Abstract
Objective: The underestimation of thyroid uptake measurements with iodine isotopes has been a generally accepted theory in patients with substernal goiter due to attenuation by the chest wall. The extent of this underestimation is not well known. In this study, we calculate the attenuation of the emissions from iodine-123 utilizing a cadaver chest wall with a thyroid probe in order to better understand the potential severity of this underestimation. Methods: A capsule of 11.1 Megabequerels (MBq) of iodine-123 was measured using a thyroid probe directly, in a standard neck phantom and behind a cadaveric chest wall that included the soft tissues and bony structures (sternum). Results: Using the thyroid probe, the calculated attenuation of the iodine capsule by the neck phantom was 18% and by the chest wall was 35%. Conclusion: Thyroid uptake in the case of substernal goiter may be underestimated by standard techniques using a neck phantom. The composition of the chest wall can vary greatly and substernal extent of the goiter would be difficult to calculate with a high level of accuracy on a routine basis. Direct comparison with a cadaveric specimen leads to similar issues but does give us a rough estimation of the extent of this issue. This study suggests that the attenuation by the chest wall can be substantial. Knowledge of the extent of the substernal component of the thyroid gland may be useful if the uptake measurement is used to calculate doses for the treatment of hyperthyroidism in patients with a substernal goiter.
