Tall Cell Carcinoma of the Breast With Reverse Polarity: Expanding the Molecular Spectrum With IDH1 R132 Mutation.

Tall cell carcinoma of the breast with reversed polarity (TCCRP) is a rare type of invasive breast cancer with overlapping histopathological features with papillary thyroid carcinoma and a characteristic immunohistochemical (IHC) and molecular profile. Case Presentation: A 72-year-old female presented with a left breast mass and purulent nipple discharge. Imaging showed a BI-RADS 4 lesion on mammography and a heterogeneous, lobulated mass on computed tomography (CT), confined to the breast. Core biopsy suggested low-grade invasive carcinoma. She underwent a left simple mastectomy with sentinel lymph node biopsy. Histopathology was consistent with features of TCCRP. IHC and molecular analysis revealed a hormone receptor (HR)-positive TCCRP with IDH1 R132 mutation with low proliferative index. The patient received adjuvant radiotherapy (40 Gy/15 fractions) and is under 5-year letrozole therapy. At 18-month follow-up, she remained disease-free, with no recurrence or metastasis. This case underscores the clinical, histopathological, immunophenotypic, and molecular features of TCCRP, including the rare IDH1 R132H mutation. Timely surgical and hormonal therapy ensures excellent prognosis, and future studies on targeted IDH1/IDH2 and PIK3CA therapies may expand treatment options.