Lymphoma comprises two groups of diseases: Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL). Within both subsets are numerous variations with distinct biologic, molecular, and cytogenetic characteristics. The World Health Organization (WHO) classification of NHL, for example, now identifies several dozen broad entities and nearly 60 unique clinicopathologic subtypes. In addition to pathologic heterogeneity, there is clinical diversity within lymphomas, with some patients achieving cure, others...