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IRSN, Institut de radioprotection et de sûreté nucléaire

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Radioprotection du personnel autour d'installations de radiothérapie. Contribution aux doses du rayonnement neutronique secondaire


Congress title :SFPM - 47èmes Journées Scientifiques
Congress town :Marseille
Congress date :04/06/2008

Summary

PURPOSE Linear accelerators operating between 4 and 25 MV are widely used for medical application. X-rays produced by high-energy (larger than 6 MeV) electrons produce a secondary neutron radiation field mainly by photonuclear reactions (,xn) inside the materials of the accelerator head, the patient and the walls of the therapy room. In France, there are more than 350 external radiotherapy facilities covering all types of techniques and designs. This set of facilities is under continuous development associated for example with the replacement of former 60Co facilities by high-energy linear accelerators and the use of the intensity-modulated radiotherapy (IMRT) technique. The radiation protection of workers around these installations has to be assessed, particularly the contribution of secondary neutrons to the doses. With this aim the IRSN carried out a measurement campaign for a selected set of facilities. MATERIALS AND METHODS Six installations were investigated, associated with four manufacturers (Varian, Elekta, General Electric and Siemens), for several energies, and for conventional and IMRT techniques. Different types of bunkers were considered: standard (or recent) and reconfigured ones (former 60Co facility, for example), i.e. adapted by reinforcing some walls (with steel or lead). The experimental conditions were chosen such as they were associated to the maximum doses to workers. Photon and neutron contributions were taken into account and the measurements were carried out at different points: outside the treatment room during treatments for the leakage radiation and inside the room between treatments to assess the contribution to the doses arising from the decay of activation products. Complementary measurements were performed inside the room during treatments to collect spectrometric data for neutrons and to assess door efficiencies. RESULTS Assuming reasonably conservative hypotheses (accelerators operating at their maximum energy of 15, 18 or 25 MV depending on the case), the annual occupational effective dose from external exposure is estimated to 0.9 mSv. The dose due to the radiations (X, , ) emitted by the decay of activation products and received between treatments represents 54 % of the total effective dose and 7 % are associated to the exposure to the neutrons emitted during treatments when the operator is at the console control panel. CONCLUSION From the results obtained from the 6 investigated facilities, a personal dose monitoring with gamma and beta dosemeters appears sufficient. However, before putting into operation a facility radiation protection controls shall include neutron measurements, particularly in the instance of reconfigured bunkers and high-energy (above 18 MV) accelerators.
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