Stage IIIC triple-negative breast cancer with pembrolizumab-related HLH/DIC and pathologic complete response

What happened, in summary

This 38-year-old woman was diagnosed in November 2022 with locally advanced left breast cancer, cT1N3M0, clinical Stage IIIC. The tumor was hormone receptor-negative and HER2-negative, and germline testing identified a pathogenic BRCA1 mutation. Because of her young age, stage, and triple-negative biology, treatment began with preoperative carboplatin, paclitaxel, and pembrolizumab. The cancer responded quickly: by the third course, the breast mass had become non-palpable and was assessed clinically as cT0. Treatment then became complicated by immune-related toxicity. She developed persistent diarrhea consistent with immune-related colitis, first improving with 5-ASA therapy and later requiring prednisone. Pembrolizumab was discontinued, while chemotherapy with carboplatin and paclitaxel continued. Liver dysfunction then worsened despite steroids, leading to hospitalization for suspected immune-related hepatitis. After liver biopsy, she developed bleeding with extremely high FDP and D-dimer levels, hyperferritinemia, elevated LDH, and a hemophagocytic pattern on pathology. She was diagnosed with secondary hemophagocytic lymphohistiocytosis and disseminated intravascular coagulation. Treatment included nafamostat, fresh frozen plasma, mycophenolate mofetil, methylprednisolone pulse therapy, tocilizumab, cyclosporine, and gradual tapering of immunosuppression. She recovered enough to proceed to breast cancer surgery on day 209. The surgical specimen showed pathologic complete response, ypT0ypN0. Prednisone was later stopped, and she was monitored without additional cancer-directed treatment. She remained recurrence-free after this high-risk treatment course. Her course combined a strong cancer response with a severe immune complication requiring intensive immunosuppression. The final cancer outcome was favorable because surgery showed no residual breast tumor or nodal disease, and she continued close follow-up after stopping prednisone and other immunosuppression.