Anaplastic large cell lymphoma

Anaplastic large cell lymphoma (ALCL) was first recognized in the 1980s and defined as a distinct clinicopathologic entity based on common histological (pleiomorphic cells) and immunophenotypic (CD30 expression) features. However, neither the presence of pleiomorphic cells nor CD30 positivity proved to be entirely specific, as heterogeneity was observed in morphology and antigen profile of the tumour cells as well as in the clinical presentation of the disease. Subsequently, a characteristic cytogenetic abnormality was identified, the t(2;5), that led to the identification of the anaplastic lymphoma kinase (ALK) gene (which is involved in most translocations of ALCL) and insights into the pathogenesis. The use of reverse transcription polymerase chain reactions assays for the detection of ALK transcripts and the development of antibodies against the ALK protein made a new classification possible between ALK-positive and ALK-negative ALCLs. The morphology, phenotype, genotype and clinical features of ALK-positive ALCL have been described very well by now, and it is regarded as a distinct entity within the broad spectrum of ALCLs, while a remaining issue for the future is a better characterization of ALK-negative ALCL.