Bunker shielding design scheme for high energy medical linear accelerator: Comparison between Chinese and international radiation shielding standards for radiotherapy facilities

Objective: To compare the differences between Chinese and two international radiation shielding standards for radiotherapy facilities in the bunker shielding design for high energy medical linear accelerator, so as to provide reference for revising and improving the current national standard. Methods: The required bunker shielding for a high energy (6 MV and 10 MV) medical electron linear accelerator to treat an average of 105 patients (90% with IMRT technology) per day was calculated and compared according to the NCRP report 151 (America), IRR 17 legislation (British) and Chinese national standard GBZ/T 201, respectively. The changes of the required shielding thickness of each concern point according to the change of workload were analyzed. A novel method of shielding effect evaluation based on the conservative estimation of instantaneous dose-equivalent rate was proposed. Results: The values of concrete shielding thickness required for point A and B (secondary barrier of the primary wall), C and D (primary barrier of the primary wall), E (secondary barrier of the side wall), F (primary barrier of the roof) and G (secondary barrier of the roof) according to the NCRP report 151 and IRR 17 legislation were 89, 115, 162, 183, 113, 163, 86 cm, and 104, 130, 215, 213, 128, 207, 105 cm, respectively. The values for these points according to the GBZ/T 201 national standards were 136, 153, 243, 265, 131, 207 and 105 cm, respectively. Compared with the NCRP 151 shielding scheme, the indoor use area and floor height of the treatment room were significantly decreased by 14.01% and 8.68%, respectively, while the roof load-bearing increased significantly by 24.01% by GBZ/T. The final values of shielding thickness of either IRR 17 or GBZ/T 201 shielding schemes were mainly dominated by the instantaneous dose equivalent rates, generally without changing with the increase of high-energy X-ray workload. The values of shielding thickness based on the conservative estimates of instantaneous dose equivalent rate GBZT_-IDR2 were larger than those calculated in terms of weekly dose control limit GBZT_-D_w with relative small difference compared with that between GBZT_-D_w and GBZT_IDR. Conclusions: The differences in shielding thickness for high energy linear accelerator were obvious among different standards due to different evaluation indices, especially the dose equivalent rate limit used. The conservative estimates of instantaneous dose equivalent rate are safe, reasonable, easy to use, and available as evaluation index for shielding calculation. (authors)