Implications of follicular neoplasms, atypia, and lesions suspicious for malignancy diagnosed by fine-needle aspiration of thyroid nodules.

OBJECTIVE: To break out subcategories of atypical and suspicious cytologic interpretations of thyroid aspirations and correlate those with final histologic diagnosis.

SUMMARY BACKGROUND DATA: Fine-needle aspiration (FNA) of thyroid nodules has become the primary diagnostic tool in the initial evaluation of thyroid nodules. Management of thyroid nodules is based on statistical data correlating a specific cytologic reading with the probability of malignancy. Two particular cytologic readings that frustrate both cytopathologists and surgeons are follicular neoplasms and cells that show atypia. In most reported series, follicular neoplasms and cells showing atypia are included in a broad "suspicious" category, with reported overall malignancy rates generally between 20% and 30%. However, there is interest in determining whether these suspicious lesions can be subcategorized, allowing a more accurate assessment of the risk of malignancy. In addition, there is recent evidence that the incidence of follicular cancer may be declining in this country, possibly decreasing the probability that a "follicular neoplasm" will prove to be a follicular cancer.

METHODS: From January 1994 through December 2000, 709 thyroid FNAs were performed at a single institution. Those interpreted as suspicious and the subsequent histologic reports were reviewed. From this set, four specific categories were defined, and the clinical records for patients whose cytology matched these categories form the subject of this study. These four categories are follicular neoplasms without atypia, follicular neoplasms with atypia, atypia, and suspicious for malignancy. In addition, lesions with nondiagnostic cytology were reviewed. Only lesions for which there was histologic follow-up were considered.

RESULTS: Ninety-eight aspirates were categorized as follicular neoplasms without atypia, follicular neoplasms with atypia, or atypia alone. Of 74 follicular neoplasms without atypia, only 5 (6.8%) were malignant, and none of these were follicular thyroid cancers. Nine of the lesions were follicular neoplasms with atypia, and four (44.4%) of these were malignant, including two that were invasive follicular cancers. Of 15 lesions showing atypia alone, 3 (20%) were malignant, all of which were papillary thyroid cancers. Twenty-five additional patients had lesions highly suspicious for malignancy. Twenty-one (84%) of these lesions were malignant, the majority being papillary thyroid cancers. Only one was a follicular cancer. Fifty patients with nondiagnostic cytology had subsequent action taken. Thirty-one of these lesions were resected, with five (16.1%) proving to be malignant.

CONCLUSIONS: Thyroid nodules whose FNA is diagnosed as highly suspicious for malignancy should be resected unless there are significant contraindications to a surgical procedure. The extremely low rate (2%) of invasive follicular cancers among all follicular neoplasms may reflect changing histologic criteria for follicular carcinoma, a true change in the disease frequency, or both. For thyroid nodules whose cytology shows a follicular neoplasm without atypia, malignancy rates of 8% or less may allow nonsurgical options, including reevaluation in selected populations. Continued efforts to correlate malignancy rates to specific cytologic criteria will allow patients to make more informed decisions regarding their medical care.