Dosimetry-Guided Radioactive Iodine Treatment in Patients with Metastatic Differentiated Thyroid Cancer: Largest Safe Dose Using a Risk-Adapted Approach

Abstract

This study is a retrospective analysis of 124 differentiated thyroid cancer patients who underwent dosimetric evaluation using MIRD methodology over a period of 15 y. The objectives of the study were to demonstrate the clinical use of dosimetry-guided radioactive iodine ([RAI] ¹³¹I) treatment and the safe and effective application of a 3-Gy bone marrow (BM) dose in patients with differentiated thyroid cancer. Methods: Tumor and BM dose estimates were obtained. The administered activity that would deliver a maximum safe dose to the organ at risk (red BM or lungs) was determined as well as the resulting doses to the metastases. The clinical benefit of an individual RAI treatment was predicted on the basis of the dose estimates and the expected therapeutic response. Each patient’s response to treatment was assessed clinically and by monitoring the hematologic profile. Results: One hundred twenty-four patients underwent 187 dosimetric evaluations. One hundred four RAI treatments were performed. A complete response at metastatic deposits was attained with absorbed doses of >100 Gy. No permanent BM suppression was observed in patients who received absorbed doses of <3 Gy to BM. The maximum administered dose was 38.5 GBq (1,040 mCi) with the BM dose limitation. Conclusion: Dosimetry-guided RAI treatment allows administration of the maximum possible RAI dose to achieve the maximum therapeutic benefit. Estimation of tumor dose rates helps to determine the curative versus the palliative intent of the therapy.