A case of prominent immunoglobulin G4‐positive lymphadenopathy in response to microscopic lung cancer

Immunoglobulin G4 (IgG4)‐related disease is established as a new clinical entity, characterized by high levels of plasma IgG4 and IgG4‐positive plasma cell infiltration. However, the elevation of plasma IgG4 and infiltration of IgG4‐positive cells have been observed in other diseases, including malignancy. We experienced a case of prominent IgG4‐positive lymphadenopathy, which was diagnosed as a reactive lesion in response to lung cancer. The cancerous lesion was so small in size that it was difficult to reveal the coexisting lung cancer. Surgical lymph node biopsy and endobronchial ultrasound‐guided transbronchial needle aspiration did not reveal lymph node metastasis of cancer. Mediastinal lymph node dissection finally revealed it. After the right upper lobectomy, the patient underwent postoperative chemotherapy and remains cancer‐free after 1 year. Our case suggests that close examination and careful follow‐up are necessary when IgG4‐positive lymphadenopathy is observed.