SU‐E‐T‐365: Estimation of Neutron Ambient Dose Equivalents for Radioprotection Exposed Workers in Radiotherapy Facilities Based On Characterization Patient Risk Estimation

Purpose: Previous measurements with Bonner spheres 1 showed that normalized neutron spectra are equal for the majority of the existing linacs 2 . This information, in addition to thermal neutron fluences obtained in the characterization procedure 3 3, would allow to estimate neutron doses accidentally received by exposed workers, without the need of an extra experimental measurement. Methods: Monte Carlo (MC) simulations demonstrated that the thermal neutron fluence distribution inside the bunker is quite uniform, as a consequence of multiple scatter in the walls⁴. Although inverse square law is approximately valid for the fast component, a more precise calculation could be obtained with a generic fast fluence distribution map around the linac, from MC simulations⁴. Thus, measurements of thermal neutron fluences performed during the characterization procedure 3 , together with a generic unitary spectra 2 , would allow to estimate the total neutron fluences and H*(10) at any point⁵. As an example, we compared estimations with Bonner sphere measurements 1 , for two points in five facilities: 3 Siemens (15–23 MV), Elekta (15 MV) and Varian (15 MV). Results: Thermal neutron fluences obtained from characterization, are within (0.2–1.6×10⁶) cm− 2 •Gy −1 for the five studied facilities. This implies ambient equivalent doses ranging from (0.27–2.01) mSv/Gy 50 cm far from the isocenter and (0.03–0.26) mSv/Gy at detector location with an average deviation of ±12.1% respect to Bonner measurements. Conclusion: The good results obtained demonstrate that neutron fluence and H*(10) can be estimated based on: (a) characterization procedure established for patient risk estimation in each facility, (b) generic unitary neutron spectrum and (c) generic MC map distribution of the fast component. [1] Radiat. Meas (2010) 45: 1391 – 1397; [2] Phys. Med. Biol (2012) 5 7:6167–6191; [3] Med. Phys (2015) 42:276 ‐ 281. [4] IFMBE (2012) 39: 1245–1248. [5] ICRU Report 57 (1998)