Aggressive follicular cell derived thyroid carcinoma: what do you need from the pathologist.

Differentiated thyroid carcinomas (DTC), including papillary thyroid carcinoma (PTC) and follicular carcinoma (FC), account for most thyroid malignancies and are generally associated with excellent prognosis. However, a subset of DTC demonstrate aggressive clinical behavior characterized by increased recurrence, distant metastasis (DM), radioactive iodine (RAI) resistance, and reduced survival. Identification of these tumors is critical for appropriate risk stratification and management. A number of pathologic features have been shown to predict aggressive behavior, including vascular invasion (VI), capsular invasion (CI), and extrathyroidal extension (ETE), as well as high-grade histologic features such as increased mitotic activity and tumor necrosis. In the 2022 World Health Organization (WHO) Classification for Endocrine and Neuroendocrine Tumours, high-grade features are acknowledged as prognostically significant, and DTC meeting these criteria are classified as high grade differentiated thyroid carcinoma (HGDTC), a category with a prognosis similar to PDTC. In addition, certain histologic subtypes of PTC, including the tall cell, hobnail, and columnar cell subtypes, have traditionally been associated with more aggressive clinical outcomes. However, the prognostic significance of histologic subtype alone remains debated, as tumor behavior is often influenced by the presence of invasive features and high-grade morphology. Recent advances in molecular profiling have further refined risk stratification. While the independent prognostic significance of the BRAFV600E mutation remains controversial, TERT promoter mutations have consistently been associated with aggressive tumor biology, including DM, RAI resistance, and decreased survival. Importantly, the coexistence of BRAFV600E and TERT promoter mutations identifies a subset of thyroid carcinomas with particularly poor outcomes. This review summarizes the key pathologic and molecular features associated with aggressive behavior in DTC, highlighting their diagnostic criteria, prognostic significance, and implications for clinical management. Comprehensive pathologic evaluation integrating morphologic and molecular findings remains essential for accurate risk stratification and multidisciplinary care of patients with thyroid carcinoma.