Diffuse large B-cell lymphoma
|Diffuse large B cell lymphoma|
|Classification and external resources|
Diffuse large B-cell lymphoma (DLBCL or DLBL) is a cancer of B cells, a type of white blood cell responsible for producing antibodies. It is the most common type of non-Hodgkin lymphoma among adults, with an annual incidence of 7-8 cases per 100,000 people per year. This cancer occurs primarily in older individuals, with a median age of diagnosis at approximately 70 years of age, though it can also occur in children and young adults in rare cases. DLBCL is an aggressive tumour which can arise in virtually any part of the body, and the first sign of this illness is typically the observation of a rapidly growing mass, sometimes associated with fever, weight loss, and night sweats.
The causes of diffuse large B-cell lymphoma are not well understood. Usually DLBCL arises from normal B cells, but it can also represent a transformation of other types of lymphoma or leukaemia. An underlying immunodeficiency, such as AIDS, is a significant risk factor. Infection with Epstein-Barr virus has also been found to contribute to the development of some subgroups of DLBCL.
Diagnosis of DLBCL is made by removing a portion of the tumour through a biopsy, and then examining this tissue using a microscope. Usually an experienced hematopathologist makes this diagnosis. Several subtypes of DLBCL have been identified, each having a different clinical presentation and prognosis. However, the usual treatment for each of these is chemotherapy, often in combination with an antibody targeted at the tumour cells. Through these treatments, more than half of patients with DLBCL can be cured, and overall survival for older adults at five years is around 58%.
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. The World Health Organization (WHO) classification system defines 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.
Within cellular morphology three variants are most commonly seen: centroblastic, immunoblastic, and anaplastic. Most cases of DLBCL are centroblastic, having the appearance of medium-to-large-sized lymphocytes with scanty cytoplasm. Oval or round nuclei containing fine chromatin are prominently visible, having two to four nucleoli within each nucleus. Sometimes the tumour may be monomorphic, composed almost entirely of centroblasts. However, most cases are polymorphic, with a mixture of centroblastic and immunoblastic cells.
Immunoblasts have significant basophilic cytoplasm and a central nucleolus. A tumour can be classified as immunoblastic if greater than 90% of its cells are immunoblasts. This distinction can be problematic, however, because hematopathologists reviewing the microscope slides may often disagree on whether a collection of cells is best characterized as centroblasts or immunoblasts. Such disagreement indicates poor inter-rater reliability.
The third morphologic variant, anaplastic, consists of tumour cells which appear very differently from their normal B cell counterparts. The cells are generally very large with a round, oval, or polygonal shape and pleomorphic nuclei, and may resemble Hodgkin cells or Reed-Sternberg cells.
Gene expression profiling studies have also attempted to distinguish heterogeneous groups of DLBCL from each other. These studies examine thousands of genes simultaneously using a DNA microarray, looking for patterns which may help in grouping cases of DLBCL. Many studies now suggest that cases of DLBCL, NOS can be separated into two groups on the basis of their gene expression profiles; these groups are known as germinal centre B-cell-like (GCB) and activated B-cell-like (ABC). Tumour cells in the germinal centre B-cell-like subgroup resemble normal B cells in the germinal centre closely, and are generally associated with a favourable prognosis. Activated B-cell-like tumour cells are associated with a poorer prognosis, and derive their name from studies which show the continuous activation of certain pathways normally activated when B cells interact with an antigen. The NF-κB pathway, which is normally involved in transforming B cells into plasma cells, is an important example of one such pathway.
Another notable finding of recent gene expression studies is the importance of the cells and microscopic structures interspersed between the malignant B cells within the DLBCL tumour, an area commonly known as the tumour microenvironment. The presence of gene expression signatures commonly associated with macrophages, T cells, and remodelling of the extracellular matrix seems to be associated with an improved prognosis and better overall survival. Alternatively, expression of genes coding for pro-angiogenic factors is correlated with poorer survival.
With the apparent success of gene expression profiling in separating biologically distinct cases of DLBCL, NOS, some researchers examined whether a similar distinction could be made using immunohistochemical staining (IHC), a widely used method for characterizing tissue samples. This technique uses highly specific antibody-based stains to detect proteins on a microscope slide, and since microarrays are not widely available for routine clinical use, IHC is a desirable alternative. Many of these studies focused on stains against the products of prognostically significant genes which had been implicated in DLBCL gene expression studies. Examples of such genes include BCL2, BCL6, MUM1, LMO2, MYC, and p21. Several algorithms for separating DLBCL cases by IHC arose out of this research, categorizing tissue samples into groups most commonly known as GCB and non-GCB. The correlation between these GCB/non-GCB immunohistochemical groupings and the GCB/ABC groupings used in gene expression profiling studies is uncertain, as is their prognostic value. This uncertainty may arise in part due to poor inter-rater reliability in performing common immunohistochemical stains.
Standard treatment is CHOP-R, also referred to as "R-CHOP", an improved form of CHOP with the addition of rituximab (Rituxan), which has increased the rates of complete responses for DLBCL patients, particularly elderly patients. R-CHOP is a combination of one monoclonal antibody, 3 chemotherapy drugs, and one steroid: rituximab (Rituxan), cyclophosphamide (Cytoxan) doxorubicin(Hydroxydaunorubicin), vincristine (Oncovin), and prednisone. Chemotherapy is most often administered intravenously and is most effective when it is administered multiple times over a period of months (e.g. every 3 weeks, over 6 to 8 cycles). For someone who will be receiving chemotherapy it is suggested they have a PICC (Peripherally inserted central catheter) in their arm near the elbow, and the catheter will extend through one of the large veins to within centimeters of the heart. Radiation is another common treatment and is most effective if done prior to the chemotherapy, or as the last treatment after chemotherapy has been completed.
A second regimen under evaluation is R-EPOCH (rituximab - etoposide-prednisone-vincristine-doxorubicin-cyclophosphamide), which demonstrated a 5-year PFS of 79% in a phase II trial. A phase III trial, CALGB 50303, is now comparing R-EPOCH with R-CHOP in patients with newly diagnosed DLBCL. 
For children with diffuse large B-cell lymphomas, most studies have found 5-year survival rates ranging from about 70% to more than 90%.
- Primary mediastinal B-cell lymphoma - a subgroup of diffuse large B-cell lymphoma arising in the mediastinum of young adults
- Germinal center B-cell like diffuse large B-cell lymphoma - a subgroup of diffuse large B-cell lymphoma which seem to arise from normal germinal center B-cells
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