INTRODUCTION: Immune thrombocytopenic purpura (ITP) is one of the immune-related adverse events (irAEs) related to immune checkpoint inhibitors (ICIs). Here, we report a case of a 51-year-old woman with triple-negative breast cancer (TNBC) who experienced severe thrombocytopenic purpura during the neoadjuvant chemotherapy (NAC), including pembrolizumab. CASE PRESENTATION: A 51-year-old woman was diagnosed with Stage II B TNBC and underwent NAC using pembrolizumab + paclitaxel + carboplatin. Her blood test on cycle 4, day 15 (C4D15) showed a significant decrease in platelets to <
2000/μL accompanied by overt bleeding tendency. She was hospitalized for further investigation and treatment. Her platelet count recovered after platelet concentrate transfusion and corticosteroid administration. Her bone marrow examination showed normal cellularity, and she was judged as ITP. Due to the event and good clinical response to NAC, she underwent a right partial mastectomy and axillary lymph node dissection without completion of the planned NAC. The surgical specimen showed a complete pathological response. CONCLUSIONS: Thrombocytopenia is known as one of the hematologic irAEs
however, severe thrombocytopenia with a bleeding tendency is rarely reported. Sufficient explanations to patients and appropriate referral to other related departments are important for earlier detection and treatment of irAE.