[Classification and differential diagnosis of NK/T-cell lymphomas]

The first classifications deviding B- and T-cell lymphomas were given with the Kiel classification and in the classification of Lukes and Collins. Those entities first described like T-zone lymphomas or T-CLL are the rarest types of the known entities today. The updated Kiel classification contained a large number of especially nodal peripheral T-cell lymphomas and seperated lymphoblastic ones from those which derive from peripheral T-cells. The increasing knowledge on T-cell differentiation allowed a more detailed description of entities according either to their primary organ manifestation (subcutaneous panniculitis-like T-cell lymphoma), their immunophenotype (NK/T-cell lymphomas with expression of CD56) or their rearrangement of the T-cell receptor (predominantly gamma/sigma-rearrangement in hepatosplenic T-cell lymphoma). The today's nodal and extranodal NK/T-cell lymphoma entities are summarized by the WHO classification. They can be seperated in primary leukemic ones, primary nodal ones, primary extranodal and primary cutaneous ones. In extranodal sites some lymphoma entities show a characteristic picture which is unique for this localisation. It has become clear that morphology alone is not able to lead to a reproducable classification of T-cell lymphomas. Today immunhistochemistry together with the distinct primary organ localisation are the major tools for classifying peripheral NK/T-cell lymphomas. In this paper distinct examples which are interesting in the differential diagnosis or which are distinct clinico-pathological entities are described. Moreover, an indication is given on some new rare entities which have not yet been listed in the WHO classification.