New Concepts in Radiotherapy Vault Shielding –NCRP 151 and IAEA Reports

Raymond K. Wu, PhD, Medical Physics Department, OhioHealth Hospitals, Columbus, OH 43214, USA

Recently several NCRP (National Council on Radiation Protection and Measurements) reports were published, replacing the NCRP Report 49 which has been the radiation facilities shielding standard for the last 30 years. The IAEA will publish a Technical Document on Radiotherapy Facility Shielding soon. The new reports were published or will be published one after another quite quickly after the publication of NCRP Statement Number 10 which clarifies the situations in which the 0.25 factor described in NCRP 116 does and does not apply.

The Reports 147 for diagnostic radiological and 151 for therapy facilities established a new set of Occupancy Factors for different types of areas based on the more up-to-date utilization of surrounding areas in clinical settings.

The Report 151 which is the focus of this talk established the parameter Time Averaged Dose Rate (TADR) for several time intervals to clearly define the term Instantaneous Dose Rate (IDR), the in-any-hour rate (R_h), and the weekly TADR (R_w). The IAEA report is expected to include the same set of TADRs. In this talk, the relationships between the Design Dose Limit and the different TADRs will be explained. Using these simple relationships, the shielding evaluations and surveys may be performed more easily and will be illustrated with examples.

The NCRP 151 Report extends the range of applicable facilities to include high energy accelerators with photon energies up to 25 MV. The empirical methods to estimate dose equivalent due to neutrons and capture gamma rays for high energy linacs will be described and illustrated with examples for typical cases. The report also describes the methods to evaluate shielding barriers for several new therapy procedures including IMRT, robotic units, and tomotherapy facilities and will be covered in the presentation.