Objective: To evaluate the relationships between thyroid remnant (131)I uptake, radiation thyroiditis and remnant ablation success rate between lower (1110 MBq) and higher (3700 MBq) initial ablative (131)I dose for post-surgical ablation therapy for differentiated thyroid cancer.
Design: Patients having post-surgical administration of 1110 MBq (68 patients) or 3700 MBq (115 patients) (131)I were retrospectively reviewed. Thyroid remnant (131)I uptake on a 48 h post-administration scan was correlated with neck symptoms experienced. Patients were classified as having insignificant, mild or severe thyroiditis based on symptoms. Absent thyroid bed (131)I uptake on a follow-up 74 MBq (131)I study was considered successful ablation.
Results: 183 patients were included. Median (131)I remnant uptake was 37 MBq. 21% (39/183) of patients developed thyroiditis. Incidence and severity of thyroiditis increased with increasing remnant (131)I activity (P <or= 0.0001). Risk of thyroiditis increased by 64% for every 37 MBq increase in remnant (131)I uptake. OR = 1.64 [1.36-1.99]. Severe thyoriditis was only seen with remnant (131)I uptake > 73 MBq. For patients treated with 1110 MBq and 3700 MBq, incidence of thyroiditis was 12% and 27% (P = 0.02) and remnant ablation success rate was 76% and 84% (P = NS), respectively. Occurrence of thyroiditis did not correlate with successful ablation.
Conclusions: Incidence and severity of radiation thyroiditis following (131)I remnant ablation therapy is directly related to thyroid remnant (131)I uptake. As 1110 MBq (131)I is associated with a significantly lower frequency of thyroiditis but similar remnant ablation rate to 3700 MBq, it warrants consideration for thyroid remnant ablation particularly in patients with low risk disease.