Relationship between internal radiation dose and outcome of radioiodine ablation of differentiated thyroid cancer

Objective: To investigate the relationship between the absorbed dose to the thyroid remnant and the outcome of ¹³¹I ablation in patients after operation for DTC. Methods: Seventy-two patients (14 males, 58 females; age range; 16-67 years, average age: (41 ± 16) years) with DTC, prepared for thyroid remnant ablation, were prospectively recruited from September 2009 to September 2011. Scintigraphic images of the head and neck were serially acquired after oral administration of 3.7 GBq of ¹³¹I to assess the biokinetics of this tracer in thyroid remnant. Ultrasound was used to measure the mass of the thyroid remnant. Calculation of absorbed dose to thyroid remnant was based on the Medical Internal Radiation Dosimetry formula of the Society of Nuclear Medicine. The outcome of 1311 ablation was evaluated on the basis of stimulated Tg measurements and neck ultrasound at 6-9 months after ablation. Patients with stimulated Tg levels less than 1 μg/L and with no detectable thyroid tissue on neck ultrasound were considered successful ablation of thyroid remnant. Two-sample t test was used to analyze the data. Results: The ¹³¹I uptake ratios at 24 h were 0.9%-6.3% in 72 patients and the effective half-lives of ¹³¹I were 12.0-146.4 h. The thyroid remnant masses were 1.0-6.9 g; absorbed doses were 23-2197 Gy, and absorbed dose rates at 24 h were 0.5-8.1 Gy/h. No significant difference was observed in absorbed doses to thyroid remnants in 43 patients having successful ablation and 29 patients having unsuccessful ablation ((363 ± 148) Gy (341 ± 167) Gy, t = 15.097, P > 0.05). However, the absorbed dose-rates of thyroid remnants at 24 h were significantly higher in patients with successful ablation than those in patients with unsuccessful ablation ((3.7 ± 2.1) Gy/h vs (2.9 ± 1.6) Gy/h, t = 7.908, P < 0.05). Conclusion: A successful ablation is strongly dependent on the absorbed dose-rate of thyroid remnant. (authors)