The Japanese Society of Internal Medicine Acta Medica Okayama 0918-2918 60 17 2021 Marginal Zone Lymphoma and Lung Adenocarcinoma with an EGFR Exon 19 E746-S752del Mutation in a Patient with IgG4-related Disease 2831 2837 EN Sachi Okawa Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Kammei Rai Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Nobuharu Fujii Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Yuka Gion Division of Pathophysiology, Okayama University Graduate School of Health Sciences Kiichiro Ninomiya Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Yuka Kato Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Akihiko Taniguchi Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Toshio Kubo Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Eiki Ichihara Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Kadoaki Ohashi Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Nobuaki Miyahara Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Katsuyuki Hotta Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Masahiro Tabata Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Yoshinobu Maeda Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Katsuyuki Kiura Department of Hematology, Oncology and Respiratory Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences A 68-year-old man presented with a solid mass at the left renal pelvis and ureter with multiple systemic lymphadenopathies and a mass with a cavity in the right lower lobe of the lung. While a transbronchial lung biopsy revealed no malignancy, a biopsy of the renal pelvis showed marginal zone lymphoma with polyclonal IgG4-positive cells. The serum IgG4 level and presence of a bilateral orbital mass suggested Mikulicz disease. The lesions shrank following the administration of steroids. A rebiopsy confirmed lung adenocarcinoma, and its background showed IgG4-positive cells a year later. IgG4-related diseases require careful follow-up because they can be complicated by malignancy. No potential conflict of interest relevant to this article was reported.