Adult T-cell Leukemia/Lymphoma (ATLL): Understanding Your Pathology Report

Section Editor: David Li MD
June 24, 2026

Adult T-cell leukemia/lymphoma (ATLL) is a rare type of blood cancer that develops from T cells, a type of white blood cell that helps the immune system fight infection and regulate immune responses. When ATLL mainly affects the blood and bone marrow, it behaves like a leukemia. When it grows as a solid mass in lymph nodes or other tissues, it behaves like a lymphoma. Many patients have features of both, which is why the disease is called leukemia/lymphoma. ATLL occurs only in people who have been infected with a virus called human T-cell lymphotropic virus type 1 (HTLV-1), and it can involve the blood, lymph nodes, skin, liver, and spleen.

This article will help you understand the findings in your pathology report for adult T-cell leukemia/lymphoma, what each term means, and why it matters for your care.

What causes adult T-cell leukemia/lymphoma?

Adult T-cell leukemia/lymphoma (ATLL) is caused by long-term infection with the HTLV-1 virus. However, only a small number of people infected with HTLV-1 (about 3 to 5 percent) ever develop ATLL, usually after carrying the virus for many years. The virus is more common in some parts of the world, including southern Japan, the Caribbean, parts of South America, and parts of Africa. A few factors increase the risk:

Long-standing infection — Being infected at a young age, such as during infancy, and carrying the virus for decades.
A high level of infected cells — A larger number of HTLV-1-infected cells in the blood raises the risk.
Other factors — Older age and male sex are associated with a higher risk.

In most people with HTLV-1, the infected T cells stay quiet and cause no problems. Over many years, some infected cells acquire additional genetic changes that allow them to grow without control, and ATLL may eventually develop.

What are the symptoms of adult T-cell leukemia/lymphoma?

The symptoms of adult T-cell leukemia/lymphoma (ATLL) depend on the type of disease and which parts of the body are affected. Common symptoms include:

Swollen lymph nodes — Often in the neck, armpits, or groin.
Fever, night sweats, and unexplained weight loss.
Fatigue and weakness — Because the bone marrow cannot make enough normal blood cells.
Skin changes — Rashes, lesions, or lumps.
Abdominal discomfort or bone pain — Often related to an enlarged liver or spleen.
High calcium levels in the blood — A common feature of ATLL that can cause nausea, vomiting, constipation, or confusion, and can become serious if not treated.

What are the four types of adult T-cell leukemia/lymphoma?

Adult T-cell leukemia/lymphoma (ATLL) is divided into four types based on how the disease behaves and which parts of the body are affected. The type strongly influences treatment and outlook.

Acute — The fastest-growing form. It usually involves the blood, lymph nodes, skin, liver, and spleen, and symptoms often appear suddenly and progress quickly.
Lymphomatous — Mainly involves the lymph nodes and other tissues, with few cancer cells in the blood. It tends to grow and spread quickly, like the acute type.
Chronic — Grows more slowly and usually affects the blood, skin, and lymph nodes. It may remain stable for a time, but in some patients it can progress to a faster-growing form.
Smoldering — The slowest-growing type. It often causes few or no symptoms and may be found only on routine blood tests, but it still needs monitoring because it can progress over time.

How is the diagnosis made?

The diagnosis of adult T-cell leukemia/lymphoma (ATLL) usually requires a combination of tests. A biopsy of an affected tissue, such as a lymph node or skin lesion, is often the first step, and a pathologist examines the tissue under the microscope to look for abnormal T cells. A blood test may show elevated numbers of abnormal lymphocytes, and in some patients, leukemia cells may be seen circulating in the blood.

Flow cytometry and immunohistochemistry use antibodies to detect proteins on and inside the cells. In ATLL, the cells usually carry T-cell markers such as CD2, CD3, and CD5, often lack CD7, and are typically positive for CD4 (a helper T-cell marker) and negative for CD8. The protein CD25 is also commonly present. Together, these findings confirm that the cancer cells are T cells. Because ATLL occurs only in people infected with HTLV-1, a blood test for the virus is required to confirm the diagnosis, and tests that show the abnormal T cells all come from a single original cell (clonality) help distinguish ATLL from a reaction to infection.

What does adult T-cell leukemia/lymphoma look like under the microscope?

Under the microscope, the appearance of adult T-cell leukemia/lymphoma (ATLL) depends on where the cancer is located. In the lymph nodes, the cancer cells usually replace the normal structure and may spread through the large channels called sinuses. In the skin, cells may collect in the upper layer, forming small clusters. When ATLL cells circulate in the blood, they often have nuclei (the part of the cell that contains the genetic material) that are deeply folded into a shape that pathologists describe as a “flower cell,” a characteristic of this disease.

The cancer cells vary in size and shape. In slower-growing types, they may appear smaller and more normal, whereas in faster-growing types, they tend to be larger and more irregular. In some cases, larger immune cells infected with another virus, the Epstein-Barr virus (EBV), may also be seen, a sign that the immune system is weakened.

Biomarker and molecular testing in adult T-cell leukemia/lymphoma

Beyond confirming the diagnosis, testing for specific surface proteins on adult T-cell leukemia/lymphoma (ATLL) cells can guide treatment. Biomarkers are features of cancer cells that help predict how the disease will behave or which treatments may be effective. Two are especially important in ATLL:

CCR4 — Many ATLL cells carry a protein called CCR4. When it is present, a drug called mogamulizumab, an antibody that targets CCR4, may be used to help the immune system attack the cancer cells.
CD30 — Some ATLL cells carry a protein called CD30. When CD30 is present, a drug called brentuximab vedotin, which delivers chemotherapy directly to CD30-positive cells, may be considered.

Because these proteins determine whether certain targeted drugs are likely to help, your report may note whether the cancer cells are positive for CCR4 and CD30.

What is the prognosis for adult T-cell leukemia/lymphoma?

The outlook for adult T-cell leukemia/lymphoma (ATLL) depends mainly on the disease type. The smoldering and chronic types grow slowly and can remain stable for years, although they may change into a faster-growing form. The acute and lymphomatous types grow rapidly and are more difficult to treat; historically, outcomes for these types have been poor, though treatment continues to improve. Several features are linked with a less favorable outlook:

Disease type — Acute and lymphomatous ATLL have a less favorable outlook than the chronic and smoldering types.
High calcium in the blood — Associated with a worse outcome.
A high number of cancer cells in the blood.
Older age and poor overall health at diagnosis.

For patients with faster-growing ATLL who are well enough, an allogeneic stem cell transplant (using blood-forming cells from a donor) offers the best chance of long-term survival, although only some patients are able to receive it. Your prognosis depends on your own combination of these factors, which your care team can explain in the context of your specific report.

What happens after a diagnosis of adult T-cell leukemia/lymphoma?

Once adult T-cell leukemia/lymphoma (ATLL) is confirmed, treatment is planned based on the type of disease, biomarker results, the patient’s age, and overall health. The pathology findings help guide several decisions:

Monitoring — For the smoldering type and some chronic cases without high-risk features, treatment may not be needed right away. The disease is watched closely, and skin-directed treatments may be used for skin involvement.
Antiviral therapy — A combination of the antiviral drug zidovudine and interferon-alpha can be helpful, particularly for the leukemic forms of the disease.
Combination chemotherapy — For the acute and lymphomatous types, multi-agent chemotherapy is the usual treatment, sometimes with additional therapy directed at the brain and spinal cord to prevent spread there.
Targeted and antibody drugs — Mogamulizumab (against CCR4) and, in CD30-positive cases, brentuximab vedotin may be added based on the biomarker results.
Stem cell transplant — An allogeneic transplant may be considered for faster-growing disease in patients who are well enough, with the goal of long-term remission.

Care is provided by a team that usually includes a hematologist or oncologist and other specialists. Because ATLL can be difficult to treat, clinical trials of newer therapies are an important option to discuss. Decisions about which approaches apply are made by the treatment team together with the patient, based on the specific findings in the report.

Questions to ask your doctor

What type of ATLL do I have (acute, lymphomatous, chronic, or smoldering)?
Did my tests confirm HTLV-1 infection?
What did the flow cytometry and immunohistochemistry show about the markers on the cells?
Are the cancer cells positive for CCR4 or CD30?
How quickly is my disease likely to grow, and what does that mean for my treatment?
Is my blood calcium level high, and does it need treatment?
Do I need treatment now, or is close monitoring reasonable for my type?
Would antiviral therapy, chemotherapy, or a targeted drug such as mogamulizumab be part of my treatment?
Am I a candidate for a stem cell transplant?
Will I need treatment directed at the brain and spinal cord?
What is my prognosis based on my specific results?
Are there clinical trials that I should consider?