A study of thyroid radioiodine monitoring by Monte Carlo simulations: implications for equipment design

Abstract

Monte Carlo simulations have been performed to evaluate the design of collimated detectors used to measure 125I or 131I in the thyroid gland. Two detector sizes were simulated for each radioisotope: (i) for 125I monitoring 2.54 cm diameter and 7.62 cm diameter and 0.2 cm thickness and (ii) for 131I 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 body in varying amounts to assess the efficacy of collimation. The results show that the detector should be sufficiently large so that its solid 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 within the subject's body is not normally required. It may be of more value as a positioning device and spacer ensuring an appropriate and constant neck to detector distance than in cutting down counts from extrathyroidal activity. In specifying a sensitive detector system for monitoring intrathyroidal radioiodine, a wide angle of acceptance and sufficient detector crystal thickness take precedence over collimation and shielding.

Relevance of accurate Monte Carlo modeling in nuclear medical imaging

“…Monte Carlo simulations have also been performed to evaluate the design of collimated detectors used to measure 125 I or 131 I in the thyroid gland. 124 Two detector sizes were simulated for each radioisotope and activity was placed in both the gland and the remainder of the body in varying amounts to assess the efficacy of collimation. This study showed that a wide angle of acceptance and sufficient detector crystal thickness take precedence over collimation and shielding.…”

Investigating the Impossible: Monte Carlo Simulations

“…For example, Monte Carlo simulations have been used in the following: help users determine the optimum detector size for thyroid monitoring of workers occupationally exposed to either 125 I or 131 I (1); to determine the effect of counting geometry on thyroid monitoring for 131 I (2); to determine the effect of a heterogeneous aerosol deposition on the activity estimate obtained using a lung counter that was calibrated assuming a homogeneous deposition (3); and to determine the effect of counting geometry on thyroid monitoring for 125 I (4). Some other examples (5,6) of the impossible that have been investigated by the HML include: removing the ribs from a phantom's chest to assess the impact of photon blockage by ribs at various photon energies; using photon energies for which there is no physical equivalent to fill in areas covered by physical equivalents; changing the chest depth profile of a phantom; extending the adipose content to values not physically achievable; and evaluating the effect of 131 I distribution on a counting system. In all of the above studies the code used was MCNP (Monte Carlo NParticle transport system).…”