Thyroid volume's influence on energy deposition from 131I calculated by Monte Carlo (MC) simulation Ali Asghar Mowlavi1,2, Maria Rosa Fornasier1, Mario de Denaro1 1 Department of Medical Physics, A.O.U. "Ospedali Riuniti di Trieste", Trieste, Italy 2 Physics Department, School of Sciences, Sabzevar Tarbat Moallem University, Sabzevar, Iran Received 12 May 2010 Accepted 2 December 2010 Correspondence to: Maria Rosa Fornasier, Department of Medical Physics, Via della Pieta 19, 34129 Trieste, Italy. Phone: +390403992381; E-mail: mariarosa.fornasier@aots.sanita.fvg.it Disclosure: No potential conflicts of interest were disclosed. Background. It is well known that the success of the radiomethabolic 1311 treatment of hyperthyroidism could depend on the absorbed dose to the thyroid. It is, thus, very important to calculate the individual radiation dose as accurately as possible for different masses of thyroid lobes. The aim of this work is to evaluate the influence of thyroid volume on the energy deposition from beta and gamma rays of 131I by Monte Carlo (MC) simulation. Materials and methods. We have considered thyroid lobes having an ellipsoidal shape, with a density of 1.05 g/ cm3 and the material composition suggested by International Commission on Radiological Protection (ICRP). We have calculated the energy deposition of 131I rays for different volumes of thyroid lobes by using the MCNPX code, with a full transport of beta and gamma rays. Results and conclusions. The results show that the total energy deposition has a significant difference, till 11%, when the lobe's volume varies from 1 ml to 25 ml, respect to the value presented in MIRDOSE for a 10 g sphere. The absorbed energy fraction increases by volume, because the increasing volume to surface ratio of ellipsoidal lobe causes the decrease of beta ray fraction escaping from the lobe. Key words: thyroid gland; 131I radionuclide; total energy deposition; MCNPX code Introduction Thyroid gland consists of two linked lobes and is located in the middle of the low neck, overlying the trachea. Radioactive iodine 131I has become the most widely used therapy for patients with hyper-thyroidism due to Graves' disease.1 This kind of therapy has largely replaced surgery as the definitive treatment for such benign disease in contrast with malignant ones2-4, because it is easier than surgery to perform and has proved to be more effective. A number of dosing regimens have been proposed, ranging from those based on thyroid volume evaluation and Iodine test-activity uptake determination - for high precision dosimetry -, to large, fixed activities of 131I administration, intended to cause hypothyroidism soon after treatment.1-3 Physicians generally determine the 131I activity on an empirical basis: the decision is based on the volume of the thyroid evaluated by scintigraphy, SPECT, MRI or ultrasonic methods and, sometimes, on the basis of 131I/123I test-activity uptake at 24 hours post-administration.5 It is well known that the success of this therapy could depend on the absorbed dose to the thyroid: it is thus very important to calculate the individual radiation dose as accurately as possible for different mass of thyroid lobe. Many authors have developed algorithms for the calculation of the radiation absorbed dose to a target organ, starting from a basic absorbed dose rate equation represented by the Medical Internal Radiation Dose (MIRD) models.6 Traino et al. evaluated the influence of the volume reduction on the calculation of the absorbed dose to the thyroid by presenting a mathematical mod-el.1 The aim of this work is to evaluate the influence of thyroid volume on the energy deposition from 131I by Monte Carlo (MC) simulation. 0.8- 0.6- 0.4 J= 0.2- ® 0.0 —•— The real beta spectrum of 131l The average beta spectrum in MIRD ^ 1 0.2832 0.0039 0.08894 0.00646 0.06936 0.0208 0.09662 0.0723 ,J 0.19160 0.896 I I I I I I I I I I I I I I I I I I I I I I I I f I T I P I "I 1 1 1 1 I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ep(MeV) ® 0.8- ^_ 1 0.7- 0.08 0.051 0.164 0.051 0.6- 0.177 0.002 0.284 0.051 0.5- 0.326 0.002 0.365 0.853 0.4- 0.503 0.003 0.637 0.069 0.3- 0.723 0.016 0.2- 0.1- 00 I —1—1—1—1— I —i—i—i—i—r 1 —i—i—i—i— 1 . —i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i— 0.1 0.2 0.3 0.4 0.5 0.6 ^(MeV) 0.7 FIGURE 1. Radiation spectra of 131I radionuclide: a) the real beta spectrum and the average beta spectrum used in MIRD, b) the photons spectrum. Volume of one thyroid lobe(ml) FIGURE 2. Variation of total energy deposition per decay against the volume of thyroid lobe. Materials and methods MCNPX is a general purpose, continuous and discrete energy, generalized-geometry, time-dependent code to simulate particles transport, based on Monte Carlo method. It is an extremely useful tool for radiations transport simulation and tracks about 40 particles including some light ions.7 The code is written in Fortran 90 and contains flexible source and tally options; it utilizes the latest nuclear cross section libraries with a data library of photons cross-section ranging from 1 keV to 100 GeV. This code has been used to calculate the energy deposition from beta and gamma rays of 131I for a thyroid lobe of ellipsoidal shape, with the major axis two times of the minor axis, 1.05 g/cm3 density and with a mass varying from 1 g to 25 g. In running MCNPX code, we have considered the "full transport" for both gamma and beta rays; that is, we have considered that beta rays do not deposit their energy in a starting point, but they undergo many Coulomb interactions, so that a significant portion of their energy, near the surface of lobes, escapes and is stored out of the thyroid lobes. Figure 1A shows the real beta spectrum of 131I that we have used for our simulation, and the average beta spectrum used in MIRD, according to the Evaluated Nuclear Structure Data File (ENSDF) decay data. In the MIRD format, the beta spectrum includes 5 discrete lines, each representing the average beta energy and the yield for 131I beta radiations.8 As well as, the gamma spectrum is presented in Figure 1B. The adult 70 kg human MIRD5 phantom has been used: the source organ was the thyroid gland with a uniform 131I distribution; the neck has been simulated with more detailed organs including skin, adipose layer under the skin, bone, spinal cord, thyroid lobes, and the remaining part as soft tissue. We have considered for soft tissue 1.05 g/ cm3 density and the ICRP composition. As it is well known, the basic formula for absorbed dose rate used in MIRD formulation is: ' \ A dD dt ■ = w \ i [i] m where w is a proportional constant, A is the radionuclide activity within the source organ, n is the number of radiations with energy emitted per one decay, &i is the fraction of energy emitted in the source that is absorbed in the target organ, and m is the mass of the target. When the thyroid is considered both as source and target organ, the beta and gamma rays absorption fraction (&) de pends on thyroid volume. We have selected a as proper parameter to eval uate, by rewriting of Equation [1]: 'Ea.W dD A — = /and it increases by volume, because the increasing volume to surface ratio of ellipsoidal lobe causes the decrease of radiations fraction escaping from the lobe (Figure 2). The calculated value of a against the thyroid volume lobe has been presented in Figure 3. It can be seen that a has a significant difference with the previous constant value, ranging from 10% to -1% when the lobe's volume varies from 1 ml to 25 ml. For a 10 g lobe, our calculation shows about 2.2% difference with MIRDOSE3 a value. This difference comes from two main sources: the first is the beta spectrum, as we have used the spectrum of 131I taken from a reference published by Eckerman et al. in 1994 in Health Physics910, with a mean beta energy of 0.1822 MeV per disintegration; the second is due to considering in our calculation the full beta and gamma transport in an ellipsoidal thyroid lobe (Figure 3). We have used the photon energy deposition tally, called F6:p in MCNPX code, to calculate the photon energy deposition per unit of mass, in the other organs of the body, due to a decay in the source organ. It is clear that the result is proportional to the dose organ per one decay in the source. The energy deposition in other organs of neck as a function of the thyroid lobe volume per decay 1.0x10^1—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i-0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thyroid lobe volume(ml) FIGURE 4. The variation of the energy deposition in other organs of neck respect to the thyroid lobe volume, per decay. 3.0x10s- Head Body Legs 2.5x10s- 3 2.0x10s-1 "X 1.5x10s- O- 1.0x10s-£ 5.0x107- 0 0 | , , 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thyroid lobe volume(ml) FIGURE 5. The variation of the energy deposition to head, body and legs respect to the thyroid lobe volume, per decay. of 131I has been shown in Figure 4. As it is predictable, by increasing the lobe volume the dose in the bone and spinal cord increases but for other organs it decreases. The energy depositions per decay to organs far from the thyroid, including head, body and legs have been presented in Figure 5. Conclusions The result shows that considering the lobe volume or mass has a significant effect over the absorbed dose calculation in thyroid gland. So, an accurate determination of the active volume of thyroid is very important in activity evaluation for radi-omethabolic therapy by Iodine-131. As well as, according to our calculation, we suggest re-evaluating the value for gamma and beta sources when the source organ is the same as target and its volume or mass variation among different patients is considerable. Acknowledgements The authors would like to thank Prof. G. Furlan and Prof. D. Treleani head of TRIL program at ICTP, Trieste, Italy, for their support to this work. References 1. Traino AC, di Martino F, Lazzeri M, Stabin MG. Influence of thyroid volume reduction on calculated dose in radioiodine therapy of Graves' hyperthyroidism. Phys Med Biol 2000; 45: 121-29. 2. Becker DV. Choice of therapy for Graves' hyperthyroidism. N Eng J Med 1989; 311: 454-66. 3. Farrar JJ, Toft AD. Iodine-131 treatment of hyperthyroidism. Clin Endocrinol Oxf 1991; 35: 207-12. 4. Vardar E, Erkan N, Bayol U, Yilmaz C, Dogan M. Metastatic tumours to the thyroid gland: report of 3 cases and brief review of the literature. Radiol Oncol 2011; 45: 53-8. 5. 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