The dose from a semi-infinite cloud of a photon-emitting radionuclide is an idealization of an exposure situation, but it is a useful one, particularly for noble gases of long radioactive decay half-times, e.g., 85 Kr. This paper presents an analysis of absorbed dose to body organs for monoenergetic photons emitted in an infinite cloud. Twelve photon energies in the range 0.01 to 4 MeV were used in these calculations, and the concentration in the cloud was considered uniform. The energy spectrum of scattered photons is first calculated by the method of Dillman, and the resulting fluence is used as the source on an anthropomorphic phantom, essentially as considered in MIRD Pamphlet No. 5. The dose to the various organs is obtained by the Monte Carlo technique.Also a method is described which provides estimates of absorbed dose to organs by use of calculated depth dose profiles in the phantom.This method is used for those organs for which the coefficient of variation of the Monte Carlo estimate exceeded 30%. It is expected that dose from photons emitted in the decay of a real radionuclide could be obtained by interpolation from these results.

Yittrium-90 microsphere brachytherapy procedures have increased in number due to their efficacy in treating some unresectable metastatic liver tumors. The discovery of long-lived impurities in two microsphere products, first reported between 2006 and 2007, has resulted in some radiation safety concerns. Since then, microsphere production processes have been refined, which reportedly lead to a reduction in detectable by-products. In this study unused vials of TheraSphere and SIR-Spheres, manufactured in early January 2011, were analyzed to identify and quantify the low-level radioactive impurities. Absorbed dose calculations were performed to assess the potential increased dose to the patient due to long-lived impurities. Results showed that while the SIR-Spheres vials contained no detectable impurities (contrary to other published results in the literature), the TheraSphere vials contained 17 radionuclides in one sample and 15 in the other. The dominant impurities were Y and Y, with specific activities ranging from 0.99 ± 3.40 × 10 kBq mg to 6.30 ± 0.40 kBq mg at vendor assay date. Other impurities were on the order of Bq mg. Based on Medical Internal Radiation Dose (MIRD) liver and lung dose estimates, the long-lived impurities would be expected to increase an administered dose by less than 0.1% from the prescribed dose.

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The c o e f f i c i e n t of v a r i a t i o n f o r these values i s greater than 50%.