Linear accelerator bunker shielding for stereotactic radiotherapy.

Shielding protocols such as NCRP 151, IAEA SRS 47 and IPEM 47 are commonly used for the design of radiotherapy facilities. Some work has been accomplished in updating the basic formula with the advent of IMRT but little consideration has been made for unflattened beams and stereotactic techniques apart from for facilities housing devices like the CyberKnife. The inevitable scenario of a stereotactic-only linear accelerator was considered in this study. The necessary shielding requirements were determined based on stereotactic data from a year's worth treatments from one clinic as well as further measurements of leakage, scatter and use factors. These values were compared to recommendations in the literature. While tenth value layer amounts, workload and barrier widths could be kept at the status quo, major changes could be made to the parameters of leakage, scatter and use factors while still maintaining safety. Some differences could also be seen for the use of IMRT factors. Current shielding protocols were found to inadequately describe methodology for the shielding of a stereotactic-only radiotherapy linac bunker, producing overly-conservative wall thicknesses which is in disagreement with the principles of ICRP.