Background: Elderly patients with differentiated thyroid cancer (DTC) tend to have more advanced disease at presentation, for which high activities of radioiodine (RAI) are often recommended. However, the 2015 American Thyroid Association guidelines recommend that empirically administered activities of RAI >150 mCi should be avoided in patients >70 years of age, based on calculated bone-marrow exposure according to two dosimetry-based studies. This study aimed to evaluate the effect of RAI treatment on bone-marrow function in elderly DTC patients. Methods: DTC patients ≥70 years of age who received RAI treatment and on whom a complete blood count was performed before and after treatment were included. Blood counts within one year before RAI and one year following treatment were compared in order to assess for marrow suppression. The impact of demographic, clinical, and laboratory variables on complete blood count were assessed. Results: One hundred fifty-three treatments in 122 patients met inclusion criteria, with a mean patient age of 76 ± 4.3 years, and 75% were women. High-risk features at presentation included T4 disease in 17%, lymph node metastases in 34%, and distant metastases in 14%. Mean RAI activity was 136.8 ± 48 mCi (82% ≥ 100 mCi, 66% ≥ 150 mCi). Of 153 RAI treatments analyzed, 114 (74%) were first treatments, 28 (18%) second treatments, seven (5%) third treatments, and four (3%) fourth treatments. At 0-3 months after RAI treatment, there was a statistically significant decrease in platelets (238 ± 66 vs. 216 ± 69 × 109/L, 10% decrease; p < 0.001), white blood cells (WBC; 6.9 ± 2 vs. 6.1 ± 1.9 × 109/L, 13% decrease; p < 0.001), and hemoglobin (Hb) in women (12.8 ± 1.1 vs. 12.4 ± 1.1 g/dL, 3% decrease; p = 0.01). Mean platelets, WBC, Hb in women, and lymphocytes remained decreased (but within the reference range) one year after treatment. Subgroup analysis demonstrated platelet suppression only with activities ≥100 mCi, and WBC and Hb suppression only with activities ≥150 mCi, with mean values within the reference ranges. There were no clinically significant cytopenia events during follow-up. Conclusions: Empiric RAI treatment in elderly patients causes mild bone-marrow suppression, with little clinical significance. Activities of 150-200 mCi can be safely used when indicated.
Keywords: bone marrow; complete blood count; differentiated thyroid cancer; elderly; radiation; radioiodine.