Tall cell carcinoma of the breast with reversed polarity - a report of three cases with a review of the literature.

Tall cell carcinoma with reverse polarity (TCCRP) is a rare special type of breast epithelial neoplasm presented by columnar cells with opposite nuclear polarity, solid and solid-papillary architecture, and frequent IDH2 gene alterations. Hereby, the authors present three cases of TCCRP in women aged 56, 66 and 67 years with maximum tumour sizes of 29 mm, 10 mm and 8 mm. Tumours showed histomorphological characteristics of TCCRP supported by immunohistochemical profile of tumour cells, in which positive expression of CK7, CK5/6, GCDFP15, mammaglobin, GATA3 and calretinin and negativity of CK14, p63, TTF1, thyroglobulin and neuroendocrine markers were demonstrated. Two tumours were triple negative, and in one tumour, only weak focal ER expression was noted along with PR and HER2 negativity. Pathogenic somatic variants in mutational hotspot region p.R172 in IDH2 gene were detected using NGS technology in all three tumours. Moreover, in two of these tumours, the most common pathogenic variants p.E545A and p.H1047R of PIK3CA were identified. TCCRP represents a rare breast neoplasm of low malignant potential, the incidence of which will probably increase due to the more clearly defined histomorphological, immunohistochemical and molecular-genetic characteristics, which were all responsible for including this entity into the 5th edition of WHO classification breast tumours.