Increasing efficacy and safety of treatments of patients with well-differentiated thyroid carcinoma by measuring body retentions of 131I.

Unlabelled: There is no consensus on the amount of (131)I for treatment of patients with well-differentiated thyroid carcinoma; usual amounts vary widely. Body retention of (131)I has been shown to be a valuable index of radiation toxicity. If a broad range of body retentions occurs among patients, then high and low retentions will be a basis for modifying the usual prescriptions for (131)I to ensure safety and increase efficacy.
Methods: After withdrawal of thyroid hormone in 87 patients, the fractional retention of diagnostic (131)I in each body was measured at 2 d by a scintillation probe. In 43 patients, the retention was measured 2 d after therapeutic (131)I.
Results: Diagnostic retention varied from 0.01 to 0.51, with a median of 0.15. These retentions did not correlate with any index of health, thyroid hormone, or carcinoma status. Seventeen patients, previously treated with (131)I, exhibited a significantly lower mean retention. In 43 patients, retention of diagnostic (131)I was highly correlated with retention of therapeutic (131)I: diagnostic predicted therapeutic retention with a mean error of 0.04. In 10 patients receiving thyroxine, the mean retention of diagnostic (131)I after recombinant human TSH (rhTSH) was strikingly lower, 0.06, with a range of 0.016-0.16.
Conclusion: Body retentions of (131)I are easily measured and vary considerably among patients. Because increased therapeutic (131)I will impart greater irradiation of tumor, and body retention has been accepted as an index of toxicity from (131)I, the use of body retention could enable prescriptions of therapeutic (131)I that enable increased efficacy while ensuring safety. If tumor retention is not proportionally decreased with the body retention of (131)I after rhTSH, then rhTSH may enable increased therapeutic efficacy.