Primary lung cancer presenting a temporary reduction by corticosteroid treatment for IgG4-related disease

Immunoglobulin G4-related disease (IgG4-RD) is a systemic autoimmune disease comprised of a spectrum of complex fibroinflammatory changes with various manifestations mimicking malignant neoplasms. Pulmonary manifestations of IgG4-RD include interstitial involvement and a thickening of bronchovascular bundles and solid nodules that is called a 'pseudotumor.' A patient's positive response to treatment with a corticosteroid can help with the differential diagnosis. We report the case of a patient with primary lung cancer presenting a temporary reduction by corticosteroid treatment for IgG4-RD. Clinicians should be aware of the possible co-existence of IgG4-related lung disease and lung cancer, both of which may be reduced by corticosteroid treatment.