Gastric plasmacytoma with rod-shaped intracytoplasmic inclusions. Report of a case studied by light and electron microscopy and immunohistochemistry.

A 78-year-old man with primary gastric plasmacytoma showing massive rod-shaped intracytoplasmic inclusions is presented. The patient was initially diagnosed as having a benign gastric ulcer and underwent partial gastrectomy. Around the ulcer of the resected stomach specimen, diffuse proliferation of plasma cells with massive rod-shaped intracytoplasmic inclusions was noticed by light microscopy. Immunohistochemically, these cells were found to contain IgM-kappa-type immunoglobulin upon peroxidase-antiperoxidase (PAP) staining. Electron microscopy showed that the intracytoplasmic inclusions were localized in the rough endoplasmic reticulum and had a lattice structure with a periodicity of about 120 A.