Leukemia and lymphoma are two types of cancer that affect the blood. Both cancers typically affect the white blood cells. Lymphoma is a cancer of a specific white blood cell called a lymphocyte. Leukemia is a broader term describing cancers of blood cell precursor cells and starts within the bone marrow. Pets with lymphoma and leukemia have very similar clinical signs and laboratory test results, and even the most astute pathologist can easily confuse the two diagnoses. The prognosis and treatment options vary greatly; therefore, it’s extremely important to have an accurate diagnosis.

Lymphoma

Lymphoma is a cancer of lymphocytes, which are a type of white blood cell. There are several different forms of lymphomas in dogs and cats, but the most common form involves an excessive proliferation of lymphoblasts (immature lymphocytes) within lymph nodes and organs of the body. Lymphoma is typically classified as either being of a B-lymphocyte or T-lymphocyte origin.

Leukemia

Leukemia refers to several different types of cancers arising from the different blood cell elements within bone marrow. Animals can develop leukemia of white blood cells, red blood cells, or platelets. Acute leukemias are first classified into one of 2 categories: acute lymphoid leukemias (ALL), which arise from immature lymphocytes (and can be of either a B-cell or T-cell origin), and acute non-lymphoid leukemias (also referred to as acute myeloid leukemias or AML), which arise from all other immature blood cell precursors in the bone marrow.

Origins

White blood cells are created in the bone marrow via a complicated hierarchy of cell division. Stem cells are the most primitive forms of the blood cell elements. These cells divide and give rise to slightly more specialized cells, which continue to progressively differentiate, until all of the finalized mature blood elements are created and “ready” to be released into the blood stream.

One of the main break off points during the maturation of blood cells in the bone marrow occurs when cells are slated to mature into what are known as lymphoid cells or myeloid cells.

Those destined to the lymphoid path start out as lymphoblasts and will further develop into different types: B-lymphocytes, T-lymphocyte, or plasma cells. Those destined to the myeloid pathway also start out as blasts and will further develop into one of the other four types of white blood cells (neutrophils, monocytes, eosinophils, or basophils), red blood cells, or platelets.

When examining bone marrow cells prior to their specialization towards a specific type of cell (i.e., the “blast” cells), they are virtually indistinguishable from each other based on appearance alone. There are no accurate ways to simply look at a very primitive blast cell and know whether it is destined to become a lymphocyte, a neutrophil, or a monocyte.

In leukemia, somewhere along the maturation process within the bone marrow, a single cell begins dividing uncontrollably and the cells are released into the blood stream where they can cause the total white blood cell count to rise and also accumulate within lymph nodes, where they can then cause these organs to enlarge. The tricky part is the same changes (abnormal cells in circulation and enlarged lymph nodes) are seen with pets with lymphoma as well.

These abnormal cells are often picked up on routine blood tests or can be noticed via an aspirate of an enlarged lymph node. The abnormal results are usually noticed, and a lab technician or clinical pathologist will evaluate a blood smear and validate results.

Testing

Bone marrow cytology is a test considered part of routine staging for pets with any hematological (blood-borne) cancer. Bone marrow analysis provides information as to what percent of this tissue is comprised of cancerous blast cells, which is useful in distinguishing lymphoma from acute leukemia. Most dogs with lymphoma have a low level of cancer cells in their bone marrow; however, if the percent of blast cells exceeds >20–30 percent of the entire sample, it is more typical for a case of leukemia.

Bone marrow cytology can be inaccurate in determining the exact cell of origin of the abnormal cells in question.

Flow cytometry is designed to look for specific markers located on the surface of cancer cells to aid in determining their origin (e.g., whether lymphoid or non-lymphoid in origin). This test can be performed on blood, bone marrow, and also fine needle aspirates of tissues (e.g., lymph nodes). One of the main markers this test can examine for is called CD34. In general, cells of bone marrow origin will express CD34, whereas those located in the periphery of the body will not. If detected, the presence of CD34 strongly supports the diagnosis of an acute leukemia.

PCR for antigen receptor rearrangement (PARR) is a DNA based test. PARR is only valuable for testing lymphocytes, so when we choose this test, we must be at least reasonably certain the cells in question in our samples are lymphocytes. Additionally, PARR cannot distinguish lymphoma from acute leukemia of lymphocyte origin. Essentially, what PARR tells us is 1) If the sample is from a cancerous condition of lymphocytes, and 2) if it is of a B-lymphocyte or a T-lymphocyte origin.

Cytochemistry staining is similar to flow cytometry. This type of test looks for markers on the surface of, or within, white blood cells. An equivalent of this test on a biopsy sample would be called immunohistochemistry.

Treatment

Lymphoma treatment uses chemotherapy as the most common treatment. It can be administered orally or intravenously. It is common to have hair loss, experience vomiting and diarrhea, and have low blood cell counts. Because chemotherapy causes immunosuppression, there is an increased risk for infection. Radiation treatments may be recommended instead of or in combination with chemotherapy as a more targeted treatment. Whole or half-body radiation is used since the cancer is not usually contained to one area. Surgery may also be an option for localized lymphoma that only affects a single lymph node. Usually this has the best chance for treatment success since it is more likely that the lymphoma has been caught early on before metastasis.

Acute leukemia can be treated with aggressive chemotherapy, while continued monitoring may be recommended for chronic leukemia.