WHO classification

Classification

Lymphoma

Lymphoma is a type of blood cancer that occurs when B or T lymphocytes divide faster than normal cells or live longer than they are supposed to. Lymphoma may develop in the lymph nodes, spleen, bone marrow, blood or other organs and eventually they form a tumor.

Typically, lymphoma presents as a solid tumor of lymphoid cells. These malignant cells often originate in lymph nodes, presenting as an enlargement of the node (a tumor).

It can also affect other organs in which case it is referred to as extranodal lymphoma. Extranodal sites include the skin, brain, bowels and bone.

Lymphomas are closely related to lymphoid leukemias, which also originate in lymphocytes but typically involve only circulating blood and the bone marrow and do not usually form static tumors.

There are many types of lymphomas, and in turn, lymphomas are a part of the broad group of diseases called hematological neoplasms.

Lymphomas are the third most common pediatric malignancy. Lymphomas account for 11% of all cancers diagnosed in children and more than twice as many cases are diagnosed in boys as in girls. Lymphomas are rare before the age of two and incidence increases with age thereafter such that lymphomas account for nearly a fifth (19%) of all childhood cancers diagnosed in 10-14 year-olds.

Because the whole system is part of the body's immune system, patients with a weakened immune system such as from HIV infection or from certain drugs or medication also have a higher incidence of lymphoma.

Lymphoma is the most common form of hematological malignancy, or "blood cancer", in the developed world.

Several classification systems have existed for lymphoma. These systems use histological findings and other findings to divide lymphoma into different categories.

Lymphomas are divided into two broad categories, depending on the appearance of their cancerous (malignant) cells. These are known as Hodgkin's lymphoma (HL) and non-Hodgkin lymphoma (NHL). Together, they are the third most common type of cancer in children.

Note! Classification systems generally classify lymphoma according to:

v Whether or not it is a Hodgkin lymphoma.

v Whether the cell that is replicating is a T-cell or B-cell.

v The site that the cell arises from.

The current accepted definition is the WHO classification, updated in 2008, is the latest classification of lymphoma and is based upon the foundations laid within the "Revised European-American Lymphoma classification" (REAL). This system attempts to group lymphomas by cell type (i.e. the normal cell type that most resembles the tumor) and defining phenotypic, molecular or cytogenetic characteristics. There are three large groups: the B-cell, T-cell, and natural killer cell tumors.

Subtypes according to the WHO classification:

* Mature B-cell neoplasms

* Mature T-cell and natural killer (NK) cell neoplasms

* Hodgkin Lymphoma

* Immunodeficiency-associated lymphoproliferative disorders