Patients with high-risk differentiated thyroid cancer have a lower I-131 ablation success rate than low-risk ones in spite of a high ablation activity.

Objective To examine success rates in strictly defined high-risk differentiated thyroid cancer ( DTC) patients who received a high-activity (≥5550 MBq) adjuvant postoperative I-131 therapy and compare these to the rates found in highest risk and low-risk patients. Design Retrospective database study. Patients We examined 377 patients with DTC who received I-131 ablation. Patients with distant metastases were classified as very high risk. Patients with primary tumours >4 cm, extensive extrathyroidal invasion (pT4a or pT4b in accordance with the 7th edition of the TNM system), and patients with ≥5 lymph node metastases or any lateral compartment lymph node metastases were considered high risk. All other patients were considered low risk. Measurements Ablation success rate at first TSH-stimulated follow-up. Results The ablation success rate was 72·6% in low-risk patients, 51·7% in high-risk patients and 13·8% in highest risk patients (all differences P < 0·001). In none of the groups, a significant difference in the initial I-131 activity was found between patients with successful and unsuccessful ablation (low risk: P = 0·16, high risk: P = 0·91 and highest risk: P = 0·48). Furthermore, there was no difference in ablation success between patients who received <5550 MBq and those who received ≥5550 Mbq (low risk: P = 0·31, high risk: P = 0·69 and highest risk: P = 0·22). Conclusions Patients with high-risk DTC have a significantly reduced I-131 ablation success rate compared to low-risk ones in spite of high initial I-131 activities. As successful ablation is prognostically important, efforts should be made to improve outcome in these patients. [ABSTRACT FROM AUTHOR]