Reactive Intravascular B-Cell Immunoblastic Proliferation: A Recently Described Entity and a Mimicker of Intravascular Lymphoma

Reactive intralymphovascular immunoblastic proliferation (ILVIP) is a rare and diagnostically challenging entity that can closely mimic intravascular large B-cell lymphoma (IVLBCL). We report the comprehensive clinicopathologic features of two patients with B-cell lineage ILVIP identified in bowel resection specimens. Both patients presented with small bowel obstruction requiring surgical intervention, and one patient was initially erroneously diagnosed with IVLBCL. Neither patient had systemic findings suggestive of lymphoma, such as lymphadenopathy, hepatosplenomegaly, or B symptoms. Histologic evaluation demonstrated focal ILVIP composed of intermediate-to-large B-lineage immunoblasts positive for CD45, CD79a, and MUM1 with polytypic light-chain expression, and negative for CD20, PAX5, CD138, Epstein–Barr virus, and HHV8. The immunoblasts showed a high proliferation index (80–100%) in both cases. Recognition of ILVIP in specimens resected for bowel obstruction in otherwise healthy patients is essential to avoid misinterpretation as intravascular lymphoma and prevent unnecessary treatment.