Blastic NK-cell lymphoma is an aggressive systemic neoplasm commonly involving the skin. Although the term 'blastic NK-cell lymphoma' implies a derivation from natural killer cells, there is a considerable body of evidence pointing to a derivation from a common myeloid and lymphoid cell precursor, identified recently as the plasmacytoid type-2 dendritic cell (DC2, 'plasmacytoid monocyte') [1-4]. Malignant transformation would occur at a very early stage of differentiation. Recently, there has been a proposal to rename this entity as 'early plasmacytoid dendritic cell leukaemia-lymphoma' .
Blastic NK-cell lymphoma is not included in the European Organization for Research and Treatment of Cancer (EORTC) classification but is included within the precursor T-cell lymphomas as a neoplasm of uncertain lineage and stage of differentiation in the World Health Organization (WHO) classification [6,7]. Cases with histopathological and phenotypical features similar to those of blastic NK-cell lymphoma have also been termed 'agranular CD4+ CD56+ haematodermic neoplasms' [2,4,8-10].
Although a relationship between blastic NK-cell lymphoma and myelogenous leukaemia has been postulated [11-15], with evolution into myelogenous leukaemia being documented in a few patients , typical cases of acute myeloid leukaemia involving the skin usually have a different phenotype, being almost always negative for CD56 (see Chapter 18) . However, overlapping myeloid and lym-phoid features are known also in other neoplasms such as chronic myelogenous leukaemia, in which blast crisis in 10% of the cases reveals a B- or, more rarely, a T-cell phenotype.
In most cases, at presentation, blastic NK-cell lymphoma is confined to the skin or skin lesions are the first manifestation of the disease [17-22]. Leukaemic spread after variable periods of time is the rule , indicating that primary cutaneous cases most likely represent examples of so-called 'aleukaemic leukaemia cutis' . The recent identification of plasmacytoid monocytes resident within the skin provides a theoretical background to the frequent occurrence of these lymphomas with lesions confined to the skin .
In contrast to true NK-cell neoplasms, which are more frequently reported in Asians, blastic NK-cell lymphoma has been observed in all races. Another different feature from lymphomas of true NK-cell origin is the constant absence of Epstein-Barr virus (EBV) in blastic NK-cell lymphoma.
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