Isolated pulmonary light chain deposition disease

Background: Pulmonary involvement is a significant aspect of many autoimmune diseases. Different pulmonary manifestations of autoimmune diseases encompass a variety of conditions that impact the lungs, including interstitial lung disease, cystic lung disease, pulmonary hypertension, and pleural involvement. One rare manifestation is light-chain deposition disease. Our case report aims to highlight the condition's rarity and presentation variability. Case presentation: F-fluorodeoxyglucose-avid pulmonary nodules with large pneumatoceles, but no evidence of metastatic disease, which increased in size over the next two years, increasing the concern for progressive lung disease. Subsequently, a lung biopsy revealed monotypic kappa light-chain deposition with plasma cell infiltration, establishing the diagnosis of light-chain deposition disease. Despite the radiographic progression, the patient remained asymptomatic and stable. She was managed conservatively with serial paraproteinemia and image-based monitoring. Conclusion: Pulmonary light chain deposition disease is an infrequent condition requiring a high index of suspicion for diagnosis. Imaging findings can help identify it, but biopsy remains the definitive diagnostic tool. Due to the lack of established management guidelines, treatment is personalized based on disease extent and organ involvement. Further research is needed to understand better this rare entity's prognosis and optimal treatment approaches. LEARNING POINTS: Pulmonary light-chain deposition disease is a rare and under-recognized manifestation.Clinicians must have a high index of clinical suspicion while evaluating cystic lung disease, particularly in the background of an autoimmune disease.This case reinforces the importance of considering light chain deposition disease as a differential in evaluation of unexplained lung nodules or cysts.