[A Case of Pembrolizumab-Associated Fulminant Type One Diabetes Mellitus during Neoadjuvant Chemotherapy for Early-Stage Breast Cancer].

A woman in her 50s was diagnosed with Stage ⅡA triple-negative breast cancer (TNBC) and neoadjuvant chemotherapy with pembrolizumab, carboplatin, and paclitaxel was initiated. On day 125 of treatment, the patient presented to the emergency department with fever and impaired mobility. After a thorough examination, she was diagnosed with fulminant type 1 diabetes mellitus (T1DM) due to an immune-related adverse event (irAE). Prompt initiation of fluid resuscitation and insulin therapy led to symptom resolution. The patient was discharged on day 17 of hospitalization. One month later, adjuvant doxorubicin and cyclophosphamide (AC) therapy was initiated and completed for 4 courses. Subsequently, the patient achieved a clinical complete response (cCR) following neoadjuvant therapy, and subsequently underwent total mastectomy and sentinel lymph node biopsy of the left breast. The final pathological examination revealed a pathological complete response (pCR; ypT0/ypN0). Pembrolizumab was not administered postoperatively. One year after surgery, no evidence of recurrence was observed. Fulminant T1DM is a rare irAE with an incidence of < 0.1%. However, this condition rapidly progresses to diabetic ketoacidosis. When pembrolizumab is administered, early detection through regularly monitoring the patient's symptoms and blood glucose should be required.