Estimation of neutron dose contributions to personnel working around high-energy medical linear accelerators for radiation therapy

Abstract: Purpose: Medical linear accelerators that operate above 10 MeV produce neutrons by photonuclear reactions which present a potential radiation hazard to the personnel. The purpose of this study was to estimate neutron dose contributions to the personnel working with external radiotherapy at Malmö University Hospital (UMAS), compare different kind of neutron detectors/dosimeters, and evaluate how to translate the results of the measurements to an effective dose. By estimating the neutron doses received by the personnel one can decide if better shielding is required.Method: Two accelerators that operate above 10 MeV, Varian Clinac 2100 C/D and Elekta precise, were investigated. Measurements with area monitoring devices (BF3 and 3He based instruments) were performed outside the treatment room (in experimental form), to make a survey of the neutron dose equivalent rate around the radiotherapy facility. Measurements were also performed inside the control rooms when the accelerators were in clinical use (during patient treatment). The personnel were carrying different kind of personal neutron dosemeters (electronic dosemeters, bubble detectors and etched-track detectors) during their time of work. The relation between the operational quantities and the protection quantities were studied.Results and Discussion: It was found that the highest neutron dose rates outside the treatment room when irradiating a phantom with 18 MV photon beams from the Elekta machine, were outside the treatment door (~150 μSv/h), at the hallway between the control rooms (~90 μSv/h), and inside one of the control rooms (~40 μSv/h). Furthermore from this machine, the neutron contribution to the measured dose equivalent rate was higher than the photon component. The estimated neutron dose equivalent varied up to a factor of two and occasionally even more for the different measuring devices. The highest personal dose equivalent from the personal dosemeters was estimated to be in the order of about1 mSv/year. The personal dose equivalent is the most appropriate operational quantity for estimating the effective dose while the ambient dose equivalent is a rough approximation of the effective dose.Conclusions: This study shows that the neutron component from the bunker with the Elekta machine needs to be considered when high-energy photon beams are used. The measure that could be taken in order to reduce the neutron dose to the personnel is to avoid 18 MV treatments in that bunker. Another solution is to build an additional “neutron stopping door” with hydrogen-containing shielding material inside the treatment room that has to be closed during 18 MV treatments.