Cytopathology of follicular and oncocytic follicular thyroid neoplasms: A Bethesda System perspective.

The third edition of The Bethesda System for Reporting Thyroid Cytopathology includes category IV, follicular neoplasm (FN), which is used to classify fine-needle aspirates of thyroid nodules that may correspond to invasive follicular-derived neoplasia other than papillary thyroid carcinoma. This diagnosis is infrequently rendered, and may represent a challenge for pathologists. This review presents a practical approach to FN and its subtype oncocytic follicular neoplasm (OFN). First, minimal criteria for the diagnosis must be achieved, namely sufficient cellularity, architectural features consistent with neoplasia, and follicular cell or oncocytic cytomorphology. Second, select diagnoses that are common or important differential diagnoses for FN or OFN must be ruled out, via a combination of morphological findings and limited ancillary tests, when available. These include follicular nodular disease, parathyroid sampling, metastatic carcinoma, noninvasive follicular thyroid neoplasm with papillary-like nuclear features, medullary thyroid carcinoma, certain subtypes of papillary thyroid carcinoma, and lymphocytic thyroiditis. This approach should allow for a careful selection of cases where diagnostic thyroid lobectomy is an appropriate therapeutic modality.