Purpose: To revise current NCRP 151 workload, use factors, and field-size recommendations utilized in linear accelerator bunker shielding calculations based on delivered modern clinical treatment techniques.
Methods: Relevant clinical data was accrued from six linear accelerators over a 12 month time frame. These machines included four Varian TrueBeam, one Varian Trilogy, and one Varian Edge. This data was compiled and sorted to calculate workloads, use factors as a function of gantry angle, and maximum jaw-defined equivalent square field-size per arc/field. The use factors were calculated in both 90 degree and 45 degree gantry intervals. Workload and field-size factors were calculated for the following two cases: the SRS/SBRT focused Varian Edge and all five non-Edge machines. TBI treatments and QA were not included as a part of this study.
Results: The single maximum workload for non-Edge machines for energies 6, 10, and 15 MV were 311 Gy/week, 196 Gy/week, and 37 Gy/week, respectively, with maximum Edge 6 MV and 10 MV workloads of 466 Gy/week and 163 Gy/week. Use factors calculated in 90 degree gantry increments resulted in a use factor 6.2 percent lower for a posterior-anterior field than values listed in Table 3.1 of NCRP 151. 45 degree gantry increments resulted in a use factor 13.8 percent lower for a posterior-anterior field. The maximum field size for all machines excluding the Edge was less than 25x25-cm², while the maximum field size for all treatments on the Edge was less than 20x20-cm².
Conclusion: The results of this study show that the current metrics used for shielding calculations in many facility designs could be considered overly conservative. The reduction in both maximum field sizes and workloads for modern treatment techniques and fractionations will likely reduce the required shielding and subsequent costs for building linear accelerator vaults.
Funding Support, Disclosures, and Conflict of Interest: MCS also reports honoraria and consulting fees with Varian Medical Systems, Inc. and consulting fees with Lifeline Software, Inc. outside of the submitted work.
Shielding, Radiation Protection, Linear Accelerator
TH- External Beam- Photons: radiation protection and shielding