A systematic experimental study of parameters influencing 131-iodine in vivo spectroscopic measurements using age-specific thyroid phantoms.

In case of nuclear accident, the internal exposure monitoring of the population will preferably focus on the detection of 131I in the thyroid by in vivo monitoring. In most cases, the calibration of in vivo monitoring is performed with an adult thyroid phantom, which raises doubts regarding the relevance of child exposure assessment. In this study, the influence on the calibration of the thyroid volume, the counting distance and the positioning variations are studied experimentally in a systematic way. A NaI and a germanium detector along with a realistic age-specific set of four thyroid phantoms were used to carry out this study. The thyroid phantom volumes correspond to the following ages: 5, 10, 15 and adult. It was found that the counting efficiency varies linearly with the thyroid volumes for both detectors and whatever the phantom-detector distance is. The variation in counting distance strongly influences the measurement. Whatever the thyroid volume, a 30% difference in efficiency was found between the measurement at the contact and 1 cm for the NaI detector. A mathematical model giving the variation of the counting efficiency as a function of phantom-detector distance is provided. The study of positioning uncertainty has shown that the lateral/vertical displacements induce negligible efficiency variations and that it is relatively independent of the thyroid volume. The counting distance is a major parameter, which must be considered to assess the uncertainty of the subjects' measurements. The data reported here might serve to extract useful orders of magnitude when similar detectors are used. For other detectors, a similar trend might be expected and the information provided here could reduce the amount of experimental work needed to obtain it.

Development and test of sets of 3D printed age-specific thyroid phantoms for 131I measurements.

In the case of a nuclear reactor accident the release contains a high proportion of iodine-131 that can be inhaled or ingested by members of the public. Iodine-131 is naturally retained in the thyroid and increases the thyroid cancer risk. Since the radiation induced thyroid cancer risk is greater for children than for adults, the thyroid dose to children should be assessed as accurately as possible. For that purpose direct measurements should be carried out with age-specific calibration factors but, currently, there is no age-specific thyroid phantoms allowing a robust measurement protocol. A set of age-specific thyroid phantoms for 5, 10, 15 year old children and for the adult has been designed and 3D printed. A realistic thyroid shape has been selected and material properties taken into account to simulate the attenuation of biological tissues. The thyroid volumes follow ICRP recommendations and the phantoms also include the trachea and a spine model. Several versions, with or without spine, with our without trachea, with or without age-specific neck have been manufactured, in order to study the influence of these elements on calibration factors. The calibration factor obtained with the adult phantom and a reference phantom are in reasonable agreement. In vivo calibration experiments with germanium detectors have shown that the difference in counting efficiency, the inverse of the calibration factor, between the 5 year and adult phantoms is 25% for measurement at contact. It is also experimentally evidenced that the inverse of the calibration factor varies linearly with the thyroid volume. The influence of scattering elements like the neck or spine is not evidenced by experimental measurements.