Breast Tumor Resembling Tall Cell Variant of Papillary Thyroid Carcinoma: A Solid Papillary Neoplasm With Characteristic Immunohistochemical Profile and Few Recurrent Mutations.

Objectives: Breast tumor resembling tall cell variant of papillary thyroid carcinoma (BTRPTC) is a rare breast lesion that is unrelated to thyroid carcinoma. Morphologically, it shows a solid papillary lesion with bland cytology, eosinophilic/amphophilic secretions, nuclear grooves, reversal of nuclear polarity (recently described), and nuclear inclusions. Clinical course is often uneventful with few exceptions reported in the literature. Herein, we report three additional cases.

Methods: Immunohistochemical staining and next-generation sequencing was performed on all three cases.

Results: The lesional cells on all cases were positive for cytokeratin 5 and S100, with weak expression/lack of estrogen receptor. No staining was observed for myoepithelial markers (p63 and myosin heavy chain) around the lesion. IDH2 mutations were identified in two cases at nucleotide 172 (cases 1 and 3). ATM gene mutation was identified in cases 2 and 3 and PIK3CA mutation in case 3. All patients are currently without disease.

Conclusions: BTRPTC is a slow-growing neoplastic lesion that needs to be distinguished from other papillary lesions for optimizing therapy.