Encapsulated Papillary Oncocytic Neoplasms of the Thyroid: Morphologic, Immunohistochemical, and Molecular Analysis of 18 Cases

Encapsulated papillary oncocytic neoplasms (EPONs) of the thyroid are rare tumors, whose relationship to other thyroid tumors has not been thoroughly elucidated. Earlier, they have been regarded as variants of papillary thyroid carcinoma (PTC), hyperplastic lesions, and follicular neoplasms. Eighteen EPONs were retrieved from our surgical pathology files and reviewed for defining morphologic features. Cases having the typical nuclear features of PTC were excluded. Immunohistochemistry (IHC) for CK19, HBME1, and CD56 was carried out. Microdissection, polymerase chain reaction, and sequencing of exon 15 of the BRAF gene were completed. Cases were evaluated for rearranged in transformation/papillary thyroid carcinoma RET/PTC rearrangement by fluorescent in situ hybridization (FISH). The majority of the tumors exhibited a distinctive histologic appearance. They were composed of true papillae lined by a single layer of predominantly cuboidal cells with oncocytic cytoplasm; hobnailing was typically prominent. Three tumors showed taller cells with uniformly apical nuclei and no hobnailing. Ten of 18 cases showed vascular and/or capsular invasion; hence, if the diagnostic criteria used to evaluate follicular neoplasms are applied, more than half of the tumors would be considered minimally invasive carcinomas. No cases were immunoreactive with antibodies to HBME1, whereas only 1 of 13 was immunoreactive for CK19. Six of 7 interpretable cases were immunoreactive for CD56. No BRAF point mutations or RET/PTC rearrangements were identified in the examined cases. All patients were alive at the time of last follow-up and no locally recurrent disease had been reported; however, 1 case was remarkable for a lymph node metastasis. Our results confirm that EPONs are histologically, immunohistochemically, and molecularly distinct from papillary thyroid carcinoma and seem to be most related to follicular neoplasms.