Clinical Significance of Distinguishing Between Follicular Lesion and Follicular Neoplasm in Thyroid Fine-Needle Aspiration Biopsy

Subclassifying indeterminate thyroid fine-needle aspiration (FNA) biopsy findings as follicular lesion or follicular neoplasm has been suggested as useful in triaging patients to observation or surgery, respectively. However, terminology and therefore the probability of malignancy vary between pathologists and institutions. The purpose of this study was to evaluate a single institution's experience with indeterminate thyroid FNA results to determine if subclassification (neoplasm versus lesion) aids in identifying patients at higher risk for malignancy. From 1990 to 2006, all patients with indeterminate thyroid FNA results (follicular lesion or neoplasm) at The University of Texas M.D. Anderson Cancer Center were evaluated for FNA correlation with the surgical specimen diagnosis. Patients with FNAs suspicious for papillary thyroid carcinoma or with definitive malignant disease (i.e., metastases) were excluded. Indeterminate FNA results were present in 540 patients, including 410 as follicular lesion and 130 as follicular neoplasm. Two hundred ninety-seven (55.0%) patients underwent surgical resection: 199 (48.5%) follicular lesions and 98 (75.4%) follicular neoplasms. Incidence of malignancy was higher in thyroid nodules classified as neoplasm compared with lesion (21.4% versus 7.0%, respectively; P = 0.0005) and increased in follicular neoplasms with nodule size (37.5% malignant if nodule was > 4 cm, P = 0.03). Subclassification of indeterminate thyroid FNA biopsy results into neoplasm and lesion successfully defines high- and low-risk nodules, respectively. These findings support surgical resection for follicular neoplasms, selective use of surgical intervention for follicular lesions at our institution, and continued efforts to define unified terminology between institutions.