Definition: Intravascular large B-cell lymphoma (IVLBCL) is a rare subtype of non-Hodgkin lymphoma in which the tumor cells selectively grow within the blood vessel lumina. Usually there are no circulating neoplastic cells in the peripheral blood. It has been known by many names, including intravascular lymphomatosis, angiotropic lymphoma and (in the past) by the term malignant angioendotheliomatosis. Clinical: It is usually seen in 6th or 7th decades of life and most commonly involves the central nervous system (CNS), skin, kidney, lungs, adrenals, and liver but may be seen in other organs. In the Western countries, it mainly presents with CNS and cutaneous symptoms, where as in patients from Asian countries, the main major manifestation is hemophagocytic syndrome. Morphology: Medium or large-sized atypical lymphoid cells are found within small or intermediate-sized blood vessels, especially capillaries and post-capillary venules. They are usually admixed with red blood cells and may be trapped within fibrin thrombi. The tumor cells may sometimes palisade along the vessel wall. When tightly packed within vessel lumina, they may appear like cohesive islands of tumor mimicking carcinoma. The surrounding native tissue may show infarction and hemorrhage. The tumor cells express CD45 and pan B-cell markers and show immunoglobulin gene rearrangements. The differential diagnosis includes acute leukemia and carcinomatosis. References: Jaffe, ES et al. (2011), Hematopathology, Saunders/Elsevier, Philadelphia, PA. Ponzoni, M et al. Definition, Diagnosis, and Management of Intravascular Large B-cell Lymphoma: Proposals and Perspectives From an International Consensus Meeting. J Clin Oncol 25:3168-3173, 2007.