Morphology remains the mainstay of lymphoma diagnosis since it is, in effect, the collective expression of the immunophenotype, genotype, and normal cell counterpart. Once an entity has been defined on the basis of its collective properties, morphology on its own is often sufficient for a definitive diagnosis. However, lymphomas of identical morphology, but arising in different sites, may constitute different disease entities and some single disease entities may be morphologically heterogeneous. For example, anaplas-tic large cell lymphoma (ALCL) arising in lymph nodes behaves much more aggressively than the morphologically identical tumor arising in the skin,6 while the cytological features of enteropathy-type T-cell lymphoma are highly variable and do not influence its clinical behavior.7 Histological grade alone, which should not be confused with clinical aggressiveness, is no longer considered a basis for the separation of lymphomas into broad groups. Many entities may transform from low- to high-grade morphology as part of their natural history and can present de novo as either low- or high-grade lesions.

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