Intravascular NK/T-Cell Lymphoma: What We Know about This Diagnostically Challenging, Aggressive Disease

SIMPLE SUMMARY: Intravascular lymphoma is a neoplasm with tumor cells localized exclusively within blood vessel lumina. Most cases are of B-cell origin; only rare cases of NK or T-cell lineage. Unlike intravascular large B-cell lymphoma, which is a well-recognized entity in the WHO classification, intravascular NK/T-cell lymphoma is not considered a specific entity and there is still a debate about whether it is closer to aggressive NK leukemia or to extranodal NK/T-cell lymphoma. Our aim was to summarize the clinical, pathological, and molecular data on intravascular NK/T-cell lymphoma, which is a challenge both in terms of diagnosis and treatment. Recent molecular studies improved our understanding of the mechanism of lymphomagenesis, showing that multiple genetic events associated with EBV infection are required for the pathogenesis of this aggressive lymphoma. Promising therapeutic results may be offered by immune checkpoint inhibitors due to the high PD-L1 expression, which is possibly related to EBV infection. ABSTRACT: Intravascular lymphoma is a form of lymphoid malignancy characterized by neoplastic cells growing almost exclusively within the lumina of small- to medium-sized blood vessels. Most cases are of B-cell origin with rare cases of natural killer or T-cell lineage. Extranodal sites are affected, mainly the skin and central nervous system, although any organ may be involved. Intravascular NK/T-cell lymphoma deserves special attention because of its clinicopathologic features and the need for adequate immunophenotyping combined with clonality test for a proper diagnosis. Moreover, intravascular NK/T-cell lymphoma is strongly linked to Epstein–Barr virus (EBV), which is considered to play a role in tumorigenesis and to be responsible for the aggressive behavior of the disease. In this paper, we review the current knowledge on this rare lymphoma and, in particular, the most recent advances about its molecular landscape. The main distinguishing features with other EBV-related entities, such as extranodal NK/T-cell lymphoma, EBV-positive primary nodal T/NK-cell lymphoma, and aggressive NK-cell leukemia, are discussed to help pathologists obtain the correct diagnosis and consequently develop an adequate and prompt therapy response.