Purpose: The aim of this study was to determine if thyroid cancer patients with low percentage neck uptake of iodine on postoperative thyroid scans can be treated with lower doses of 131Iodine while maintaining a high ablation rate.
Materials and methods: We reviewed the records of 58 patients with differentiated thyroid cancer treated with 131I at the Princess Margaret Hospital. The activity of 131I was prescribed based on the 48 h percentage neck uptake in postoperative thyroid scans. Patients with < or =2% uptake received 1.07 GBq, patients with 2.1-4% uptake received 1.85 GBq, patients with 4.1-6% uptake received 2.80 GBq, patients with 6.1-8% uptake received 3.70 GBq and patients with >8% uptake received 4.60 GBq. When the scan suggested cervical lymph node metastases or residual tumor, 7.40 GBq was prescribed. Follow-up scans were performed at least 5 months after 131I therapy. Successful ablation was defined as the absence of visible uptake in the neck above background.
Results: Forty-nine patients were included in this analysis. The ablation rate according to the prescribed activity was as follows: 1.07 GBq, 16/20 (80%); 1.85 GBq, 4/5 (80%); 2.80 GBq, 1/1 (100%); 3.70 GBq, 0/1 (0%); 4.60 GBq, 7/8 (88%); 7.40 GBq, 13/14 (93%). The ablation rate for all patients treated on the protocol was 41/49 (84%, 95% CI 70-93%). For the group treated for remnant ablation, the overall ablation rate was 28/35 (80%, 95% CI 63-92%). Twenty-two (38%) of the 58 eligible patients received 1.07 GBq as outpatients. This saved 38 hospitalization days compared to a policy of treating all patients requiring remnant ablation with 3.70 GBq.
Conclusions: We conclude that patients with less iodine uptake in postoperative thyroid scans can receive lower activities of 131I, allowing a significant proportion of patients to be treated on an outpatient basis while maintaining a high ablation rate.