Breast Tumor Resembling the Tall Cell Variant of Papillary Thyroid Carcinoma: Molecular Characterization by Next-Generation Sequencing and Histopathological Comparison With Tall Cell Papillary Carcinoma of Thyroid.

Recent publications have brought attention to the histopathological, immunohistochemical, and molecular aspects of the rare breast tumor resembling the tall cell variant of papillary thyroid carcinoma (BrTC). Nine archived cases of this entity were retrieved, reviewed, and compared with randomly selected tall cell variants of papillary thyroid carcinoma (ThTC). Seven of the BrTC cases as well as 5 cases of solid papillary carcinoma of breast were analyzed by Oncomine next-generation sequencing. BrTC and ThTC were histologically distinguishable by the presence of solid architecture, luminal histiocytes, and reverse polarity in the former, and psammoma bodies, giant cells, and optically clear nuclei in the latter. Sequencing revealed IDH2 R172 single-nucleotide variants in all 7 BrTCs, 6 of which had concurrent PIK3CA mutations. None of the conventional solid papillary carcinomas demonstrated IDH2 mutation. BrTC bears superficial resemblance to other papillary tumors but is unique in terms of histology and molecular profile.