Indolent T-Cell/Natural Killer-Cell Lymphomas/Lymphoproliferative Disorders of the Gastrointestinal Tract d What Have We Learned in the Last Decade?

Primary gastrointestinal (GI) T-cell and natural killer (NK) ecell lymphomas/lymphoproliferative disorders (LPD) are uncommon, and they are usually aggressive in nature. However, T-cell and NKcell lymphoma/LPD of the GI tract with indolent clinical course has been reported over the past 2 decades. Indolent T-cell LPD was formally proposed a decade ago in 2013 and 4 years later recognized as a provisional entity by the revised fourth edition of WHO Classi fication of Tumours of Haematopoietic and Lymphoid Tissues in 2017. Indolent T-cell LPD of the GI tract has been changed to indolent T-cell lymphoma of the GI tract as a distinct entity by the fifth edition of WHO Classi fication of Haematolymphoid Tumours , but the International Consensus Classi fication of mature lymphoid neoplasms prefers indolent clonal T-cell LPD of the GI tract instead. In the past decade, indolent lymphoma/LPD of the GI tract has been expanded to NK cells, and as such, indolent NKcell LPD of the GI tract was recognized as an entity by both the fifth edition of WHO Classi fication of Haematolymphoid Tumours and the International Consensus Classi fication. The underlying genetic/ molecular mechanisms of both indolent T-cell lymphoma/LPD of the GI tract and indolent NK-cell LPD of the GI tract have been recently discovered. In this review, we describe the history; salient clinical, cytohistomorphologic, and immunohistochemical features; and genetic/genomic landscape of both entities. In addition, we also summarize the mimics and differential diagnosis. Finally, we propose future directions with regard to the pathogenesis and clinical management. Published by Elsevier Inc. on behalf of the United States & Canadian Academy of Pathology.