Determination of uncertainties associated to the in vivo measurement of iodine-131 in the thyroid

•Uncertainties associated to the in vivo measurement of 131I were evaluated.•Both systems presented reliable techniques for occupational monitoring purposes.•Activity distribution and overlaying structures are the most critical parameters.•The uses of a mechanical coordinate system is recommended to guarantee reproducibility.•Geometric deviation values are comparable to international standards.

Intakes of radionuclides can be estimated through in vivo measurements, and the uncertainties associated to the measured activities should be clearly stated in monitoring program reports. This study aims to evaluate the uncertainties of in vivo monitoring of iodine 131 in the thyroid. The reference values for high-energy photons are based on the IDEAS Guide. Measurements were performed at the In Vivo Monitoring Laboratory of the Institute of Radiation Protection and Dosimetry (IRD) and at the Internal Dosimetry Laboratory of the Regional Center of Nuclear Sciences (CRCN-NE). In both institutions, the experiment was performed using a NaI(Tl) 3′′3″ scintillation detector and a neck-thyroid phantom. Scattering factors were calculated and compared in different counting geometries. The results show that the technique produces reproducibility equivalent to the values suggested in the IDEAS Guide and measurement uncertainties is comparable to international quality standards for this type of in vivo monitoring.