Shielding design calculations for a radiotherapy vault of a 6 and 10-megavoltage medical linear accelerator operating with or without a flattening filter
This study focuses on upgrading the shielding design of a Co-60 radiotherapy unit to accommodate a linear accelerator operating at 6 and 10 megavoltage, which can function in both flattening filter (FF) and flattening filter-free (FFF) modes. Shielding calculations were performed using analytical methods from the National Council on Radiation Protection and Measurements Report No. 151 and International Atomic Energy Agency Safety Reports Series No. 47, considering standard (40 patients/day) and heavy (60 patients/day) workloads. Barrier thicknesses were determined to ensure the instantaneous dose rate (IDR) behind primary barriers is ≤7.5 μSv/h in FF mode and ≤20 μSv/h in FFF mode, as recommended in the Institute of Physics and Engineering in Medicine Report No. 75. Results showed that workload had no significant effect on the primary barrier thickness. Moreover, strict adherence to the IDR criteria increased shielding demands ranging from 11% to 46% while reducing the personal equivalent dose to 1.2 – 13.4% of the dose constraints. This highlights a potential overdesign that could substantially increase raise construction costs. Therefore, applying the IDR criteria violates the radiation protection principles, namely, “optimization.” An alternative approach has been proposed to optimally use the IDR criteria by including the patient’s transmission factor in the current IDR criteria or by adopting higher values of IDR strictly to verify the shielding efficiency. Higher IDR adoption would result in lower linear accelerator vault costs and more realistic shielding calculations consistent with operating conditions.
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