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Empiric Radioactive Iodine Dosing Regimens Frequently Exceed Maximum Tolerated Activity Levels in Elderly Patients with Thyroid Cancer

¹ Endocrinology Service, Department of Medicine, Memorial Sloan-Kettering Cancer Center, New York, New York; and ² Nuclear Medicine Service, Departments of Medicine and Radiology, Memorial Sloan-Kettering Cancer Center, New York, New York

Correspondence: For correspondence or reprints contact: R. Michael Tuttle, MD, Endocrinology Service, Department of Medicine, Memorial Sloan-Kettering Cancer Center, Schwartz Bldg., Room 713, 1275 York Ave., New York, NY 10021. E-mail: tuttlem{at}mskcc.org

Although ¹³¹I-iodine (RAI) therapy is a mainstay in the treatment of metastatic thyroid cancer, there is controversy regarding the maximum activity that can safely be administered without dosimetric determination of the maximum tolerable activity (MTA). At most institutions, a fixed empiric dosing strategy is often used, with administered activities ranging from 5.55 to 9.25 GBq (150–250 mCi). In our experience with dosimetry, we have observed that this empiric dosing strategy often results in administered RAI activities exceeding the MTA safety limit of 200 cGy (rads) to the blood or bone marrow in many patients with metastatic thyroid cancer. Methods: We retrospectively analyzed 535 hypothyroid dosimetry studies performed as part of routine clinical care in 328 patients with apparently normal renal function. Results: The MTA was less than 5.18 GBq (140 mCi) in 3%, less than 7.4 GBq (200 mCi) in 8%, and less than 9.25 GBq (250 mCi) in 19%. Analysis of MTA values by age at the time of dosimetry revealed little change in the MTA until the age of 70 y, when a significant decrease occurred. An empiric administered activity of 7.4 GBq (200 mCi) would exceed the MTA in 8%–15% of patients less than 70 y old and 22%–38% of patients 70 y old or older. However, administration of 9.25 GBq (250 mCi) would exceed the MTA in 22% of patients less than 70 y old and 50% of patients 70 y old or older. Factors associated with a lowering of MTA to less than 9.25 GBq (250 mCi) were age at dosimetry greater than 45 y, the female sex, subtotal thyroidectomy, and RAI-avid diffuse bilateral pulmonary metastases. Conclusion: Administered RAI activities of less than 5.18 GBq (140 mCi) rarely exposed blood to more than 200 cGy except in the very elderly. However, administered activities of 7.4–9.25 GBq (200–250 mCi) frequently exceeded the calculated MTA in patients 70 y old or older. Therefore, dosimetry-guided RAI therapy may be preferable to fixed-dose RAI treatment strategies in older patients with thyroid cancer and in patients with RAI-avid diffuse bilateral pulmonary metastases, even when renal function is normal.

Key Words: dosimetry • radioactive iodine • therapy • thyroid cancer

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