On methods for radiometric surveying in radiotherapy bunkers

Abstract

Radiometric surveys in radiotherapy bunkers have been carried out in Brazil for many years, both by the same radiotherapy facility for verification of shielding as by the regulatory agency for licensing and control purposes. In recent years, the Intensity Modulated Radiation Therapy (IMRT) technique has been gradually incorporated into many facilities. Therefore, it has been necessary to consider the increased leakage component that has an important impact on the secondary walls. For that, a radiometric survey method has been used that considers an increased ‘time of beam—on’ for the secondary walls. In this work we discuss two methods of doing this: the first considers that this ‘time of beam—on’ affects the sum of the two components, leakage and scattered. In another method it is considered that only the leakage component is affected by this extended ‘time of beam—on’. We compare the methods and show that for secondary walls with U = 1 the first method overestimates dose rates by important percentages and for secondary walls with U < 1 it can both overestimate or underestimate the dose rates, depending on the parameters of the project. An optimized procedure is proposed, according to the use factor (U) of the secondary wall to be measured.