Case Report: Aplastic anemia associated with parvovirus B19 infection following neoadjuvant pembrolizumab-based chemoimmunotherapy for triple-negative breast cancer.

Immune checkpoint inhibitors are increasingly incorporated into curative treatment strategies for early-stage triple-negative breast cancer. Although hematologic immune-related adverse events are rare, they may be severe and difficult to distinguish from other causes of bone marrow failure. We report a 54-year-old woman with triple-negative breast cancer treated with neoadjuvant pembrolizumab plus chemotherapy according to the KEYNOTE-522 regimen. After breast-conserving surgery and achievement of a pathologic complete response, she developed a rapidly progressive severe pancytopenia (grade 4 febrile neutropenia, grade 4 thrombocytopenia and grade 3 anemia) in the early postoperative period. Bone marrow biopsy showed generalized bone marrow suppression. An immune-related hematologic toxicity was suspected; high-dose corticosteroids were initiated. However, no hematologic response was observed. Further tests detected ongoing parvovirus B19 infection (positive plasma viral DNA and IgM and IgG serology). Intravenous immunoglobulin therapy was therefore initiated, resulting in rapid hematologic recovery. Pembrolizumab was discontinued, and the patient subsequently completed adjuvant radiotherapy. At a 20-month follow-up, she remained free of disease recurrence. This case highlights the importance of broad diagnostic evaluation of severe cytopenias after pembrolizumab-based chemoimmunotherapy and underscores the need to distinguish infectious bone marrow suppression from primary immune-related hematologic toxicity, as this distinction may directly influence management.