Radioiodine therapy for metastatic differentiated thyroid cancer: Determination of the maximum safe dose until 48 hours after the administration of a tracer dose

Objectives In radioiodine therapy for differentiated thyroid cancer, the maximum safe dose (MSD) to be administered to patients is based on considering an absorbed dose limit of 2-3Gy in bone marrow and maximum body retention of 120mCi at 48h. The objective of this study is to compare the calculated MSD considering the tracer dose kinetic in a study of 96h versus study of 48h.

Methods 39 patients with differentiated thyroid cancer and treated with MSD were included in this retrospective study. The iodine-131 retention rates in the blood and whole-body were measured at 0, 4, 24, 48, 72, and 96h after tracer dose administration with additional measurement of whole-body after 2h. MIRD methodology was used in order to estimate the absorbed dose in bone marrow. The MSD was limited with radiation of 2Gy and 3Gy to the bone marrow and whole-body retention of 120mCi at 48h.

Results The mean tracer administered dose was 2.16mCi. The average retention rate of iodine-131 in the blood samples at 24, 48, 72 e 96 h were 2.9%, 1.0%, 0.5% e 0.3%, and in the whole-body was 30%, 13%, 6% e 4%. The mean absorbed dose in bone marrow was 6.3mGy/mCi in the study of 96h, and 5.2mGy/mCi in 48h. The average of MSD was 624mCi to the limit of 2Gy in bone marrow, and 839mCi to 3Gy. The ratio between the MSD using the study of 48h to 96h was 1.12 to the limit of 2Gy and 0.86 to 3Gy.

Conclusions The tracer activity kinetic study of 48h demonstrated to be comparable to the study of 96h for the largest safe dose determination.