Modifications to the ORNL Phantom Series in Simulation of the Responses of Thyroid Detectors

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Abstract
Childhood thyroid cancer appears to be a major consequence of the Chernobyl accident. To quantify the dose-response relationship, reliable estimates of the radiation dose to the gland are needed. A large number of direct measurements of radioiodine in the thyroid gland of individuals were made in Belarus in May-June 1986 following the accident. Data were obtained using a variety of instruments and the measurements were conducted under various conditions. Mathematical simulation of the measurements has been undertaken to calibrate the instruments for the typical human-detector geometry and to quantify the confounding of the measurements by radionuclides present both within and upon the body of the subjects. During the course of this work, it was found necessary to modify the Oak Ridge National Laboratory (ORNL) mathematical phantom series. The modifications were undertaken to provide a more realistic representation of the upper thorax and neck regions of the phantoms. The major modification reshaped the thorax by intersecting the elliptical cylinder of the trunk with a parabolic sheet which forms the shoulders. This modification required that the upper rib cage be modified and that the clavicles be repositioned. The geometric model of the thyroid was also modified to simplify the equations defining the gland. The paper discusses these modifications and provides the equations defining the phantom in the study.

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