[An unusual cause of cystic lung disease: light chain deposition disease].

INTRODUCTION Light chain deposition disease is a rare clinical entity characterized by deposition of monoclonal immunoglobulin light chains in organs. The kidneys are almost always affected, while the lung manifestations that have been reported, including nodular or diffuse disease, especially cystic lesions, are unusual. CASE REPORT We report the case of a 60-year-old man with a diffuse infiltrative lung disease characterized by numerous apical cysts. The diagnosis of light chain deposition cystic lung disease was obtained by surgical lung biopsy. Light chain deposits in the salivary glands were the only extrapulmonary manifestation. Despite 12 chemotherapy cycles, the patient's lung function and radiological appearances worsened. CONCLUSION This is the fourth case describing a cystic lung disease due to light chain deposition in the literature. It highlights the need for comprehensive investigations so as not to miss this rare cause of cystic lung disease, which appears to be related to a primary pulmonary lymphoproliferative disorder. The only treatment that appears to be effective is lung transplantation.