Comparative evaluation of gamma cameras performance for internal monitoring of workers exposed to I-131 in nuclear medicine services

• Published in: Brazilian Journal of Radiation Sciences 2020-06, Vol.8 (1A)
• Main Authors: Salomão Marques de Oliveira, Ana Letícia Almeida Dantas, Bernardo Maranhão Dantas
• Format: Article
• Language: English
• Subjects: internal monitoring; iodine 131; nuclear medicine; radiation protection
• ISSN: 2319-0612
• DOI: 10.15392/bjrs.v8i1A.996

Introduction: In the nuclear medicine practice, a variety of radionuclides are handled for diagnostic and therapy which represents a significant risk of internal and external exposure for workers. Such occupational exposure requires the implementation of a routine monitoring plan, including intakes of radionuclides. Currently, in Brazil, approximately 90 Nuclear Medicine Services (NMS) are authorized by the Nacional Nuclear Energy Commission of Brazil to handle 131I for therapy purposes [1], resulting in a significant number of workers routinely exposed to internal exposures. 131I remains as one of the most frequently used radionuclides in NMS, and presents higher risks of intake and internal doses. The International Atomic Energy Agency recommends implementing an internal monitoring program of this group of workers. However, in Brazil there are not qualified laboratories to attend the demand of internal monitoring [2]. The IRD has developed several studies proposing the use of detectors available in NMS for 131I occupational monitoring. Among such detectors, the gamma camera stands out to present the highest sensitivities for the proposed application. This work presents a comparison of the performances of gamma cameras available in six NMS in Brazil for 131I in vivo monitoring purpose.   Methodology: The technique consists basically in (i) determining the calibration factors for the measurement of 131I in the thyroid, (ii) estimation of the minimum detectable activities and the corresponding Minimum detectable intakes and effective doses. The IRD neck-thyroid phantom containing a 133Ba source certified by the LNMRI of IRD was used to calibrate the medical devices evaluated in this study. The standard geometry consisted in positioning the phantom at 12 cm distance to the Gamma Cameras (GC) detectors. The count time was determined according to detectors sensitivity for a measurement performed at 1 and 7 days after the intake, considering a weekly generic monitoring frequency, resulting in 48 monitoring periods per year.   Calibration of the Detection System   The Calibration Factor (CF) is calculated as follows: CFctg/Bq = (total counts – total background counts) / 131I Eq Ac Where: 131I Eq Ac = Equivalent activity of 131I in neck-thyroid phantom, corrected for the calibration date.   Evaluation of the Sensitivity of the Methods   The Minimum Detectable Activity (MDA) [3] is calculated as follows: MDABq = (4,65 x √N) / CF Where: N = Total background counts