Constipated plasma cells in extranodal marginal zone lymphoma of the lung

A 68 year-old woman with a history of Sjögren syndrome presented with a lung nodule on positron emission tomography scan. The lung wedge biopsy showed an infiltrate largely composed of atypical plasma cells with prominent eosinophilic, refractile globular cytoplasmic inclusions, displacing the nucleus to the periphery of the cell (panel A; hematoxylin and eosin [H&E] stain, original magnification ×100). Abnormal plasma cells infiltrated the bronchial epithelium, producing lymphoepithelial lesions (panel B; H&E stain, original magnification ×400). Atypical plasma cells were positive for CD79a (panel C; original magnification ×200), but negative for CD20, positive in a lesser number of small B cells (not shown). The cytoplasmic inclusions stained for immunoglobulin G (panel D; original magnification ×200). In situ hybridization was positive for κ light chain RNA (panel E; original magnification ×200), but negative for λ RNA (panel F; original magnification ×200).

The morphological appearance raised the possibility of crystal-storing histiocytosis, in which crystallized immunoglobulin molecules accumulate within the cytoplasm of histiocytes. In this case, the globular inclusions are confined to the cytoplasm of plasma cells, a finding usually related to defective secretion of the immunoglobulin product and most often seen in lymphoplasmacytic neoplasms. The unusual morphologic appearance is a clue for the diagnosis of extranodal marginal zone lymphoma.