Progress in the Classification of Hematopoietic and Lymphoid Neoplasms: Clinical Implications

In this chapter we provide a summary overview on classification of neoplasms in myeloid, lymphoid, and histiocytic linages, emphasizing changes that have had an effect on practice guidelines along with highlights to significant findings that have been identified in these entities since the publication of the most recent World Health Organization (WHO) classification in 2016. The myeloid neoplasms diagnosis does not rest solely with the routine microscopic examination. Ancillary studies, such as cytogenetic and molecular analysis, as well as other more specific laboratory evaluations, are just as important in formulating the correct diagnosis. The most recent 2016 WHO revisions developed a focus on early or in situ lesions, as well as definition of the earlier steps of neoplastic transformation, age as a disease-defining feature (e.g., diffuse large-cell lymphoma of the older adult), and site-specific impact on disease definition. In addition, there was an emphasis on overlapping or borderline entities, with fuzzy demarcation of morphologic, molecular, and genetic characteristics as areas of diagnostic challenge. Excisional biopsies are often required to render the diagnosis of histiocytic disorders. Morphologic review and an adequate battery of immunohistochemical stains are the most important tools in making an accurate diagnosis of these entities and in differentiating them from other, often-mistaken categories, most commonly non-Hodgkin lymphomas.