A STUDY OF THYROID RADIOIODINE MONITORING BY MONTE-CARLO SIMULATIONS - IMPLICATIONS FOR EQUIPMENT DESIGN

Monte Carlo simulations have been performed to evaluate the design of collimated detectors used to measure I-125 or I-131 in the thyroid gla nd. Two detector sizes were simulated for each radioisotope: (i) for I -125 monitoring 2.54 cm diameter and 7.62 cm diameter and 0.2 cm thick ness and (ii) for I-131 monitoring 2.54 cm diameter, 3.2 cm thickness and 7.62 cm diameter, 6.4 cm thickness. The virtual thyroid gland was 20 g. Activity was placed in both the gland and the remainder of the b ody in varying amounts to assess the efficacy of collimation. The resu lts show that the detector should be sufficiently large so that its so lid angle of acceptance when placed 15 cm anterior to the skin surface will include the whole of a moderately enlarged thyroid gland. Heavy collimation to reduce the contribution of extrathyroidal radioiodine w ithin the subject's body is not normally required. It may be of more v alue as a positioning device and spacer ensuring an appropriate and co nstant neck to detector distance than in cutting down counts from extr athyroidal activity. In specifying a sensitive detector system for mon itoring intrathyroidal radioiodine, a wide angle of acceptance and suf ficient detector crystal thickness take precedence over collimation an d shielding.