Increasing Efficacy and Safety of Treatments of Patients with Well-Differentiated Thyroid Carcinoma by Measuring Body Retentions of ¹³¹I

Abstract

There is no consensus on the amount of ¹³¹I for treatment of patients with well-differentiated thyroid carcinoma; usual amounts vary widely. Body retention of ¹³¹I 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 ¹³¹I to ensure safety and increase efficacy. Methods: After withdrawal of thyroid hormone in 87 patients, the fractional retention of diagnostic ¹³¹I in each body was measured at 2 d by a scintillation probe. In 43 patients, the retention was measured 2 d after therapeutic ¹³¹I. 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 ¹³¹I, exhibited a significantly lower mean retention. In 43 patients, retention of diagnostic ¹³¹I was highly correlated with retention of therapeutic ¹³¹I: diagnostic predicted therapeutic retention with a mean error of 0.04. In 10 patients receiving thyroxine, the mean retention of diagnostic ¹³¹I after recombinant human TSH (rhTSH) was strikingly lower, 0.06, with a range of 0.016–0.16. Conclusion: Body retentions of ¹³¹I are easily measured and vary considerably among patients. Because increased therapeutic ¹³¹I will impart greater irradiation of tumor, and body retention has been accepted as an index of toxicity from ¹³¹I, the use of body retention could enable prescriptions of therapeutic ¹³¹I that enable increased efficacy while ensuring safety. If tumor retention is not proportionally decreased with the body retention of ¹³¹I after rhTSH, then rhTSH may enable increased therapeutic efficacy.