Presumed Second Focus of Lung Immunoglobulin G4-Related Disease Found to be Adenocarcinoma

We describe a patient presenting with bilateral radiologically similar lung lesions initially diagnosed as immunoglobulin (Ig) G4-related disease from biopsy of one lesion, but radiographic changes 6 months later prompted biopsy of the second lesion and showed adenocarcinoma. No case of lung IgG4-related disease and a distant lung malignancy has been previously reported. This is notable because lung IgG4-related disease often manifests in multiple thoracic locations but is diagnosed from a representative biopsy specimen.