IgG4 ‐Related Lung Disease in a Former Smoker: A Challenging Diagnosis

ABSTRACT Immunoglobulin G4‐related disease (IgG4‐RD) is a rare systemic fibroinflammatory condition that may affect virtually any organ, including the lungs. Thoracic involvement often occurs with non‐specific imaging findings and requires histopathological confirmation. We present the case of a 71‐year‐old man with a history of T‐cell large granular lymphocytic leukaemia and MGUS, who presented with smoking‐related interstitial lung disease on high‐resolution chest computed tomography (CT) and left lower lobe (LLL) consolidation. Positron emission tomography (PET)/CT revealed hypermetabolic activity in lung consolidations and mediastinal and diaphragmatic lymph nodes. A right diaphragmatic lymph node biopsy showed increased IgG4‐positive plasma cells (IgG4/IgG > 40%). Subsequent transbronchial biopsies confirmed the presence of dense lymphoplasmacytic infiltrates, fibrosclerosis and IgG4‐positive plasma cells in lung tissue. Given the patient's diabetes, rituximab (RTX) was chosen over corticosteroids. RTX, consisting of four weekly infusions, led to significant clinical improvement and complete metabolic response on follow‐up PET/CT. Reporting this case, we would like to emphasise the importance of considering IgG4‐RD in the differential diagnosis of atypical thoracic infiltrates and highlight the role of RTX as an effective therapeutic option in patients for whom corticosteroids are contraindicated.