Incidence of radiation thyroiditis and thyroid remnant ablation success rates following 1110 MBq (30 mCi) and 3700 MBq (100 mCi) post-surgical 131I ablation therapy for differentiated thyroid carcinoma.

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.