To evaluate the neutron and photondose equivalent rate ( and ) 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 was measured using an Andersson–Braun neutron Rem meter and the photondose equivalent was measured using a Geiger Müller and an ion chamber -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 Falcão 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 by about two to four times (calculation/measurement ). Falcão’s method largely overestimates the (calculation/measurement ). 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 and the at maze entrance were decreased by 41% and 59%, respectively.Conclusions:
This work indicates that the Kersey method overestimates the neutrondose equivalent for a Varian Clinac 23EX accelerator. The Falcão method overestimates the 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 . 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.
The authors are deeply indebted to the staff of Department of Environmental Health and Safety, students and technicians at Cancer Therapy Research Center, at University of Texas Health Science Center at San Antonio: Christy Shriver, Jennifer Watson, Robert O. Moreno, Dennis Tyll, Nestor Ruiz-Gonzalez, and Jeffrey Peyton.
II. METHODS AND MATERIALS
II.B. Survey meters and measurement parameters
III.A.1. Neutrondose equivalent
III.A.2. Photondose equivalent
III.A.3. Maximum reading setup
III.B. Semiempirical calculations
III.C. Dose measurement for shielding
IV.A. Maximum reading setup in measurements
IV.B. Photondose equivalent components
IV.B.1. Components in the calculations
IV.B.2. Neutron capture -rays in measurements
IV.B.3. Primary beam scatteringphotons in measurements
IV.B.4. Counteractive effect of neutrons capture γ-rays and primary beam scatteringphotons
IV.C. Comparison of measurement and semiempirical calculations
IV.C.1. Photondose equivalent calculations
IV.C.2. Neutrondose equivalent calculations
IV.D. Dose measurement for shielding
Data & Media loading...
Article metrics loading...
Full text loading...