International Journal of Radiation Research, April 2013 Volume 11, No 2 Calculation of beta absorbed fractions for iodine isotopes in ellipsoidal thyroid lobe * Corresponding author: Dr. Ali Asghar Mowlavi, Fax: +98 571 4003170 E‐mail: aa_mowlavi@yahoo.com Received: Feb. 2012 Accepted: May 2012 Int. J. Radiat. Res., April 2013; 11(2): 121-126 INTRODUCTION There are many different iodine isotopes (up to 37) and some of them have an important usage for medical applications (1‐20) . Radioactive iodine isotopes have the same physical properties as stable iodine‐127 and thyroid gland cannot distinguish between radioactive and non‐radioactive iodine. Iodine‐131, with a gamma emission of 365keV and a physical half‐ life of 8.05 days, is readily available and has a low cost. Therefore, it is widely used for a number of nuclear medicine procedures, including monitoring and tracing the ϐlow of thyroxin from the thyroid, imaging of thyroid, evaluation of the iodine uptake curve, therapy of thyroid benign diseases or cancer (1‐4) . Iodine‐ 123 has a proper gamma emission (159 keV) for thyroid imaging by gamma camera with a physi‐ cal half‐life of 13.1 hours. Therefore, it is used A.A. Mowlavi 1* , M. Mirzaei 2 , M.R. Fornasier 3 , M. de Denaro 3 1 Physics Department, School of Sciences, Hakim Sabzevari University, Sabzevar, Iran 2 Physics Department, faculty of Shahid Chamran, Technical & Vocational University, Kerman, Iran 3 S.C. di Fisica Sanitaria, A.O.U. “Ospedali Riuniti” di Trieste, Trieste, Italy ABSTRACT Background: The thyroid gland absorbs nearly all the iodine in the blood, independently of its isotopic composiƟon. When a large enough acƟvity of radioacƟve iodine like 131 I, 132 I, 133 I is taken into the body, it can destroy the healthy thyroid gland cells as well as the cancer’s cells. In fact, as it is well known, some isotopes of iodine are used not only for acquiring thyroid images but also for curing thyroid cancer or hyperthyroidism due to Graves’ disease. Moreover, some of them are released in nuclear accidents. The aim of this work is to evaluate the absorbed fracƟon of beta rays from different iodine radioisotopes in thyroid lobe, using Monte Carlo method. Materials and Methods: We have applied the MCNPX code to calculate the beta absorbed fracƟons for the most important iodine radioisotopes in the thyroid lobes, supposed to have an ellipsoidal shape, with the volume varying from 1 ml to 25 ml and the material composiƟon suggested by ICRP. The beta rays spectra of iodine isotopes have been taken from the LBNL website. Results: The result showed that the volume lobe variaƟon had a significant effect on the absorbed fracƟon for beta rays in thyroid gland, up to 25% for 124 I. The absorbed fracƟons of beta rays were decreasing from 123 I to 131 I, 130 I, 126 I, 135 I, 133 I and 124 I respecƟvely. Conclusions: Decreasing of the absorbed fracƟon might be related to the beta rays energy spectra of the isotopes. Moreover, for 131 I results obtained for beta absorbed fracƟon in spheres was in agreement with previously published results. Keywords: Thyroid gland; Beta absorbed fraction; Iodine isotopes; MCNPX code. ► Original article