Retroperitoneal schwannoma with monoclonal plasma cell infiltration: an exceptionally rare collision tumor?

Collision tumors within the retroperitoneum are rare. We present the case of a 54-year-old man with an incidental finding of a well-defined iliopsoas mass. He underwent marginal resection via an anterior superior ilio-inguinal approach. Histological examination revealed typical features of a schwannoma surrounded by a notable plasma cell infiltrate. On immunohistochemistry stains, the spindled cells displayed diffuse, strong nuclear and cytoplasmic positivity for S100, whereas epithelial membrane antigen, glial fibrillary acidic protein, and neurofilament stains were uniformly negative. The plasma cell infiltrate was diffusely positive for CD 138, with a majority of them demonstrating positive staining for lambda light chain and negative for kappa light chain. Hematological review found no evidence of marrow plasmacytosis and multiple myeloma was ruled out. At 12 month follow-up, the patient remains free of any recurrence. To our knowledge this is the second case of a schwannoma coexisting with a monoclonal plasma cell infiltrate and the first in the retroperitoneum.