Dosimetry-guided high-activity 131I therapy in patients with advanced differentiated thyroid carcinoma: initial experience

In patients with advanced differentiated thyroid carcinoma (DTC), therapy with the highest safe I-131 activity is desirable to maximize the tumour radiation dose yet avoid severe myelotoxicity. Recently, the European Association of Nuclear Medicine (EANM) published a standard operational procedure (SOP) for pre-therapeutic dosimetry in DTC patients incorporating a safety threshold of a 2 Gy absorbed dose to the blood as a surrogate for the red marrow. We sought to evaluate the safety and effectiveness in everyday tertiary referral centre practice of treating advanced DTC with high I-131 activities chosen primarily based on the results of dosimetry following this SOP. We retrospectively assessed toxicity as well as biochemical and scintigraphic response in our first ten patients receiving such therapy for advanced DTC. The 10 patients received a total of 13 dosimetrically guided treatments with a median administered activity of 14.0 GBq (range: 7.0-21.4 GBq) I-131. After 6 of 13 treatments in 6 of 10 patients, short-term side effects of I-131 therapy, namely nausea, vomiting or sialadenitis, were observed. Leukocyte and platelet counts dropped significantly in the weeks after I-131 treatment, but returned to pre-treatment levels by 3 months post-therapy. Serum thyroglobulin levels decreased after 12 of 13 treatments (median reduction: 58%) in 9 of 10 patients. In our initial patient cohort, high-activity I-131 therapy for advanced DTC based on pre-therapeutic blood dosimetry following the EANM SOP was safe and well tolerated. Such treatment almost always produced a partial biochemical tumour response.