Fine needle aspiration biopsy of Hashimoto's thyroiditis - Sources of diagnostic error

OBJECTIVE: To determine the accuracy of cytologic interpretation in the dia gnosis of Hashimoto's thyroiditis (HT). STUDY DESIGN: At Ottawa Hospital from 1987 to 1994, 1,638 fine needle aspri ation biopsies (FNABs)from thyroid were performed. HT was suggested in 184 FNAB samples taken from 157 patients. Of the 184 aspirates diagnosed with H T, 39 had corresponding surgical specimens taken from 31 patients. A retros pective review of these FNABs and surgical pathology slides formed the basi s of this study. RESULTS: In 27 (69%) aspirates, HT tons diagnosed on both the FNAB and surg ical specimens. In 10 of 27 FNABs an associated lesion was not sampled by F NAB. In four of these 10 aspirates some of the cellular features of HT were misinterpreted, and the possibility of an associated neoplasm could not be ruled out. This resulted br four false positive diagnoses. In 12 (31%) FNA Bs from nine patients, the cytologic diagnosis of HT was not confirmed hist ologically. These cases included five Hurthle cell adenomas and one case ea ch of follicular adenoma, nodular goiter, macrofollicular adenoma and malig nant lymphoma. This resulted in five false negative diagnoses. CONCLUSION: These results support the value of FNAB in the diagnosis of HT. The presence of hyperplastic follicular cells on FNAB samples from HT may mimic a follicular,neoplasm and result in a false positive interpretation. Adequate sampling of the thyroid is important, particularly when there is a n associated lesion. The diagnosis of lymphocytic thyroiditis should not be made when only a few lymphocytes are present. Finally, pleomorphic Hurthle cells may be present in aspirates from Hurthle cell neoplasms and underdia gnosed as HT, especially when they are associated with a few lymphocytes.