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

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Abstract
To examine success rates in strictly defined high risk differentiated thyroid cancer (DTC) patients who received a high activity (≥5550 MBq) adjuvant post-operative I-131 therapy and compare these to the rates found in highest risk and low risk patients. Retrospective database study We examined 377 DTC patients 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), patients with ≥5 lymph node metastases or any lateral compartment lymph node metastases were considered high risk. All other patients were considered low risk. Ablation success rate at first TSH stimulated follow-up. 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). high risk DTC patients 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.

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