Effective dose as a limiting quantity for the evaluation of primary barriers for diagnostic x-ray facilities

The National Council on Radiation Protection in Report 147 of NCRP has recommended that shielding design limit for diagnostic x-ray facilities must be consistent with the guidance specified in Report 116 of NCRP. In the latter report, it is specified that the limit of exposure must be in terms of effective dose received annually. New mathematical models that are different from those in Report 49 of NCRP are reported in the recently published Report 147 of NCRP, and the design limit is specified as kerma value. In this work, to provide a means of compliance with the recommendation in Report 116 of NCRP, the effective dose that is classified as the limiting quantity in Report 57 of ICRU has been incorporated into shielding algorithms for diagnostic x-ray facilities. Also, shielding models are presented using exposure, kerma-in-air, kerma-in-tissue and ambient dose equivalent as limiting quantities. A computer program, XSHIELD, was written in FORTRAN language to execute these models. With design limits set at 1 mSv y and 0.25 mSv y (as specified in Report 116 of NCRP) and using sample distribution of workload, age of patient, field sizes at image receptor, and types of projection, computations of shielding requirements were carried out for rooms designated adult and pediatric chest rooms. For same values of respective workload and design limit, the use of exposure, kerma-in-air, kerma-in-tissue, and ambient dose equivalent as limiting quantity produces thicker barriers than the use of effective dose. By the use of effective dose as limiting quantity, the shielding requirement for the same workload is independent of size of the individual to be shielded. However, irradiating the individual who is to be shielded in posterior-anterior projection requires a thicker barrier than when irradiation is in lateral projection.