Anaplastic large-cell lymphomas of B-cell phenotype are anaplastic lymphoma kinase (ALK) negative and belong to the spectrum of diffuse large B-cell lymphomas

Timely topic: anaplastic lymphoma kinase (ALK) spreads its influence

Anaplastic lymphoma kinase (ALK) is normally not expressed in human tissues except selected sites in the nervous system. Its expression and constitutive activation as a result of a chromosomal translocation involving 2p23 plays a pivotal role in the genesis of anaplastic large cell lymphoma. ALK expression has been instrumental in defining a homogeneous subset from the category of anaplastic large cell lymphoma, characterised by occurrence in young patients, primary systemic presentation, favorable prognosis, a broad morphological spectrum, nuclear and/or cytoplasmic immunostaining for ALK protein, and a number of possible fusion partner genes such as NPM, ATIC, TFG, TPM3 and CLTCL. Recently ALK has been implicated in the genesis of another tumour type, the inflammatory myofibroblastic tumours. The ALK-positive examples occur in children and young adults, involving a variety of sites, such as the abdomen, mesentery, liver, bladder, mediastinum, lung and bone. The partner genes identified in some cases are TPM3 (tropomyosin 3) and TPM4 (tropomyosin 4). These molecular findings also further support the neoplastic nature of at least a subset of inflammatory myofibroblastic tumours.

CD30+ anaplastic large cell lymphoma: a review of its histopathologic, genetic, and clinical features

Anaplastic large cell lymphoma (ALCL) represents a generally recognized group of large cell lymphomas. Defining features consist of a proliferation of predominantly large lymphoid cells with strong expression of the cytokine receptor CD30 and a characteristic growth pattern. With the use of molecular and clinical criteria, 3 entities of ALCL have been identified: primary systemic anaplastic lymphoma kinase (ALK)+ ALCL, primary systemic ALK− ALCL, and primary cutaneous ALCL. ALK expression is caused by chromosomal translocations, most commonly t(2;5). ALK+ ALCL predominantly affects young male patients and, if treated with chemotherapy, has a favorable prognosis. It shows a broad morphologic spectrum, with the “common type,” the small cell variant, and the lymphohistiocytic variant being most commonly observed. The knowledge of the existence of these variants is essential in establishing a correct diagnosis. ALK− ALCL occurs in older patients, affecting both genders equally and having an unfavorable prognosis. The morphology and the immunophenotype of primary cutaneous ALCL show an overlap with that of lymphomatoid papulosis. Both diseases have an excellent prognosis, and secondary systemic dissemination is only rarely observed. The described ALCL entities usually derive from cytotoxic T cells. In contrast, large B-cell lymphomas with anaplastic morphology are believed to represent not a separate entity but a morphologic variant of diffuse large B-cell lymphoma. Malignant lymphomas with morphologic features of both Hodgkin disease and ALCL have formerly been classified as Hodgkin-like ALCL . Recent immunohistologic studies, however, suggest that ALCLs Hodgkin-like represent either cases of tumor cell–rich classic Hodgkin disease or (less commonly) ALK+ ALCL or ALK− ALCL.