A theoretical evaluation of the assessment of effective dose using multiple personnel dosimeters

The ability of a dose calculation algorithm, using the readings of multiple dosimeters, to accurately assess the effective dose under different photon irradiation conditions was assessed using computer simulation. The algorithm was that described in American National Standards Institute publication N13.41. Monte Carlo calculations with an anthropomorphic humanoid phantom were used to calculate the effective doses and also the expected readings of the multiple dosimeters. The irradiation geometries considered included a point source placed at several locations at a distance of 100 cm in front of the phantom, as well as an anterior-posterior plane parallel beam with a lead shield interposed between the phantom and the source. The point source energies considered were 0.05, 0.6, and 2 MeV, and the beam energy was varied between 0.03 and 10 MeV. Also considered were the estimates of effective dose based on the highest reading of the multiple dosimeters, a practice that is currently used in many work places. The results showed that use of the algorithm resulted in substantial improvements in the ability to accurately estimate effective dose. However, the results also showed that the improvements in accuracy were achievable only by using a calibration factor for the dosimetry that is different from the one obtained in current dosimetry calibration practices, and that without the use of this factor, the algorithm tended to underestimate the effective dose for nearly all the irradiation geometries considered. In addition, it appeared that this calibration factor is not constant but varies with irradiation conditions. There thus appears to be a problem of proper dosimetry calibration for use with the algorithm. This work considered only anterior posterior irradiations, and additional work is needed to assess the performance of the algorithm in other non-uniform irradiation geometries.