Low FDG Metabolic Activity of Loco-Regional Recurrence in ThyroidCancer Patients; Is Iodine-131 has a Potentiality?

Purpose: To investigate the impact of quantitative FDG-Metabolic activity of non-iodine avid Loco-regional recurrence (LRR; thyroid recurrence and/or cervical node metastasis) on therapy selection in thyroid cancer patients with elevated thyroglobulin (TG). Methods: Forty thyroid cancer (33 papillary and 7 follicular) patients who underwent FDG PET/CT were studied (with TSH>30 mU/L). Those with only LRR were classified according to maxSUV (cut-off level 5) into high and low metabolic activity lesions (HMA L their therapy outcome was compared with histopathologic findings and/or follow-up routine evaluation. Results: Only LRRs was found in 20 patients (17 papillary and 3 follicular) with diagnostic accuracy of FDG PET/CT of 100 %. 14/20 patients belonged to HMA where surgical neck exploration was done while in the remaining 6 patients with LMA empirical high dose of radioactive iodine-131 was given based on the assumption of the presence of a mixture of undifferentiated and differentiated thyroid cancer cells; though their post-therapy scan was negative but declined TG- levels were elicited in their follow-up (base-line 27.7±2.4 and at follow-up 6.8±1.4 ng/ml; P 0.02). Conclusion: Max-SUV based classification of non-iodine avid LRR might improve the diagnostic accuracy FDG PET/CT in a therapeutically relevant way in DTC-patients by precisely localizing them with subsequent surgical guidance in HMA lesions while those with LMA could benefit from further RA-131 therapy.