Two Cases of Lung Nodules Presenting With Kappa Light Chain Deposition Disease/Congo-Red Negative AL Amyloidosis.

Abstract

Light Chain Deposition Disease (LCDD) is a rare disorder characterized by the extracellular accumulation of monoclonal immunoglobulin light chains in various organs, most commonly the kidneys, but occasionally the heart, liver, and lungs. Pulmonary LCDD (PLCDD) is an exceedingly rare manifestation that can closely resemble other pulmonary diseases both radiologically and histologically. Unlike amyloidosis, LCDD does not exhibit Congo red positivity or apple-green birefringence under polarized light, which can complicate diagnosis. Mass spectrometry (MS) has emerged as a valuable diagnostic tool in such cases, providing definitive identification of monoclonal light chains when traditional histopathological techniques are inconclusive. We present two cases of pulmonary nodular LCDD, both in women with autoimmune backgrounds, including Sjögren's syndrome. Both patients presented with pulmonary nodules demonstrating FDG uptake on PET-CT scans. Histologic examination revealed amorphous eosinophilic deposits without Congo red birefringence. Proteomic analysis via laser microdissection and liquid chromatography-mass spectrometry (LC-MS) confirmed the presence of kappa light chain deposits, supporting a diagnosis of LCDD or non-Congophilic AL-type amyloidosis. One patient had no evidence of systemic disease, whereas the other showed clonal plasma cell infiltration with kappa light chain restriction, suggestive of a low-grade B-cell neoplasm. These cases underscore the diagnostic complexity of PLCDD and highlight the critical role of MS in differentiating it from pulmonary classical amyloidosis. Nodular PLCDD may follow an indolent course and often lacks systemic involvement, whereas diffuse forms carry a poorer prognosis because of extensive pulmonary and systemic involvement. Treatment remains focused on managing the underlying plasma cell disorder, though standardized therapeutic protocols are lacking because of the rarity of the condition. Our findings emphasize the need for increased clinical awareness and advanced diagnostic strategies to appropriately identify and manage PLCDD.

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