Pediatric Natural Killer Cell Malignancy

Natural killer (NK) cell malignancy is a heterogeneous disorder and rare, representing <1% of non-Hodgkin lymphomas for most of the world, except in Asia and Latin America. In Asia, especially, the incidence of NK-cell lymphomas is approximately 7%–10% of lymphomas [Au et al., 2005]. The pathogenesis of NK-cell malignancies has not yet been fully elucidated. In the 2008 World Health Organization (WHO) classification of tumors of hematopoietic and lymphoid tissues, there are two entities associated with NK cells: mature T-cell and NKcell neoplasms (including chronic lymphoproliferative disorder of NK cells, aggressive NK-cell leukemia, and extranodal NK/T-cell lymphoma, nasal type) and NK-cell lymphoblastic leukemia/lymphoma, which is included provisionally in the category of acute leukemias of ambiguous lineage (Table 1) [Swedlow et al., 2008]. What was previously described as blastic NK-cell leukemia is defined as blastic plasmacytoid dendritic cell neoplasm in the WHO 2008 classification [Swedlow et al., 2008]. In pediatric patients, the four NK cell neoplasms are rare, with blastic NK-cell lymphoma being the most prevalent.