Calibration of gamma cameras for in vivo monitoring of workers occupationally exposed to unsealed sources in nuclear medicine

The routine manipulation of unsealed sources of various radionuclides for therapy and diagnostic in nuclear medicine poses a significant risk of internal exposure to the staff. The critical group includes workers who develop a variety of wet operations such as dose fractionation, preparation of radiopharmaceuticals and administration to the patients. The internal monitoring of such workers is recommended by the IAEA for those who might be exposed to effective doses above 1 mSv per year. According to the CNEN regulations, the Radiation Safety Officer of the facility is responsible for the selection of workers to be submitted to a routine monitoring. Currently, in Brazil, internal monitoring can only be executed in a few Laboratories located in governmental institutions, which turns unfeasible to implement such control in a national basis. On the other hand, such monitoring procedures could be carried out in the nuclear medicine services by using their image diagnose equipment. Among the radionuclides used in nuclear medicine, iodine-131 (131I) is still largely applied for diagnostic and treatment of thyroid diseases. The objective of this work is to establish a calibration protocol and an in vivo measurement procedure to evaluate incorporation of 131I using gamma cameras. The calibration factors - necessary to calculate the activity deposited in the human body - of two gamma cameras installed in University Hospitals located in the city of Rio de Janeiro were determined with a neck-thyroid phantom developed at the In Vivo Monitoring Laboratory of IRD. Both equipment presented equivalent sensitivities for in vivo measurement of 131I in thyroid. The results show that the gamma camera can be used for routine monitoring of 131I since the minimum detectable effective dose for a standard count time of 10 minutes is below 1 mSv. (author)