Method for calibration of the NaI(Tl) 3”x3” detector for in vivo measurements in patients undergoing radioiodine therapy and occupationally exposed individuals to 131I

Abstract

According to studies by the Instituto Nacional do Câncer (INCA), in recent years the incidence of thyroidcancer is the fastest growing in relation to other types. For the treatment is required the exact amount of131I isotope for each patient resulting on manipulation of this isotope in nuclear medicine services byindividuals who are considered occupationally exposed (OEI). This work aims to establish themethodology for calibration of the detection system used to determine the therapeutic activity of 131I,required to release the desired absorbed dose in the thyroid gland, as well as in monitoring OEI thatmanipulate this radionuclide in nuclear medicine services. This is one of the early stages ofimplementation of Laboratório de Dosimetria Interna at the Centro Regional de Ciências Nucleares doNordeste (CRCN-NE). The detector system consists of a NaI(Tl) 3"x 3" associated with electronicdevices and Genie 2000 software. Sources of 60Co, 137Cs and 133Ba were utilized for calibration. Firstly,measurements were carried out in non-exposed individuals for obtaining the Minimum DetectableActivity (MDA) and then Minimum Detectable Incorporation (MDI) and the Minimum Effective DoseDetectable (MEDD) were determined through biokinetic models provided by the ICRP 67 and edited bythe AIDE software version 6 to workers and patients. The values for the MEDD were 3.57 x 10-2 mSv,5.56 x 10-2 mSv and 10.7 x 10-2 mSv to 1, 7 and 14 days, respectively, after 131I incorporation by workers.As for patients, MEDD were 3.44 x 10-2 mSv, 5.63 x 10-2 mSv and 10.9 x 10-2 mSv for the same periodsof time. It is applicable to the assessment of dose OEI, since it presents less than the recommended minimum of 1 mSv. The technique also has adequate sensitivity for evaluation of activity present in thethyroid of patients with thyroid dysfunction.