Non-Hodgkin's Lymphoma - Introduction

Locations of Non-Hodgkin's Lymphomas

Non-Hodgkin's lymphomas occur most often in lymph nodes in the chest, neck, abdomen, tonsils, and the skin. NHLs may also develop in sites other than lymph nodes. In Americans, this occurs most commonly in the digestive tract, although primary lymphomas of the central nervous system are on the rise. In Europeans, lymphomas outside the lymph nodes are more likely to develop around the tonsils.

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Cells Affected in NHL

About 85% of non-Hodgkin's lymphomas (NHLs) arise in B cells; the rest occur in T cells. Activation of a gene called BCL-2 is believed to be partly responsible for many B-cell lymphomas. This defect prevents apoptosis (a natural process whereby cells self-destruct) in the lymphoma cells.

Non-Hodgkin's Lymphomas Categories

There are more than 20 distinct types of non-Hodgkin's lymphomas. Most first arise in the lymph nodes, but about 20 - 30% of cases are now found outside the nodes, most often in the stomach, small intestine, skin, and brain.

Even experts disagree about the exact groupings. Lymphomas are categorized in a number of ways.

Classification by Cell Type, Appearance, and Genetic Make-up: The REAL System. Different classification systems for lymphoma have been proposed. The system used in this report is called REAL (Revised European-American Lymphoma Classification). It classifies all lymphomas by appearance, cell type, and genetic make-up:

Non-Hodgkin's lymphomas are first grouped as B cell or T cell.
Next, they are categorized by whether the B- and T-cell lymphomas were derived from immature (precursor) cells or mature (peripheral) cells.
The peripheral B and T cells are then classified by their appearance, genetic make-up, and specific chemical "markers," which further identify them.

T-cell lymphomas, Hodgkin's disease, and certain leukemias and aggressive lymphomas are covered in the REAL classification but are not discussed in any depth in this report.

Groups by Slow or Fast Growth. Each non-Hodgkin's lymphoma is further defined by its grade, or how aggressive it is:

Indolent (slow-growing), also referred to as low-grad
Aggressive (fast-growing), also referred to as intermediate- or high-grade

According to a 2002 report, half of new cases are now intermediate-grade lymphomas. Low-grade makes up 30% and high-grade 10% of all lymphomas.

Groups by Properties. Lymphomas are also grouped by certain properties:

Size (large versus small)
Shape (round versus irregular)
Whether they are or resemble blood plasma cells
Whether they are follicular (organized in round clusters) or diffuse (spread evenly throughout the lymph node)

Staging. Staging the disease is the next important step in classifying lymphomas. The stage (I through IV) of an NHL is determined by the number of tumors and whether they are still localized or have spread beyond the lymph node. In general, the higher the stage, the poorer the outcome, but other factors are important for a precise prognosis.

B-cell Lymphomas Classified by Grade

Indolent (Slow-Growing) Lymphomas (also referred to as Low-Grade Lymphomas)

Follicular lymphomas (FLs). Follicular small cleaved cell lymphoma (grade 1) and follicular mixed small and large cell lymphoma (grade II). FLs account for 70% of indolent tumors and 20% of all NHLs in industrialized countries. It is very rare in developing countries and in Asia.

Lymphoplasmacytoid/Waldenstrom's macroglobulinemia. Often found in bone marrow, lymph nodes, and spleen. Can cause blood to become viscous and "sticky."

Marginal zone lymphomas (MZL). MZLs often occur as a result of a pre-existing disorder such as hepatitis C, bacterial infection in the stomach (H. pylori ), or an autoimmune disorder (e.g., Sjögren's disease in the salivary glands or Hashimoto's thyroiditis in the thyroid gland). They may be classified as follows:

Monocytoid B-cell lymphoma, which involves only lymph nodes
Splenic marginal zone lymphoma, which affects the spleen, blood, and bone marrow
Mucosa-associated lymphoid tissue (MALT) lymphoma, which usually involves the gastrointestinal tract, thyroid, lung, breast, or skin

There is some controversy over whether MALT is a variation of MZL or a completely seperate type of lymphoma that is more suitably classified as a separate low-grade lymphoma. At this time, it is classified as an MZL.

Aggressive Lymphomas (also referred to as Intermediate- and High-Grade Lymphomas)

Diffuse large-cell lymphomas (DL). DLs are the most common NHLs, accounting for about 40% of all cases. Subtypes include the following:

Primary mediastinal large B-cell lymphoma
Follicular large cell lymphoma
Anaplastic large cell lymphoma
T-cell lymphomas (not covered in this report)

In about 40% of cases, these DL lymphomas appear in areas outside lymph nodes, including digestive tract, skin, bone, thyroid, and testes.

Burkitt's lymphoma/diffuse, small noncleaved cell lymphoma. This is the most common childhood NHL. In African children, it often involves facial bones and is associated with Epstein-Barr infection.

Mantle cell lymphoma. Mantle cell lymphomas are found in lymph nodes, the spleen, bone marrow, blood, and sometimes the gastrointestinal system (lymphomatous polyposis). It is similar to indolent lymphomas at the time of diagnosis, but it is more aggressive.

Lymphoblastic lymphoma. This lymphoma often occurs in young people. It is associated with a large mediastinal mass (occurring in chest cavity between the lungs) and carries a high risk for spreading to bone marrow and central nervous system.