A case of breast acinar cell carcinoma combined with metaplastic carcinoma.

BACKGROUND: Breast acinar cell carcinoma (AcCC) is a rare subtype of triple-negative breast cancer. Its histological origin remains controversial, and it exhibits poor sensitivity to conventional chemotherapy. High-grade transformation and co-existence with matrix-producing metaplastic carcinoma are uncommon, and their clinical and molecular genetic features are poorly understood. CASE PRESENTATION: A 42-year-old woman incidentally discovered a breast mass for 10 days. Ultrasound revealed an irregular hypoechoic lesion in the upper inner quadrant of the left breast, classified as BI-RADS 4b, leading to a clinical diagnosis of breast cancer. Microscopically, the tumor comprised two distinct components. Component one exhibited tubular or solid nested patterns with eosinophilic secretions in lumina; some cells were amphophilic, and focal areas showed marked high-grade transformation with significant nuclear atypia. Component two featured tumor cell cords and nests distributed within a chondromyxoid stromal matrix. Immunophenotypically, tumor cells in both components diffusely and strongly expressed S-100, GATA3, AE1/AE3, and EMA. Component one expressed Lysozyme (LYS) and α1-antichymotrypsin (AACT). Both components were negative for ER, PR, HER2, CK5/6, CD117, P63, and Collagen IV. The Ki-67 proliferation index was approximately 10% in the low-grade areas of component one, reached about 30% in its high-grade areas, and was approximately 70% in component two. Molecular genetics: Next-generation sequencing detected a somatic frameshift mutation in the TP53 gene with a variant allele frequency of 36.51%, as well as germline variants in APC, ATM, BRCA1, BRCA2, and others. CONCLUSION: This report describes a breast tumor exhibiting concurrent features of AcCC and matrix-producing metaplastic carcinoma, summarizing its immunophenotypic characteristics and molecular genetic alterations. This information may provide diagnostic insights for future encounters with similar cases.