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Influence of various geometric factors on the iodine-131 uptake measurement for solitary thyroid nodules

AimThe influence of various geometric factors on 131I uptake measurements for solitary thyroid nodule was systematically investigated to derive an approach, based on routinely performed ultrasound examinations, to correct for the effect of geometric variations. MethodsThe influence of size, shape, and position of a thyroid nodule, neck-to-detector distance and neck curvature on the uptake value was analyzed with a three-dimensional model. Uptake measurements using a tissue-equivalent neck phantom were carried out to verify the calculated correction factors and also to check the influence of scatter. Sonograms of 92 patients with solitary nodules were analyzed to correct for geometric variations. ResultsThe correction factors were independent of the size and shape of the nodule, and the activity distribution of the solitary nodules can be approximated by a point source. The correction factors were mainly determined by the nodular depth and by the accuracy of the neck-to-detector distance and were affected to a lesser extent by the lateral position of the nodule as well as the curvature of the neck. The effect of scatter can be neglected if the energy window largely excludes Compton scatter, as is the case in the 131I uptake measurement. The ultrasound-derived correction factors ranged from 0.85 to 1.25. ConclusionThe proposed approach is capable of correcting for the geometric variation for a solitary nodule and can be easily applied in routine clinics. The accuracy of absorbed dose in radioiodine therapy can be improved in particular for nodules located well beneath the neck surface.