Thyroid stunning in vivo and in vitro.

AIM: To review published in-vivo and in-vitro quantitative dosimetric studies on thyroid stunning in order to derive novel data applicable in clinical practice.
METHODS: A non-linear regression analysis was applied to describe the extent of thyroid stunning in thyroid remnants, as a function of the radiation absorbed dose of diagnostic radioiodine-131 (I), in thyroid cancer patients investigated in four in-vivo studies. The regression curves were fitted using individual patient absorbed doses or the mean absorbed doses for the groups of patients. Fitted curves were compared with two recent models, the first found in patients with benign thyroid disease and the second found in cultured thyroid cells after I irradiation.
RESULTS: The extrapolated absorbed doses for the onset of thyroid stunning were 0 Gy delivered to thyroid cells in vitro, and < or =4 Gy and 34 Gy delivered to thyroid cells in vivo (malignant and benign conditions, respectively). Thyroid stunning amounted to roughly 50% in the case of 2 Gy delivered to thyroid cells in vitro, and in the case of < or =30 Gy and 472 Gy delivered to thyroid cells in vivo (malignant and benign conditions, respectively).
CONCLUSIONS: There is no scintigraphically sufficient diagnostic amount of I that can be given prior to I therapy for thyroid cancer that does not cause thyroid stunning, i.e. it is not recommended to deliver pre-therapeutically more than a few gray (<5 Gy) into thyroid remnants. More investigations are required to confirm the proposed in-vitro and benign in-vivo models, but characteristic absorbed doses presented so far for in-vitro vs. in-vivo malignant vs. in-vivo benign thyroid environments differ roughly by an order of magnitude.