IgG4-related pleurisy with suspected asbestos involvement

A man in his 60s with a history of asbestos exposure had been followed for progressive right-sided pleural thickening over several years. Imaging findings on high-resolution CT and ¹⁸F-fluorodeoxyglucose positron emission tomography/CT strongly suggested malignant pleural mesothelioma. However, both CT-guided pleural biopsy and endobronchial ultrasound-guided transbronchial needle aspiration of the mediastinal lymph nodes showed no evidence of malignancy, and a definitive diagnosis could not be established. After a 2-year interruption in follow-up, pleural thickening had further progressed. Immunostaining of previously obtained tissue samples ultimately led to a diagnosis of immunoglobulin G4 (IgG4)-related pleurisy. Steroid therapy resulted in a reduction of pleural thickening. This case highlights the importance of considering IgG4-related pleurisy in the differential diagnosis of pleural disease mimicking malignant pleural mesothelioma, particularly in patients with a history of asbestos exposure.