The neutron dose equivalent evaluation and shielding at the maze entrance of a Varian Clinac 23EX treatment room

Purpose: To evaluate the neutron and photondose equivalent rate ( H n , D and H G ) at the outer maze entrance and the adjacent treatment console area after the installation of a Varian Clinac 23EX accelerator with a higher beam energy than its predecessor. The evaluation was based on measurements and comparison with several empirical calculations. The effectiveness of borated polyethylene (BPE) boards, as a maze wall lining material, on neutrondose and photondose reduction is also reported. Methods: A single energy Varian 6 MV photonlinear accelerator(linac) was replaced with a Varian Clinac 23EX accelerator capable of producing 18 MV photons in a vault originally designed for the former accelerator. In order to evaluate and redesign the shielding of the vault, the neutrondose equivalent H n , D was measured using an Andersson–Braun neutron Rem meter and the photondose equivalent H G was measured using a Geiger Muller and an ion chamber γ -ray survey meter at the outer maze entrance. The measurement data were compared to semiempirical calculations such as the Kersey method, the modified Kersey method, and a newly proposed method by Falcao et al. Additional measurements were taken after BPE boards were installed on the maze walls as a neutron absorption lining material. Results: With the gantry head tilted close to the inner maze entrance and with the jaws closed, both neutrondose equivalent and photondose equivalent reached their maximum. Compared to the measurement results, the Kersey method overestimates the neutrondose equivalent H n , D by about two to four times (calculation/measurement ratio ≈ 2.4 – 3.8 ). Falcao’s method largely overestimates the H n , D (calculation/measurement ratio ≈ 3.9 – 5.5 ). The modified Kersey method has a calculation to measurement ratio about 0.6–0.9. The photondose equivalent calculation including McGinley’s capture gamma dose equivalent equation estimates about 77%–98% of the measurement. After applying BPE boards as a lining material on the inner corner of the maze wall, the H n , D and the H G at maze entrance were decreased by 41% and 59%, respectively. Conclusions: This work indicates that the Kersey method overestimates the neutrondose equivalent H n , D for a Varian Clinac 23EX accelerator. The Falcao method overestimates the H n , D partially due to the discrepancy in the International Commission on Radiological Protection (ICRP) conversion factors caused by the uncertainties of the estimated average neutron energy. The modified Kersey method gives the closest estimation of a Varian Clinac 23EX accelerator operated at 18 MV photon mode in a maze with a similar design as in the authors’ study. However, it should be used with caution because of its tendency to underestimate the H n , D . A borated polyethylene lining can provide a cost effective method to reduce neutron and photondose equivalent at the maze door for an existing linac vault, following the installation of a higher energy linac.