What happened, in summary

This 74-year-old woman found a lump in her left breast. Ultrasound showed a 3.2 × 2.1 cm hypoechoic mass within the glandular layer, with relatively clear boundaries, slightly irregular margins, heterogeneous internal echoes, and detectable blood flow. The finding was classified as BI-RADS 4A-4B, and digital breast X-ray later classified the mass as BI-RADS 6 after biopsy confirmed carcinoma. The biopsy diagnosis was difficult to classify and was considered a low-grade malignant tumor. She underwent total mastectomy of the left breast with sentinel lymph node exploration. Final pathology diagnosed breast pleomorphic adenocarcinoma, a rare tumor type. The sentinel lymph nodes did not contain metastasis, and no distant spread was described in the case. Immunohistochemistry showed ER-negative, PR-negative, and HER2 0 status, so the tumor was triple-negative. Additional markers included Ki-67 20%, Bcl-2 positivity, focal CK7 positivity, E-cadherin positivity, S100 positivity, partial CD117 positivity, and SOX10 positivity. Next-generation sequencing of the breast tumor found a PIK3CA C420R mutation with a 41.24% mutant allele frequency, along with other genetic alterations listed in the article. The clinical picture was localized, node-negative breast pleomorphic adenocarcinoma after surgery, without a formal AJCC stage group stated. Because she declined further treatment, only 1 course of oral capecitabine was given after mastectomy. As of September 2025, she had been followed for 2 years with disease-free survival. Her story highlights a rare triple-negative breast tumor with a PIK3CA mutation and durable disease-free follow-up despite very limited postoperative chemotherapy after definitive surgery and no sentinel lymph node involvement.