Determination of age specific 131 I S-factor values for thyroid using anthropomorphic phantom in geant4 simulations Ziaur Rahman a , Syed Bilal Ahmad b , Sikander M. Mirza b , Waheed Arshed a , Nasir M. Mirza b,n , Waheed Ahmed a a Health Physics Division, Pakistan Institute of Nuclear Science & Technology, Nilore, Islamabad 45650, Pakistan b Department of Physics & Applied Mathematics, Pakistan Institute of Engineering & Applied Sciences, Nilore, Islamabad 45650, Pakistan HIGHLIGHTS  Using anthropomorphic phantom, β- and γ-rays absorbed fractions are found for I-131.  It was done for thyroid of various age groups and geometrical models with Geant4.  For β-particles, absorbed fraction increased from 0.88 to 0.97 with fetus age.  The max. difference in absorbed energy per decay for soft tissue is 7.2% for γ-rays and 0.4% for β-particles.  Two-lobe ellipsoidal model shows 3% lower value of S-factor than the ORNL data. article info Article history: Received 1 July 2013 Received in revised form 5 December 2013 Accepted 5 March 2014 Available online 12 March 2014 Keywords: Iodine-131 S-factor values Thyroid Anthropomorphic phantom Geant4 simulations abstract Using anthropomorphic phantom in Geant4, determination of β- and γ-absorbed fractions and energy absorbed per event due to 131 I activity in thyroid of individuals of various age groups and geometrical models, have been carried out. In the case of 131 I β-particles, the values of the absorbed fraction increased from 0.88 to 0.97 with fetus age. The maximum difference in absorbed energy per decay for soft tissue and water is 7.2% for γ-rays and 0.4% for β-particles. The new mathematical MIRD embedded in Geant4 (MEG) and two-lobe ellipsoidal models developed in this work have 4.3% and 2.9% lower value of S-factor as compared with the ORNL data. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Radioactive Iodine is taken up by the thyroid from the main bloodstream quite quickly. The source of this radionuclide in thyroid could be a medical procedure for the treatment of hyperthyroidism or intake from an accidental release from a nuclear facility etc. In nuclear accidents, amongst other radioactive isotopes, various isotopes of iodine are released in significant proportions in the environment (WHO, 1999). It has been estimated that the risk of an accident in a nuclear facility cannot be eliminated completely (Sovacool, 2011). Consequently, it is very important to study the dosimetry protocols, set for radioiodine intake, on the basis of a broad spectrum of population parameters including age, gender, mass of thyroid, etc. (Krajewski et al., 2008). This may allow a reliable estimation of the absorbed dose and subsequently an adequate contingency plan for the population at risk during a nuclear accident. Precise estimation of absorbed dose is also extremely important, to determine the risk versus benefit ratio, during radiotherapy treatments. The most popular methodology for dosimetry of internally administered radionuclides is the Medical Internal Radiation Dose (MIRD) schema. This system of dose calculation provides a systematic approach towards combining biological distribution data, clearance and physical properties of radionuclides in order to estimate the internal doses. The standard MIRD schema assumes a uniform deposition of activity and distribution of radiation energy within the target volume. However more advanced schema considers the non-uniform and microscopic (i.e. cellular level) distribution of radioactivity (Zanzonico, 2000). The MIRD schema defines a para- meter called S-factor (mean absorbed dose per unit cumulative Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes http://dx.doi.org/10.1016/j.apradiso.2014.03.004 0969-8043/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail addresses: nasirmm@yahoo.com, nmm@pieas.edu.pk (N.M. Mirza). Applied Radiation and Isotopes 90 (2014) 15–22