Fibrotubular Tumors of the Thyroid: An Emerging Thyroid Neoplasm Characterized by Distinctive Morphology and Recurrent OCLN::PRKCI Gene Fusions Spanning the Adenoma-Carcinoma Spectrum.
The histopathologic and genomic landscape of thyroid tumors is well characterized, although new genetic alterations and tumor types are being described. We present 2 cases of thyroid tumors originating from follicular cells that had highly unusual and distinct morphologic features and carried an OCLN::PRKCI gene fusion. These tumors were well circumscribed, encapsulated, and composed of irregularly shaped tubular and follicular structures surrounded by layers of distinct fibrocollagenous basement membrane material positive for type IV collagen and laminin; they showed no definitive nuclear features of papillary carcinoma. Importantly, whereas one of the tumors had no invasive growth, the other demonstrated tumor capsule invasion, compatible with the adenoma-carcinoma spectrum seen in thyroid follicular and oncocytic tumors. Gene expression profiles of these tumors differed from those of common types of papillary thyroid carcinoma. The unusual and reproducible histopathologic characteristics and unique molecular profiles of these tumors support their designation as a distinct type of thyroid neoplasm that exists in noninvasive and invasive forms and, therefore, can be designated as fibrotubular adenoma and carcinoma. Recognition of this distinct entity is important for improving diagnostic accuracy and avoiding overtreatment of this likely indolent type of thyroid neoplasia.