Cancer is characterized by the uncontrolled growth and spread of abnormal cells. Normally, cells follow an orderly process of growth, division, and death, but in cancer, this regulation breaks down, leading to malignant proliferation. These abnormal cells disregard the signals that govern cell behavior, allowing them to invade surrounding tissues and sometimes spread to distant sites.
Leukemia and lymphoma are distinct yet related hematologic, or blood, malignancies. The primary difference lies in where the cancer originates and how the malignant cells behave. Leukemia is generally a cancer of the blood and bone marrow, while lymphoma primarily involves the lymphatic system.
Leukemia A Cancer of the Blood and Bone Marrow
Leukemia is a cancer that begins in the bone marrow, the soft, spongy tissue inside bones. The bone marrow creates all blood cells, including red blood cells, platelets, and white blood cells. In leukemia, one developing blood cell line, often a type of white blood cell, undergoes a genetic change and multiplies uncontrollably.
This uncontrolled production results in a massive number of abnormal, immature white blood cells, referred to as leukemia cells. These malignant cells circulate freely throughout the bloodstream, a defining characteristic of the disease. As these non-functional cells build up, they crowd out the healthy blood-forming cells within the bone marrow.
The lack of normal blood cells leads to complications like anemia, infections due to non-functional white blood cells, and easy bruising or bleeding from a lack of platelets. The disease is broadly classified based on its speed of progression. Acute leukemias are characterized by a rapid increase in immature cells, requiring immediate treatment.
Chronic leukemias involve more mature cells and progress much more slowly, sometimes taking years before symptoms appear. The cancer’s primary burden remains the bone marrow and the circulating blood, distinguishing it from malignancies that form localized masses.
Lymphoma Cancer in the Lymphatic System
Lymphoma originates within the lymphatic system, a widespread network that plays a major role in immune function. This system includes lymph nodes, the spleen, the thymus, and vessels that transport infection-fighting lymphocytes. The lymphatic system also helps manage fluid balance and filter waste.
When a lymphocyte in this system undergoes a malignant transformation, it begins to multiply and accumulate, leading to the formation of a localized mass or solid tumor. The most common presentation of lymphoma is an enlargement or swelling of the lymph nodes, often in the neck, armpit, or groin, as the cancerous cells build up. Unlike leukemia, which is a cancer of the circulatory system, lymphoma is primarily a solid tumor cancer of the immune system’s tissues.
While the disease starts in the lymphatic system, the cancer cells can eventually spread to other parts of the body. This spread can involve organs outside of the primary lymph tissue, such as the lungs, liver, or bone marrow. The initial site of origin, presenting as a mass or tumor within a lymph structure, remains the key difference. The behavior of the malignant lymphocytes, which tend to aggregate and form a tumor, defines lymphoma.
The Common Ancestry of Blood Cancers
Leukemia and lymphoma are grouped together as blood cancers due to their shared cellular origin in the bone marrow’s hematopoietic system. All blood cells, including the white blood cells involved in these cancers, originate from hematopoietic stem cells (HSCs) residing in the bone marrow. These stem cells are multipotent, meaning they can differentiate into any mature blood cell type.
The hematopoietic system branches into two main developmental pathways: the myeloid lineage and the lymphoid lineage. The myeloid line gives rise to red blood cells, platelets, and white blood cells like neutrophils and monocytes. The lymphoid line produces lymphocytes (T-cells and B-cells) responsible for targeted immune responses.
Both leukemia and lymphoma arise when a cell from either of these lineages becomes cancerous. Many leukemias and almost all lymphomas involve the lymphoid lineage, affecting the same family of white blood cells. The difference between a lymphoid leukemia and a lymphoma is where the cancer establishes its primary foothold and how the abnormal cells behave.
A lymphoid leukemia occurs when the malignant lymphocytes stay mostly in the bone marrow and flood the bloodstream. A lymphoma occurs when the malignant lymphocytes cluster together to form a solid tumor, typically in a lymph node. The common ancestry explains why some cancers have overlapping features or can even transform from one type to another, such as in cases where a lymphoma spreads extensively to the bone marrow and blood, taking on characteristics of a leukemia.
Subtyping and Categorization
Once a diagnosis of leukemia or lymphoma is established, medical professionals use specific criteria to subtype the disease, which guides treatment. The first axis of classification involves the speed of cell proliferation, categorizing them as either acute or chronic. Acute forms are characterized by rapid growth and the presence of immature, non-functional cells.
The second major axis is the cell line of origin, dividing the cancers into lymphoid or myeloid types. For example, acute myeloid leukemia (AML) affects the myeloid lineage, while chronic lymphocytic leukemia (CLL) affects the lymphoid lineage. This classification dictates the specific cellular targets for therapy.
Lymphomas are divided into two primary categories: Hodgkin Lymphoma (HL) and Non-Hodgkin Lymphoma (NHL). This distinction is made by examining a tissue sample under a microscope. Hodgkin Lymphoma is characterized by the presence of the unique, large malignant Reed-Sternberg cell.
If the Reed-Sternberg cell is absent, the cancer is classified as one of the many types of Non-Hodgkin Lymphoma. Non-Hodgkin Lymphoma is far more common and encompasses a diverse group of over 70 different subtypes that can be further classified as B-cell or T-cell lymphomas. The presence or absence of the Reed-Sternberg cell is the defining feature that separates these two broad lymphoma groups.