Classification
Diffuse large B-cell lymphoma encompasses a biologically and clinically diverse set of diseases, many of which cannot be separated from one another by well-defined and widely accepted criteria. However the World Health Organization (WHO) classification system does define more than a dozen subtypes, each of which can be differentiated based on the location of the tumour, the presence of other cells within the tumour (such as T cells), and whether the patient has certain other illnesses related to DLBCL. One of these well-defined groupings of particular note is "primary mediastinal (thymic) large B-cell lymphoma", which arises within the thymus or mediastinal lymph nodes.
In some cases, a tumour may share many features with both DLBCL and Burkitt lymphoma. In these situations, the tumour is classified as simply “B-cell lymphoma, unclassifiable, with features intermediate between diffuse large B-cell lymphoma and Burkitt lymphoma”. A similar situation can arise between DLBCL and Hodgkin’s lymphoma; the tumour is then classified as “B-cell lymphoma, unclassifiable, with features intermediate between diffuse large B-cell lymphoma and Hodgkin’s lymphoma”.
When a case of DLBCL does not conform to any of the well-defined subtypes, and is also not considered unclassifiable, then it is classified as “diffuse large B-cell lymphoma, not otherwise specified” (DLBCL, NOS). The majority of DLBCL cases fall into this category. Much research has been devoted to separating this still-heterogeneous group; such distinctions are usually made along lines of cellular morphology, gene expression, and immunohistochemical properties.
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