Uncovering the face of acinic cell carcinoma of the breast with high-grade morphology: evidences from case report

Abstract Background Acinic cell carcinoma (AciCC) of the breast is a rare subtype of breast cancer. It was regarded as a low-grade triple-negative breast cancer (TNBC) with the potential to progress or transform to a high-grade lesion due to the molecular similarities with conventional aggressive TNBC in several genetic studies. Microscopically, the coexistence of classical low-grade and high-grade triple-negative components in breast AciCC is not uncommon. Nevertheless, the comparative histopathological and genetic study on both components is rarely reported at present. Case Presentation We present the case of a 34-year-old woman diagnosed initially on the preoperative biopsy with malignant small round cell tumor (undifferentiated or poorly differentiated carcinoma) that was later identified as breast AciCC with a high-grade solid component. The breast AciCC consisted of a classical acinic component and a high-grade component. The high-grade lesion in the tumor showed a solid sheet-like pattern characterized by large, round, pleomorphic or vesicular nuclei, prominent nucleoli, frequent mitotic activities. Classical acinic architectures focally merged together to form solid nests and underwent a gradual transition into high-grade areas. Notably, in the high-grade lesion, conventional immunochemical markers for breast AciCC, such as, AAT, EMA, CK, SOX10, S100, GATA3, Periodic acid–Schiff-diastase (PASD), E-cadherin were lost while CyclinD1, Vimentin were diffusely expressed. Next‑generation sequencing (NGS) analysis revealed that 43.5% of variants were identical in both classical and high-grade components. Additionally, PAK5 mutation, copy number loss of CDH1, CHEK1, MLH1, copy number gains of CDK6, HGF, FOXP1 were identified in the high-grade component. Conclusions The case offers a comparative analysis of the histopathology and genetic characteristics of classical low-grade and high-grade components within the same breast AciCC. This may serve as a morphological and molecular basis for further investigation into the molecular mechanisms underlying high-grade lesions in breast AciCC.