Abstract
Purpose:Previous measurements with Bonner spheres1 showed that normalized neutron spectra are equal for the majority of the existing linacs2. This information, in addition to thermal neutron fluences obtained in the characterization procedure33, 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 procedure3, together with a generic unitary spectra2, would allow to estimate the total neutron fluences and H*(10) at any point⁵. As an example, we compared estimations with Bonner sphere measurements1, 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)
Published Version
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