Diagnostic criteria of primary cutaneous B-cell lymphomas and pseudolymphomas.

Primary B-cell lymphomas of the skin are defined as malignant B-cell proliferations presenting with cutaneous involvement alone and no evidence of extracutaneous manifestations over a period of at least six months when complete staging has been performed. The major subtypes are follicle center-cell lymphoma, marginal zone lymphoma and large B-cell lymphoma of the leg (EORTC classification 1997). Primary B-cell lymphomas of the skin differ significantly from nodal lymphomas especially with respect to their clinical behavior. Pseudolymphomas of the skin are inflammatory diseases that simulate malignant lymphomas either clinically, histopathologically, or both. Particular pseudolymphomas may mimic cutaneous B-cell lymphomas. The most important examples are: lymphomatoid drug reactions, lymphocytoma (borrelia burgdorferi as causative agent), arthropod reactions, pseudolymphomas associated with vaccinations or tattoos and inflammatory pseudotumors. In recent years, new immunohistological and molecular techniques have added important criteria for the differentiation of cutaneous lymphomas from pseudolymphomas.