S3838 Multifocal Colonic Masses Presenting as Secondary Extramedullary Plasmacytoma in Relapsing-Refractory Multiple Myeloma: A Case Report and Review of Literature

Introduction: Multiple myeloma (MM) is a common plasma cell malignancy; however, extramedullary plasmacytoma (EMP)—plasma cell proliferation outside of bone marrow—is an aggressive and uncommon phenotype. Gastrointestinal (GI) involvement is exceedingly rare. We present a case of multifocal secondary colonic EMP in a patient with relapsed refractory multiple myeloma (RRMM) and review literature of reported GI associated EMP. Case Description/Methods: A 70-year-old man with new onset constipation and bone marrow biopsy-proven MM, who failed multiple lines of chemotherapy, underwent positron emission tomography imaging which revealed increased focal metabolic activity within the hepatic flexure and sigmoid colon. Diagnostic colonoscopy showed a partially circumferential, partially obstructing mass in the distal ascending colon involving 2-thirds of luminal circumference with a luminal diameter of 10 mm, biopsies were performed. In addition, a 17 mm sessile cecal polyp and a 4 mm transverse colonic semi-pedunculated polyp were endoscopically resected. Pathology from all sites demonstrated plasmacytomas, staining positive for CD138 and MUM1, and were kappa restricted. Subsequent labs revealed bicytopenia and atypical lymphocytes with plasmacytoid morphology, suggestive of plasma cell leukemia. He ultimately began treatment with etoposide, prednisone, vincristine, cyclophosphamide, and doxorubicin (EPOCH) as bridging therapy prior to chimeric antigen receptor (CAR)-T cell infusion for treatment of RRMM and currently remains on CAR-T therapy. Discussion: While primary EMPs are more common and arise in the absence of systemic MM, secondary EMPs are rare and are associated with 15% lower 5-year survival outcomes than those without plasmacytoma at MM diagnosis. Gastrointestinal involvement is extremely rare and endoscopic evaluation is required for definitive diagnosis. GI presentations can include constipation, bowel obstruction, and rectal bleeding. Chemotherapy is typically the mainstay of treatment and surgery is usually avoided unless colonic obstruction is present. In our literature review of secondary EMP cases, we found 11 cases reported to date of isolated GI involvement— extensive and multifocal colonic involvement, as seen in our case, was reported in only 1 other case. Extramedullary disease typically portends a higher mortality in progressive MM; therefore, prompt recognition and treatment of colonic EMPs is crucial.