Optimization of thyroid measurements with Monte Carlo simulation in in-vivo monitoring

Ionizing radiations can cause external or internal exposure to the human body. The internal exposure is typically determined in two steps. In the first step the actual activity present in the body is determined by direct or indirect monitoring methods. By direct measurements, the activity in the whole or part of the human body can be determined (in-vivo). In the second step the intake value and the associated committed dose can be estimated on the basis of measured data and considering necessary assumptions on exposure conditions (time and route of intake, chemical form etc.). In case of the in-vivo measurement the low activity of the human body, the non-standard geometry and the limited measurement times considerably increase the measurement uncertainty. Thyroid measurements are also influenced by various parameters such as the age of the patient, the shape of thyroid, position of the thyroid inside the human body, the detector distance from the body surface, the distribution of the activity within the organ. Physical phantoms can be used for efficiency calibration of the measurement system. After efficiency calibration of the thyroid counter with physical phantoms, efficiency calibration was performed by numerical methods to optimize the measurements. In order to take into account uncertainties introduced by these factors in the estimation of dose attributed to different radioactive isotopes of iodine, numerical simulations based on Monte Carlo photon transport techniques were performed, detector response and corresponding detection efficiencies were calculated. During our studies uncertainties due to differences in the distance of the thyroid from neck surface (0.6-2.2 cm), different size and shape of thyroid (e.g. variant thyroid shapes of children, teenagers and adults) and the detector distance from the body surface (4-12 cm) were analysed. All these factors affect the accuracy of dose estimation as well. The computed uncertainties due to various parameters should be taken into account while estimating the activity of iodine isotopes in the thyroid. (author)