349. IORT dedicated linac radiation protection: A novel approach

Purpose IORT dedicated Linacs operate inside not dedicated Operation Rooms. Even though there are currently more than 140 units installed worldwide, produced by different companies, currently there is no agreement on a standard approach dealing with how to define and measure their radiation protection features. A method based on NCRP 151 [1] is proposed and applied to SIT latest model, LIAC HWL. Methods Stray radiation (SR) produced by IORT dedicated Linacs have been thoroughly analysed according to NCRP 151 description (direct beam, leakage radiation, scattered radiation from the patient; scattered radiation from the walls, secondary radiations). A novel measurement method is proposed in order to create a standard and reproducible setup that provides all the information needed for forecasting SR in any installation. The SR in patient plane (direction 90°) has been measured at 3 m from the target behind drywall walls with variable thicknesses (1 to 7 cm); SR along direction 180° has been measured at a distance of 1 m from the radiant head; SR downstairs has been measured as shown in figure along planes perpendicular to beam axis at two different distances, 3 m and 2 m from the upper floor (standard concrete 30 cm). TVL along directions 0°, 90° and 180° have been determined. Furthermore, a comparison between many survey meters has been performed in order to ascertain the most adequate ones: Inovision 451 B, Fluke 451P, Mirion DMC 3000, Canberra Babyline. Results SR in patient plane results less than 0.2 uSv/Gy with 5 cm drywall wall; hotspot downstairs is less than 0,025 uSv/Gy (beam stopper diameter 70 cm, see figure); upstairs SR is less than 0,07 uSv/Gy. Conclusions Such procedure provides a reliable and effective tool for predicting SR in any installation; post installation surveys have proven the efficacy of such method. We hope the scientific community could study the subject and define new standards, so that users may benefit from specific, authoritative, and up to-date recommendations [2] .