State of the art in natural killer cell malignancies.

The recently updated World Health Organization (WHO) classification of tumors of hematopoietic and lymphoid tissues, published in 2008, has made great advances in revising the disorders previously included in the pool of natural killer (NK) cell tumors. Although NK cell neoplasms represent a relatively rare group of diseases, accounting for <5% of all lymphoid neoplasms, they include very distinctive conditions both clinically and pathologically. This family of diseases includes the most indolent clinical forms, such as the provisional new entry of chronic lymphoproliferative disorder of NK cells (CLPD-NK) in the WHO classification, as well as one of the most fatal diseases recognized in medical oncology, aggressive NK cell leukemia (ANKL), which is characterized by a prognosis of weeks, or even days. In addition, some disorders previously identified as blastic NK cell lymphoma within the NK cell system have been more properly defined and included in the blastic plasmacytoid dentritic cell neoplasms, although rare cases of bona fide immature NK lymphoid tumors (now classified as NK cell lymphoblastic leukemia/lymphoma) have been reported in the literature. This paper focuses on recent concepts and progress in morphology, pathogenesis, clinicopathological features, treatment approaches, and outcomes of NK cell malignancies.