Indian Journal of Nuclear Medicine
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Year : 2021  |  Volume : 36  |  Issue : 3  |  Page : 273-281

Prediction of Absorbed Dose to Normal Organs with Endocrine Tumors for I-131 by use of 99mTC Single Photon Emission Computed Tomography/Computed Tomography and Geant4 Application for Tomographic Emission Simulation

1 Department of Medical Physics and Radiation Sciences, Faculty of Paramedicine, Sabzevar University of Medical Sciences, Sabzevar, Iran
2 Department of Medical Physics, Faculty of medicine, Semnan University of Medical Sciences, Semnan, Iran
3 Endocrinologist, Department of Internal Medicine, Faculty of Medicine, Semnan University of Medical Sciences, Semnan, Iran

Correspondence Address:
Dr. Hadi Taleshi Ahangari
Department of Medical Physics, Faculty of Medicine, Semnan University of Medical Sciences, Semnan
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijnm.ijnm_6_21

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Introduction: This study aimed to predict the dose absorbed by normal organs with neuroendocrine tumors for 131I using single photon emission computed tomography/computed tomography (SPECT/CT) images and Geant4 application for tomographic emission (GATE) simulation. Materials and Methods: Four to 5 whole-body planar scan series, along with one SPECT/CT image, were taken from four patients following 99mTc-hynic-Tyr3-octreotide radiotracer injection. After image quantification, the residence time of each organ was calculated using the image analysis and the activity time curves. The energy deposit and dose conversion (S-value) were extracted from the GATE simulation for the target organs of each patient. Using the residence times and S-values, the mean absorbed dose for the target organs of each patient was calculated and compared with the data obtained from the standard method. Results: Very close agreement was obtained between the S-value of the self–organ irradiation. The mean percentage difference between the two methods (i.e. GATE and Medical Internal Radiation Dose [MIRD]) was 1.8%, while a weak agreement was observed for cross-organ irradiation. The percentage difference between the total absorbed doses by the organs was 2%. The percentage difference between the absorbed doses obtained for tumors and three considered normal organs estimated by the GATE method was slightly higher than the MIRD method (about 11% on average for tumors). Conclusion: Regardless of the small difference between the obtained results for the organs and absorbed doses of the tumors in the present study, patient-specific dosimetry by the GATE methods is useful and essential for therapeutic radionuclides such as 131I due to high cross-dose effects, especially for young adult patients, to ensure the radiation safety and increase the effectiveness of the treatment.

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