Mycosis Fungoides: Understanding Your Pathology Report

Section Editor: Allison Osmond MD FRCPC
June 18, 2026

Mycosis fungoides is a type of cancer called lymphoma that starts in the skin. A lymphoma is a cancer of lymphocytes, the white blood cells that help protect the body from infection. In mycosis fungoides, the cancerous cells are a type of lymphocyte called T cells, which build up in the skin and cause rashes, patches, and other skin changes. It is the most common type of cutaneous (skin) lymphoma.

This article explains what a diagnosis of mycosis fungoides means, what the findings in your pathology report describe, and how those findings guide the decisions you and your care team make together. In most people, the disease develops slowly over years, beginning with mild skin changes that can be mistaken for eczema, and many people with early disease live a normal lifespan. Over time, in some people, the cancerous T cells grow deeper into the skin to form raised tumors or spread beyond the skin to the blood, lymph nodes, or other organs.

What are the symptoms of mycosis fungoides?

The symptoms depend on the stage of the disease and usually develop slowly over months to years. Common symptoms include:

• Patches — Red, scaly, often itchy flat areas of skin (early disease).
• Plaques — Thicker, raised areas of skin (later disease).
• Tumors — Raised lumps on the skin that can grow (advanced disease).
• Spread beyond the skin — Enlarged lymph nodes or fatigue if the disease moves beyond the skin.

Because these changes develop slowly and look like common skin conditions, mycosis fungoides can be difficult to diagnose in its early stages.

What causes mycosis fungoides?

The exact cause of mycosis fungoides is not known. It is not contagious, cannot be passed from person to person, and is not inherited. The disease arises when a single population of T cells (a clone) grows uncontrollably in the skin, but its trigger is not understood. Long-term stimulation of the immune system has been suggested as a possible factor, although no specific cause has been confirmed.

How is the diagnosis made?

The diagnosis is made after a small sample of affected skin is removed in a biopsy and examined under the microscope by a pathologist. Because mycosis fungoides can resemble other skin conditions and its early changes can be subtle, multiple biopsies over time are often needed to confirm the diagnosis.

Under the microscope, the appearance changes as the disease progresses. In the early patch stage, small to medium lymphocytes with dark, folded (cerebriform) nuclei collect in the epidermis (the outer layer of the skin) and line up along its base, a feature called epidermotropism. In the plaque stage, these cells move higher in the epidermis and can cluster into small collections called Pautrier microabscesses. In the tumor stage, the cancer cells lose their connection to the surface and grow into the dermis (the deeper layer of the skin) as sheets of cells. When more than about 25% of the cells become large and more abnormal, this is called large cell transformation, which can indicate the disease is growing faster and may need more intensive treatment.

To confirm that the abnormal cells are T cells and to support the diagnosis, the pathologist uses immunohistochemistry (special stains that detect specific proteins). The cells usually carry the T-cell markers CD2, CD3, CD5, and CD4, often with loss of another T-cell marker called CD7, and stains such as TOX, PD-1, and the skin-homing markers CLA and CCR4 may also be used. A marker called Ki-67 may be measured to show how quickly the cells are dividing. The pathologist may also perform a test for T-cell receptor gene rearrangement, which looks for a single dominant (clonal) population of T cells; finding the same clone in more than one sample supports the diagnosis. One result, the CD30 marker, can also guide treatment and is described in the next section.

Biomarker and molecular testing

Biomarkers are features of the cancer that provide information beyond the diagnosis itself, such as which treatments may help. In mycosis fungoides, the most useful treatment-related biomarker is CD30.

• CD30 positive — A meaningful proportion of the cancer cells carry the CD30 protein. This result can make a targeted drug called brentuximab vedotin, which is directed against CD30, an option, particularly in more advanced or transformed disease.
• CD30 negative — Few or no cancer cells carry CD30. Brentuximab vedotin is less likely to help, and other treatments are considered. CD30 levels can change over time, so testing may be repeated on a later biopsy.

The T-cell receptor gene rearrangement test described above is a molecular test used mainly to confirm the diagnosis (by showing a clonal population of T cells) rather than to select a drug. You can read more in the Biomarkers section of this site.

How is mycosis fungoides staged?

The stage describes how far the disease has progressed. Mycosis fungoides is staged using a system developed by the International Society of Cutaneous Lymphomas (ISCL) and the European Organization for Research and Treatment of Cancer (EORTC), known as the TNMB system. It looks at the skin (T), lymph nodes (N), internal organs (M), and blood (B).

Skin (T)

• T1 — Patches, raised spots (papules), or plaques covering less than 10% of the skin (T1a: patches only; T1b: plaques and patches).
• T2 — Patches, papules, or plaques covering 10% or more of the skin (T2a: patches only; T2b: plaques and patches).
• T3 — One or more tumors at least 1 cm across.
• T4 — Redness (erythroderma) covering at least 80% of the skin.

Lymph nodes (N)

• N0 — No abnormal lymph nodes.
• N1 — Abnormal nodes with limited involvement (N1a: no clone detected; N1b: clone detected).
• N2 — Abnormal nodes with moderate involvement (N2a: no clone; N2b: clone detected).
• N3 — Abnormal nodes with extensive involvement.
• NX — Nodes appear abnormal but were not biopsied to confirm.

Internal organs (M)

• M0 — No spread to internal organs.
• M1 — Spread to internal organs, confirmed by biopsy.

Blood (B)

• B0 — Little or no blood involvement (fewer than 5% of white blood cells are cancerous). B0a: no clone; B0b: clone detected.
• B1 — Low blood involvement (more than 5% cancerous cells but below the B2 threshold). B1a: no clone; B1b: clone detected.
• B2 — High blood involvement (at least 1000 cancerous cells per microliter with a clone present).

These four parts are combined into an overall clinical stage that helps predict the likely course of the disease. In simplified terms:

• Stage IA — Patches or plaques on less than 10% of the skin.
• Stage IB — Patches or plaques on 10% or more of the skin.
• Stage IIA — Skin patches or plaques with abnormal lymph nodes that do not yet contain clear lymphoma.
• Stage IIB — One or more skin tumors.
• Stage III — Redness (erythroderma) over most of the body.
• Stage IVA — Significant lymph node or blood involvement.
• Stage IVB — Spread to internal organs.

What is the prognosis?

The prognosis depends mainly on the stage at diagnosis. In the early stages (IA, IB, and IIA), the disease is usually limited to the skin, the outlook is very good, and many people have a normal life expectancy. In the more advanced stages (IIB and beyond), the outlook becomes more serious as the disease forms tumors or spreads to lymph nodes, blood, or internal organs. Other factors linked to a less favorable outlook include older age, elevated blood lactate dehydrogenase (LDH) levels, large-cell transformation, and the presence of cancer cells in the blood. With early diagnosis and appropriate treatment, many people can control the disease for years, and palliative (supportive) care can be an important part of care at any stage to manage symptoms such as itching.

What happens after this diagnosis?

Treatment is guided by the stage and is usually coordinated by a team that may include a dermatologist, a hematologist or medical oncologist, and a radiation oncologist. For many people, the goal is to control the disease and relieve symptoms over the long term rather than to cure it, and early-stage disease can often be managed for years.

For disease limited to the skin (early stages), the team often considers skin-directed treatments, such as topical corticosteroids, topical chemotherapy (mechlorethamine gel), topical retinoids, phototherapy (light treatment, such as PUVA or narrowband UVB), and radiation targeted to individual spots. For more extensive skin disease, skin tumors, or disease that has spread, systemic treatments may be added. These can include oral retinoids (bexarotene), interferon, drugs called HDAC inhibitors (such as vorinostat or romidepsin), methotrexate, total skin electron beam radiation for widespread skin involvement, and, when there is blood involvement, extracorporeal photopheresis (a treatment that filters and treats blood cells). When the cancer cells are CD30 positive, brentuximab vedotin may be an option, and for advanced disease, a drug called mogamulizumab may be considered. Chemotherapy is generally reserved for diseases that have not responded to other treatments, or that involve internal organs, and a stem cell transplant may be considered in selected people with advanced, treatment-resistant disease. Because the skin barrier can be affected, skin care and measures to reduce the risk of infection are also important parts of long-term management.

Questions to ask your doctor

• What stage is my mycosis fungoides, and how much of my skin, lymph nodes, blood, and organs are involved?
• Do I have patches, plaques, or tumors, and what percentage of my skin is affected?
• Was large cell transformation present?
• Was my biopsy tested for CD30, and was it positive or negative?
• Were cancer cells found in my blood or lymph nodes?
• Was a T-cell receptor gene test done, and what did it show?
• Which treatments are appropriate for my stage, skin-directed or systemic?
• Is the goal of treatment to control the disease or to try to cure it?
• How often will I need follow-up and repeat testing?
• What signs of progression should I watch for?
• Should I be seen at a center that specializes in cutaneous lymphoma?
• What can I do to care for my skin and lower my risk of infection?

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