entry into the treatment room before safe decay of activation products and unintentional entry during treatment. By varying the parameters within reasonable ranges, risk has been shown to correlate with personnel training, workload, installation characteristics and operational procedures. To optimize protection, quantitative limitations have been set for human error probability, daily workload, number and quality of safety devices and waiting time before entry after a treatment with high-energy radiations.

Although the potential exposure risk for a typical radiotherapy department with standard safety devices is well below international recommended values, our results indicate that risk can be further decreased by improving personnel training, in particular relative to minimum time to entry after a high-energy treatment, to respecting warning signs and being skilled in emergency procedures. In addition, failing to install some safety devices or removing them after a failure may result in rapidly exceeding IRCP thresholds.