DICER1 Mutation-Associated Follicular Thyroid Tumor With Bizarre and Pleomorphic Nuclei.

Thyroid carcinomas comprise diverse subtypes, each characterized by distinct histomorphologies, molecular profiles, and prognoses. Follicular thyroid carcinoma (FTC) is a differentiated thyroid cancer demonstrating vascular and/or capsular invasion, while anaplastic thyroid carcinoma (ATC) is an aggressive and undifferentiated form with an exceedingly dismal prognosis. Thyroid carcinomas also can exhibit mixed histologic features, complicating the diagnosis and management of these entities. DICER1 germline and somatic mutations are associated with multiple neoplasms, including thyroid neoplasms along a morphological spectrum from benign, differentiated malignant carcinomas, and poorly differentiated carcinomas, suggesting a role for these mutations in tumor dedifferentiation and progression. We present a 31-year-old female with a large encompassing thyroid tumor harboring a somatic DICER1 "hotspot" mutation (p.E1705K) displaying predominantly follicular microarchitecture, bizarre and pleomorphic nuclear features, yet without overt invasive or poorly differentiated features and low proliferation indices. While poorly differentiated follicular thyroid carcinomas have been associated with DICER1 mutations, the morphology present in this tumor, to our knowledge, has not been reported in DICER1-associated thyroid neoplasms. This case continues to expand the morphological spectrum of thyroid tumors with DICER1 mutation and again raises questions about the role of DICER1 in thyroid tumor morphology and behavior.