Estimation of Chest Wall Attenuation of ¹²³I Emissions in Substernal Goiter: A Phantom Study

In patients with substernal goiter, the generally accepted theory is that thyroid uptake measurements with iodine isotopes are underestimated because of attenuation by the chest wall. The extent of this underestimation is not well known. In this study, we calculated the attenuation of ¹²³I emissions using a cadaver chest wall with a thyroid probe to better understand the potential severity of this underestimation. Methods: A 11.1-MBq capsule of ¹²³I was measured using a thyroid probe directly in a standard neck phantom and behind a cadaver chest wall that included the soft tissues and bony structures (sternum). Results: The calculated attenuation of the iodine capsule was 18% for the neck phantom and 35% for the cadaver chest wall. Conclusion: Thyroid uptake in cases of substernal goiter may be underestimated by standard techniques using a neck phantom. The composition of the chest wall can vary greatly, and the substernal extent of the goiter would be difficult to calculate with high accuracy on a routine basis. Comparison between the cadaveric specimen and the phantom does give us a rough estimation of the differences in attenuation. Our findings suggest that 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 treating hyperthyroidism in patients with substernal goiter.