Statistical Phantoms and Determining the Uncertainty of the Thyroid Mathematical Phantom in Internal Dosimetry Using the
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Statistical Phantoms and Determining the Uncertainty of the Thyroid Mathematical Phantom in Internal Dosimetry Using the Monte Carlo Method Vahid Ragheb, Zahra Sajjadi∗ and Saeed Mohammadi Department of Physics, Payame Noor University, P.O.Box 19395-3697, Tehran, Iran (Received 25 November 2019; revised 1 July 2020; accepted 19 July 2020) The purpose of this study was to investigate the effect of changes in the thyroid mass on its absorbed dose rate. To this end, 100 similar mathematical phantoms, which only differed in their thyroid mass, were created. Then, the thyroid was selected as the source of photon, electron and 131 I, respectively, the absorbed doses to the thyroid and other organs were estimated using the MCNPX code. The results show when the mass increases, the amount of absorbed dose in the thyroid decreased. The power of this reduction for the energies between 0.01 and 4 MeV was in the ranges of (1/3 ≤ x ≤ 1) for photons and (2/3 ≤ x ≤ 1) for electrons. Finally, analyzing the statistical distribution of the doses, the coefficient of changes in the thyroid absorption dose for photon energies ranging from 0.01 to 4 MeV was 12–30%, for electrons in the same energy range, it was between 12–23%, and per decay of 131 I, the figure turned out to be 19%. Moreover, the risk of a secondary cancer related to the non-consideration of the special body thyroid mass per 100 mCi (3.7 GBq) of the 131 I radiopharmaceutical, was estimated to be 1.2%. Keywords: Statistical phantom, Internal dosimetry, Monte Carlo, Dose uncertainty DOI: 10.3938/jkps.77.1075
I. INTRODUCTION The thyroid is a lobular gland located in the anterior part of the neck, and its mass, based on ICRP89 [1], is 20 ± 6.8 grams for human beings [2]. Iodine is an essential element in the composition of the thyroid-secreted hormones, which plays a decisive role in the metabolism of all cells present in the organs of the body [3], and it is equally important in the growth process of most organs, especially the brain [4]. 131 I is among the most suitable radioisotopes in the diagnosis and treatment of hyperthyroidism and thyroid cancers [5]. Various studies have shown that, in the long term, 131 I can be used to treat thyroid cancer and hyperthyroidism, even for patients allergic to iodine or its agents [6]. Iodine therapy is very effective, provided sufficient radiation is deposited in the thyroid [7]. The main goal of using radioisotopic 131 I treatment is to produce the highest absorbed dose in the thyroid gland and the lowest dose possible in other organs, so that it can have the highest therapeutic effect and the least side effects [8]. Although 131 I therapy is usually well tolerated and may reduce long-term morbidity and possibly improve survival in patients with differentiated thyroid cancer, potential side effects can occur, such as radiation-induced thyroiditis, sialadenitis, bone marrow depression, pulmonary fibrosis, and rarely induction of a second primary malignancy [9]. It ∗ E-mail:
has also been reported that using an additional 10 mCi of iodine in trea
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