Postoperative cutaneous metastasis of breast cancer with phenotypic conversion to triple-negative subtype: A case report.

RATIONALE: Cutaneous breast cancer metastasis carries poor prognosis. Phenotypic conversion to a triple-negative subtype presents therapeutic challenges. Re-biopsy is crucial. PATIENT CONCERNS: A 53-year-old female underwent right modified radical mastectomy for HER2-positive invasive breast carcinoma in September 2023, and developed pruritic, erythematous nodules on the right chest wall, axilla, and back 9 months later. DIAGNOSES: Histopathology revealed poorly differentiated dermal carcinoma. The primary tumor was ypT2N3aMx with 23/23 positive nodes, grade 3. IHC showed ER(-), PR(-), HER2 (3+), and Ki-67 (~40%). In October 2023, a supraclavicular lymph node biopsy revealed HER2 IHC 0, representing the first evidence of phenotypic conversion. In July 2024, biopsy of the cutaneous metastatic lesion confirmed HER2 IHC 0 (no membrane staining), ER(-), PR(-), and Ki-67 (~60%+), consistent with a persistent triple-negative phenotype. INTERVENTIONS: Treatment was switched to gemcitabine plus cisplatin combined with envafolimab (a PD-L1 inhibitor). Prior to initiation, PD-L1 IHC (SP263, Ventana) revealed TC 0%, IC+ <1%, and ICP ~10%. Six cycles were administered between August and November 2024. In January 2025, new lesions with ER 20% positivity emerged, and the regimen was adjusted to vinorelbine plus bevacizumab combined with envafolimab. OUTCOMES: The patient completed 6 cycles of gemcitabine plus cisplatin combined with envafolimab. Course was uneventful, with isolated grade 3 neutropenia (0.71 × 10^9/L) in October 2024. In January 2025, new cutaneous lesions emerged, prompting biopsy and regimen change. As of April 2025, the patient remained on active treatment with stable disease status. LESSONS: This case underlines re-biopsy importance. Conversion to a triple-negative phenotype necessitates complete change in systemic therapy. Phenotypic conversion may first manifest in lymph node metastases before becoming clinically evident in cutaneous lesions.