The assessment of the effect of thyroid size and shape on the activity estimate using Monte Carlo simulation

Monte Carlo simulations have been used to assess the uncertainty introduced into an activity estimate of radioiodine (125I and 131I) in the thyroid when the size and shape of the gland differs from that of the calibration phantom. The detector dimensions for the 125I simulations were small (diameter 2.54 cm, thickness 0.2 cm); medium (diameter 7.62 cm, thickness 0.2 cm); large (diameter 30.48 cm, thickness 0.2 cm). The detector dimensions for the 131I simulations were small (diameter 2.54 cm, thickness 3.2 cm); medium (diameter 7.62 cm, thickness 6.4 cm); large (diameter 30.48 cm, thickness 11.0 cm). Shapes simulated have included thyroid glands with a third (pyramidal) lobe, no isthmus, and rotated lobes. Sizes simulated have been 10 g, 20 g, and 40 g. The results show that the size of the uncertainty is dependent on the detector size, the neck-to-detector distance, and the type of radioiodine being measured (i.e., 131I or 125I). The worst case bias (on contact counting) for either 125I or 131I using the different sized detectors is as follows: small is between -40% and 40%; medium is between -33% and 17%; large is between -20% and 5%. If the detectors are placed at about 15 cm from the neck the bias values drop so that the uncertainty introduced if the subject has a smaller than standard thyroid becomes insignificant and becomes much improved if the thyroid is larger than the standard. The bias values for the detectors are as follows: small is between -20% and 5%; medium is between -23% and 2%; large is between -21% and 2%.