Dosimetry in thyroid follicles due to low-energy electrons of iodine using the Monte Carlo method

Objective: To evaluate the absorbed dose in thyroid follicles due to low-energy electrons such as Auger and internal conversion electrons, besides beta particles, for iodine radioisotopes (131I, 132I, 13'3I, 134I and 135I) utilizing the Monte Carlo method. Materials And Methods: The dose calculation was performed at follicular level, simulating Auger, internal conversion electrons and beta particles, with the MCNP4C code. The follicles (colloid and follicular cells) were modeled as spheres with colloid diameter ranging from 30 to 500 μm, and with the same density of water (1.0 g.cm-3). Results: Considering low-energy particles, the contribution of 131I for total absorbed dose to the colloid is about 25%, while the contribution due to short-lived isotopes is 75%. For follicular cells, this contribution is still higher achieving 87% due to short-lived iodine and 13% due to 131I. Conclusion: The results of the present study demonstrate the importance of considering low energy particles in the contribution for the total absorbed dose at follicular level (colloid and follicular cells) due to iodine radioisotopes (13'1I, 13'2I, 1'3'3I, 1'34I and 135I). (author)