Flynn, A. A., Green, A. J., Pedley, R. B., Boxer, G. M., Boden, R. and Begent, R. H. J. A Mouse Model for Calculating the Absorbed Beta-Particle Dose from 131I- and 90Y-Labeled Immunoconjugates, Including a Method for Dealing with Heterogeneity in Kidney and Tumor. Radiat. Res. 156, 28–35 (2001).
Conventional internal radiation dosimetry methods assume that the β-particle energy is absorbed uniformly and completely in the source organ and that the radioactivity is distributed uniformly in the source. However, in mice, a considerable proportion of the β-particle energy can escape the source organ, resulting in large cross-organ doses. Furthermore, the distribution of radioactivity is generally heterogeneous in kidney and tumor. Therefore, a model was developed to account for cross-organ doses and for the effects of heterogeneity in kidney and tumor in mice for two of the most important radionuclides used in therapy, 131I and 90Y. Most mouse organs were modeled as single-compartment ellipsoids or cylinders, while heterogeneity in kidney and in tumor was addressed by using two compartments to represent the cortex and the medulla and viable and necrotic cells, respectively. The dimensions of these models were taken from previous studies, with the exception of kidney and tumor, which were defined using radioluminography and mosaics of high-power microscopy images. The absorbed fractions in each compartment were calculated using β-particle point dose kernels. The self-organ dose was significantly higher for 131I compared to 90Y in all compartments, but a considerable amount of β-particle energy was shown to escape the source organ for both radionuclides, with as much as 85% and 36% escaping the marrow for 90Y and 131I, respectively. The cortex was found to occupy a greater proportion of the total kidney volume than the medulla, and consequently the self-dose was higher in the cortex. In addition, the thickness of the viable shell in the tumor increased with tumor size, as did the self-dose fractions in both necrotic and viable areas. This dosimetry model improves dose estimates in mice and gives a conceptual basis for considering dosimetry in humans.
