Stage IIB triple-negative metaplastic breast cancer with early metastatic recurrence: durable response to pembrolizumab-based therapy

What happened, in summary

This 52-year-old Japanese woman noticed a rapidly growing mass in her left breast. Examination found a 7.0 × 5.5 cm palpable mass, and imaging suggested possible pectoralis muscle invasion but no regional or distant metastasis. Biopsy showed metaplastic breast carcinoma with a triple-negative profile. Tumor cells were positive for AE1/AE3, CK5/6, and p63. Because her mother had ovarian cancer, BRCA1/BRCA2 germline testing was performed and was negative. She underwent left mastectomy with sentinel lymph node biopsy, achieving complete resection. Final pathology showed metaplastic breast carcinoma with extensive spindle-cell carcinoma, necrosis, and some liposarcoma-like components; MDM2 and CDK4 were positive in those areas. The final stage was pT3N0M0, Stage IIB. Adjuvant dose-dense epirubicin and cyclophosphamide was started, with paclitaxel planned afterward. Only 3 weeks after chemotherapy began, a new mass appeared on the left anterior chest, and biopsy confirmed recurrent metaplastic breast cancer. CT showed multiple lung metastases and sternal metastasis. The original tumor had high PD-L1 expression, with CPS 83.2, so treatment changed to pembrolizumab plus gemcitabine and carboplatin. After an early temporary increase in lesions, the metastases shrank significantly. She developed grade 2 adrenal insufficiency, rash, pruritus, and anemia; adrenal insufficiency was treated with hydrocortisone. Gemcitabine/carboplatin was stopped after 10 months because of adverse events, while pembrolizumab continued. At 30 months after chemoimmunotherapy began, PET/CT showed only mild uptake in the sternal metastasis and no progression. Because treatment options for recurrent metaplastic triple-negative breast cancer can be limited, her prolonged disease control on pembrolizumab-based therapy was clinically important.