Thyroid cytology and the risk of malignancy in thyroid nodules: Importanceof nuclear atypia in indeterminate specimens

Fine needle aspiration (FNA) cytology is the best test for malignancy in th yroid nodules. However, cytologic interpretation of FNA specimens is often difficult, especially in the presence of indeterminate microfollicular cyto logic patterns, which are thought to suggest follicular neoplasm (adenoma o r carcinoma). To assess the risk of malignancy associated with specific cyt ologic patterns, we correlated preoperative FNA cytologic patterns (n = 484 reports including repeat aspirations) with final histological diagnoses fo r 368 surgical thyroid specimens obtained during the period 1994-1998. The overall prevalence of malignancy in the surgical specimens was 31% (113 can cers, including 96 papillary and 9 follicular carcinomas). For nodules with benign FNA cytologic diagnoses of nodular goiter and chronic thyroiditis t here was a low risk of malignancy (6/99, or 6.1%). Nodules with indetermina te cytologic patterns in the absence of nuclear atypia (i.e., microfollicle s without nuclear atypia) had a similarly low malignancy risk (3/46, or 6.5 %). In contrast, 31/52 nodules with cytologic nuclear atypia consistent wit h follicular neoplasm were malignant (60%), including specimens with or wit hout microfollicular cytology. Nodules with frankly malignant cytologic pat terns were almost invariably cancer (54/55), and cytologic diagnoses of pap illary carcinoma were confirmed at surgery in all 49 cases. These results i ndicate that indeterminate microfollicular cytologic patterns in the absenc e of nuclear atypia are associated with a low risk of malignancy, at least in this series. This finding suggests that many nodules with such microfoll icular cytology might be managed conservatively with observation. In contra st, cytologic nuclear atypia consistent with a follicular neoplasm confers a high risk of cancer. In addition, frankly malignant cytologic diagnoses, especially papillary carcinoma, are highly reliable, and thus may be used a s a guide for planning surgery appropriate for thyroid cancer.