A Giant Intra-Abdominal Plasmablastic Neoplasm Favoring Lymphoma Mimicking an Exacerbation of Severe Fistulizing Crohn's Disease: A Case Report.

BACKGROUND Plasmablastic lymphoma (PBL) is a rare, aggressive large B-cell lymphoma associated with immunosuppression. In Crohn's disease, it can mimic disease exacerbation or infection. We describe a rapidly enlarging intra-abdominal plasmablastic neoplasm in a medically complex patient, emphasizing diagnostic vigilance without inferring a causal association with biological therapy. CASE REPORT A 44-year-old woman with severe fistulizing Crohn's disease treated sequentially with adalimumab, infliximab, and ustekinumab developed progressive systemic deterioration despite partial intestinal stabilization. Her course included portal hypertension secondary to portal vein thrombosis, refractory ascites, severe malnutrition, and recurrent systemic and multidrug-resistant infections. In September 2023 imaging showed a rapidly enlarging intra-abdominal/pelvic mass measuring more than 22 cm. Core-needle biopsy demonstrated a plasmablastic neoplasm positive for CD38, CD138, MUM1, and c-MYC, with equivocal CD10 and negative CD45, favoring PBL. Epstein-Barr virus-encoded RNA in situ hybridization, CD20, PAX5, Ki-67, anaplastic lymphoma kinase, human herpesvirus 8, and tissue light-chain restriction were unavailable. Bone marrow examination, serum and urine immunofixation, complete monoclonal-protein assessment, and whole-body skeletal imaging were not performed; therefore, plasmablastic myeloma could not be definitively excluded. Intensive chemotherapy was not feasible because her Eastern Cooperative Oncology Group performance status was at least 3 and she had multiorgan dysfunction, severe malnutrition, and active infections. The patient was transitioned to palliative care and subsequently died. CONCLUSIONS Discordance between apparent intestinal control and systemic deterioration should prompt urgent tissue diagnosis. The findings favor PBL within the spectrum of plasmablastic neoplasms but neither establish drug-specific causation nor definitively exclude plasmablastic myeloma.