Utility of thyroglobulin measurements in fine-needle aspirates of space occupying lesions in the thyroid bed after thyroid cancer operations.

BACKGROUND: Ultrasound, and sometimes cytology, cannot differentiate between recurrent or persistent thyroid cancer and benign forms of space occupying lesions (SOLs) in the thyroid bed, including unsuspected thyroid remnants, that are noted several months to years after thyroidectomy (Tx) for thyroid cancer. The purpose of the present study was to evaluate the hypothesis that measurement of thyroglobulin (Tg) in fine-needle aspirates from these lesions might help differentiate between benign and malignant SOLs in the thyroid bed.

METHODS: We studied 47 lesions in the thyroid bed from 43 patients who, 8-240 months previously, had 43 Txs for thyroid cancer. Eleven patients had a lobectomy and 32 patients had a total Tx. Also, some patients had radioactive iodine (RAI) ablation after their thyroid surgery and some did not. "Recurrence" was defined as the SOL, which was confirmed by cytological or histopathological results. "Benign SOL" was defined as a focal lesion, which was benign or nondiagnostic result on cytology and for which there was no RAI uptake on whole-body scintigraphy with both negative serum Tg and Tg antibodies. Diagnostic performances of fine-needle aspiration cytology (FNAC), FNA-Tg, and combining FNAC with FNA-Tg level were assessed for detection of malignant SOL. The diagnostic performance of FNA-Tg was assessed using three threshold values: 1 ng/mL, 10 ng/mL, and an FNA-Tg/serum-Tg ratio of 1.0.

RESULTS: FNA-Tg level and combining FNA-Tg levels with FNAC had higher sensitivities (100% in all three threshold values) and diagnostic accuracies (91.5%-95.7%) than FNAC alone (sensitivity of 85.3%, accuracy of 89.4%) in all threshold values. In both the RAI ablation and non-RAI ablation groups, the FNA-Tg levels and combining the FNA-Tg levels with FNAC had a higher sensitivity and diagnostic accuracy than FNAC alone with threshold values of 10 ng/mL and FNA-Tg/serum-Tg ratio of 1.0. The non-RAI ablation group did not have a different diagnostic accuracy than the RAI ablation group in all threshold values (p>0.05). FNA-Tg level showed a negative predictive value of 100% in all threshold values, in both the RAI ablation and the non-RAI ablation groups.

CONCLUSIONS: Measurement of Tg levels in the FNA of SOLs in the thyroid bed can be helpful in diagnosing tumor recurrence, because an FNA-Tg level lower than the threshold value has the added value of suggesting a benign lesion rather than tumor recurrence.