Abstract
Immunoglobulin G4-related disease (IgG4-RD) is defined as an inflammatory lymphoproliferative disorder. The relationship between malignancies and IgG4-RD remains unclear. We herein present a case of IgG4-RD that occurred during chemotherapy for advanced breast cancer. In this case, it was challenging to determine which of these diseases was responsible for the patient's mediastinal lymphadenopathy. Lymphadenopathy with IgG4-RD was diagnosed by assessing the reactivity to corticosteroids, which were used as premedication in chemotherapy, over time. The administration of prednisolone, which was initiated to treat active IgG4-RD, led to stable systemic therapy for malignancy. It is imperative to assess the disease activity and consider each treatment.