A diagnosis of Sézary syndrome or another cutaneous T-cell lymphoma can suddenly introduce an entirely new vocabulary: flow cytometry, B2 blood involvement, erythroderma, CCR4, photopheresis, TSEBT, allogeneic transplant and many more.
This glossary translates common terms into ordinary language for patients, families and caregivers. It is meant to help you follow conversations with your medical team—not replace them.
Newly diagnosed? You may also want to start with Newly Diagnosed? Start Here and Understanding Sézary Syndrome.
Medical note: These definitions are plain-language summaries based on established medical sources. Individual cases differ, terminology evolves, and your own doctors should interpret your tests, stage and treatment options.
On this page
- The basics
- Blood tests & pathology
- Markers you may hear about
- Skin & symptoms
- Lymph nodes & imaging
- Staging in plain English
- B0, B1 and B2 blood involvement
- Treatments you may hear about
- Treatment-related terms
- Stem-cell transplant terms
- Treatment response & care goals
- Sources
The Basics
Cutaneous T-cell lymphoma (CTCL)
A group of uncommon lymphomas involving T lymphocytes, a type of white blood cell. CTCL primarily affects the skin, although some forms can also involve the blood, lymph nodes or internal organs.
Sézary syndrome (SS)
A leukemic form of CTCL in which malignant T cells are present in the bloodstream. It usually causes widespread red, inflamed and often intensely itchy skin and may also involve lymph nodes.
Mycosis fungoides (MF)
The most common type of CTCL. It usually begins in the skin and may appear as patches, plaques or tumors. Mycosis fungoides and Sézary syndrome are closely related and are often discussed together, but they are not identical diseases.
Lymphoma
A cancer that begins in cells of the lymphatic or immune system. CTCL is a type of non-Hodgkin lymphoma.
T cell / T lymphocyte
A type of white blood cell that normally helps the immune system recognize and respond to infections and abnormal cells. In CTCL, a population of T cells becomes malignant.
Lymphocyte
A type of white blood cell. The major groups include T cells, B cells and natural killer cells.
Sézary cell
A malignant T cell associated with Sézary syndrome. These abnormal cells may be found in the blood, skin and lymph nodes and often have a characteristically folded or “cerebriform” nucleus under the microscope.
Blood Tests & Pathology
CBC — Complete Blood Count
A common blood test that measures red blood cells, white blood cells, hemoglobin, hematocrit and platelets.
Differential
A breakdown of the different types of white blood cells in a blood sample, including lymphocytes, neutrophils, monocytes, eosinophils and basophils.
Flow cytometry
A laboratory test that analyzes individual cells and proteins found on their surfaces. In CTCL, flow cytometry can help identify and measure an abnormal T-cell population in blood, lymph nodes or other tissue.
Immunophenotype
The pattern of proteins found on the surface of a cell. Malignant T cells may have an abnormal pattern that helps doctors distinguish them from normal T cells.
CD3, CD4, CD7 and CD26
Proteins associated with T cells. Doctors examine combinations of these markers because malignant cells in Sézary syndrome frequently show characteristic abnormalities, including loss or reduction of some normal T-cell markers.
CD4/CD8 ratio
A comparison between two major groups of T cells. A markedly abnormal ratio may provide evidence of an abnormal T-cell population, but it is not sufficient by itself to diagnose Sézary syndrome.
T-cell clonality / T-cell receptor clonality
Testing that helps determine whether a large number of T cells originated from the same parent cell. Finding a dominant abnormal T-cell clone can support a diagnosis of T-cell lymphoma.
Bone marrow biopsy
A procedure in which a small sample of bone marrow, usually from the pelvic bone, is removed and examined for abnormal cells.
LDH — Lactate Dehydrogenase
An enzyme measured in the blood. LDH can be elevated for many reasons. In lymphoma, doctors may follow it as one piece of information about disease activity, but it is not specific to cancer.
Platelets
Small blood components involved in clotting. Cancer, infection, medications and cancer treatments can all affect the platelet count.
Markers You May Hear About
CCR4
A protein found on the surface of certain immune cells and commonly expressed by malignant T cells in Sézary syndrome. Mogamulizumab targets CCR4.
CD30
A protein found on some lymphoma cells. Testing for CD30 may help determine whether a treatment directed at CD30, such as brentuximab vedotin, could be appropriate.
Biomarker
A measurable characteristic of cancer cells or the body that may provide information about diagnosis, prognosis or treatment. Cell-surface proteins such as CCR4 and CD30 are examples.
Skin & Symptoms
Erythroderma
Widespread redness and inflammation involving most of the skin. In MF/Sézary staging, redness involving at least 80% of the body surface is classified as T4 skin disease.
Pruritus
The medical term for itching. Severe itching is one of the most common and sometimes most debilitating symptoms of CTCL and Sézary syndrome.
Patch
A flat area of abnormal skin.
Plaque
An abnormal area of skin that is raised or thickened.
Tumor
In CTCL terminology, a raised or nodular skin lesion with deeper growth. The word “tumor” in this context does not necessarily mean that lymphoma has spread to an internal organ.
Body surface area (BSA)
The percentage of the body’s skin affected by disease. BSA can be important in CTCL staging.
Skin biopsy
Removal of a small piece of skin so that a pathologist can examine the cells and perform additional tests.
Skin barrier
The outer protective function of the skin. Extensive CTCL can damage this barrier, contributing to dryness, cracking, irritation and increased susceptibility to skin infection.
Emollient / moisturizer
A cream or ointment used to restore moisture and protect the skin. Moisturizing does not treat the lymphoma itself, but good skin care can be an important part of controlling symptoms and protecting damaged skin.
Lymph Nodes & Imaging
Lymph node
A small immune-system structure that filters lymph fluid. Lymph nodes can become enlarged because of infection, inflammation or lymphoma.
Lymphadenopathy
The medical term for enlarged or abnormal lymph nodes.
Lymph-node biopsy
Removal of all or part of a lymph node for examination by a pathologist. In CTCL, the biopsy—not simply the size of a lymph node—can be important in determining the N stage.
PET/CT
A combined imaging study. The CT portion shows anatomy, while the PET portion identifies areas taking up increased amounts of a radioactive glucose-like tracer.
SUV — Standardized Uptake Value
A measurement used on PET scans to describe how much tracer an area takes up. Higher uptake means greater metabolic activity, but a high SUV does not by itself prove that an area contains cancer.
Visceral involvement
Lymphoma involving an internal organ. This is different from involvement confined to skin, blood and lymph nodes.
Staging in Plain English
MF and Sézary syndrome use a system called TNMB:
- T = Skin: how much skin is affected and what type of lesions are present.
- N = Nodes: whether lymph nodes are involved and what a lymph-node biopsy shows.
- M = Internal organs: whether lymphoma has been confirmed in an internal organ.
- B = Blood: how much lymphoma is present in the bloodstream.
The simplest way to understand Stage IV
Stage IVA1 = high blood involvement.
There is B2 blood involvement, but not the most advanced N3 lymph-node category and no confirmed internal-organ involvement.
Stage IVA2 = advanced lymph-node involvement.
A lymph-node biopsy meets the criteria for N3 disease. Blood involvement may range from B0 through B2. There is no confirmed internal-organ involvement.
Stage IVB = internal-organ involvement.
Lymphoma has been confirmed in an internal organ.
An important point about “Stage IV”: Stage IV in MF/Sézary syndrome is defined by this specific TNMB system. It should not automatically be interpreted according to what “Stage IV” means in unrelated cancers. High blood involvement alone can place MF/Sézary disease in Stage IVA1.
B0, B1 and B2 Blood Involvement
B0 — No significant blood involvement
Little or no measurable involvement of the blood by the malignant T-cell population.
B1 — Low blood tumor burden
Abnormal T cells are present, but the amount does not meet the criteria for B2.
B2 — High blood tumor burden
A substantial malignant T-cell population is present in the bloodstream. Specialists use findings such as cell counts, flow cytometry, abnormal T-cell markers and evidence of a matching T-cell clone to determine whether B2 criteria are met.
Other staging terms
T4
Widespread erythroderma involving at least 80% of the body surface.
N3
The most advanced lymph-node category in the MF/Sézary TNMB system, based on the microscopic appearance of a lymph-node biopsy. An enlarged lymph node is not automatically N3.
M0
No confirmed internal-organ involvement.
M1
Confirmed lymphoma involvement of an internal organ.
Treatments You May Hear About
There is no single treatment sequence that is appropriate for every person with Sézary syndrome. Treatment can depend on the skin, blood and lymph-node burden; previous therapies; tumor markers; symptoms; overall health; transplant plans and other factors. For the treatment approach being used in my own case, see Treatment.
Skin-directed therapy
Skin-directed therapy
Treatment aimed primarily at lymphoma in the skin. Examples include topical medications, phototherapy and radiation. Because Sézary syndrome involves the blood, skin-directed treatment is commonly combined with systemic treatment rather than used alone.
Topical corticosteroids (“steroid creams”)
Anti-inflammatory medicines applied directly to the skin. They can reduce redness, inflammation and itching and can be part of skin-directed CTCL treatment. Potency, location and duration matter because prolonged or extensive use can cause side effects such as skin thinning.
Oral or systemic corticosteroids — such as prednisone or methylprednisolone
Steroids taken by mouth or given intravenously suppress inflammation throughout the body. They may be used in selected circumstances, including severe inflammatory symptoms or treatment reactions, but they are generally not considered a durable stand-alone strategy for controlling advanced Sézary syndrome.
Phototherapy
Treatment that exposes the skin to controlled ultraviolet light. Types used in CTCL include narrowband UVB and PUVA.
PUVA
A treatment combining the light-sensitizing medicine psoralen with ultraviolet A light.
Mechlorethamine / nitrogen mustard / Valchlor
A topical chemotherapy applied directly to affected skin. It treats skin disease rather than malignant cells circulating in the bloodstream.
Local radiation therapy
Radiation directed at one or several specific lesions rather than the entire skin surface.
Total Skin Electron Beam Therapy (TSEBT or TSEB)
A specialized form of radiation using electrons to treat most or all of the skin while limiting how deeply the radiation penetrates into the body.
Systemic treatments
Systemic therapy
Treatment that travels through the bloodstream and can reach lymphoma cells in multiple parts of the body. Because Sézary syndrome involves circulating malignant cells, systemic therapy is usually an important part of treatment.
Extracorporeal photopheresis (ECP)
A treatment in which some of the patient’s white blood cells are removed from the bloodstream, treated with a light-sensitizing medicine and ultraviolet A light, and then returned to the body. ECP is particularly associated with blood-involved CTCL and may be combined with other therapies.
Mogamulizumab — MOGA / Poteligeo
A monoclonal antibody that targets CCR4, a protein found on some T cells and lymphoma cells. It is used in MF and Sézary syndrome in appropriate clinical settings and is given by IV infusion.
Bexarotene — Targretin
A retinoid, a medicine related to vitamin A, used in CTCL. It can affect triglycerides and thyroid function, so laboratory monitoring and additional medicines may be required.
Interferon alfa
An immune-modulating treatment that can slow the growth of malignant T cells. It may be used alone or in combination with ECP or skin-directed therapy.
HDAC inhibitors
Histone deacetylase inhibitors alter the way genes and proteins are regulated inside cancer cells. Examples used in CTCL include vorinostat (Zolinza) and romidepsin (Istodax).
Brentuximab vedotin — Adcetris
A targeted antibody-drug conjugate directed at CD30. It is most relevant when the lymphoma expresses CD30.
Methotrexate
A medicine that interferes with cell growth and immune activity. It can be used as systemic therapy in CTCL.
Systemic chemotherapy
Medicines that interfere with cancer-cell growth or survival. Chemotherapy is one of several systemic approaches available for advanced CTCL and is not necessarily the first treatment used in every patient. Drugs patients may hear about include gemcitabine, liposomal doxorubicin and others.
Immune checkpoint inhibitors
Medicines that release some of the natural “brakes” on the immune system so immune cells can attack cancer more effectively. Pembrolizumab is one example that has been studied in CTCL; its use may be in clinical trials or selected circumstances rather than routine first-line treatment.
Clinical trial
A carefully controlled research study evaluating a treatment or treatment strategy. Because Sézary syndrome is rare and treatment continues to evolve, clinical trials can be an important option at different points in the disease.
Treatment-Related Terms
Monoclonal antibody
A laboratory-produced antibody designed to recognize a specific target on a cell. Mogamulizumab is an example.
Antibody-drug conjugate
An antibody attached to an anticancer drug. The antibody recognizes a specific target on the cancer cell and helps deliver the drug to that cell. Brentuximab vedotin is an example.
Infusion
Giving medicine directly into a vein over a period of time.
Infusion reaction
Symptoms that occur during or shortly after an IV medicine is given. They can include flushing, fever, chills, itching, dizziness, blood-pressure changes and other symptoms. Treatment teams may stop or slow the infusion and give additional medicines if necessary.
Premedication
Medicine given before another treatment to reduce the likelihood or severity of side effects such as an infusion reaction.
Washout period
A planned interval between stopping one treatment and beginning another treatment or procedure. A washout may be important when one therapy has lingering effects that could interfere with the next.
Combination therapy
Using two or more treatments together or in a planned sequence.
Stem-Cell Transplant Terms
For more about how transplant fits into my own treatment plan, see Transplant.
Allogeneic stem-cell transplant
A transplant in which blood-forming stem cells come from another person. In selected patients with advanced CTCL or Sézary syndrome, allogeneic transplant may offer long-term disease control and the possibility of cure, but it also carries substantial short- and long-term risks.
Autologous transplant
A transplant using the patient’s own previously collected stem cells. This is different from an allogeneic transplant; CTCL transplant discussions generally focus on the allogeneic approach because the donor immune system can contribute an anti-lymphoma effect.
HLA matching
Testing that compares immune-system markers between a patient and a possible donor. The degree of matching can affect transplant risks and outcomes.
Donor registry
A database of potential volunteer stem-cell donors that can be searched when an appropriate related donor is not available.
Conditioning
Chemotherapy, radiation or both given before a transplant to suppress the patient’s immune system, treat lymphoma and prepare the body to accept donor stem cells.
Reduced-intensity conditioning / reduced-intensity transplant
An allogeneic transplant approach using less intensive conditioning than traditional high-dose regimens. The donor immune system’s anti-lymphoma effect plays an especially important role.
Engraftment
The point after transplant when donor stem cells begin producing new blood cells in the patient’s bone marrow.
GVHD — Graft-versus-Host Disease
A complication of allogeneic transplantation in which donor immune cells recognize some of the recipient’s tissues as foreign and attack them. GVHD can be acute or chronic and can range from mild to life-threatening.
Graft-versus-lymphoma effect
A potentially beneficial effect of an allogeneic transplant in which the donor’s new immune system recognizes and attacks remaining lymphoma cells.
Immunosuppression
Reduction of immune-system activity. After an allogeneic transplant, medicines are commonly used to reduce immune reactions and the risk or severity of GVHD.
Treatment Response & Care Goals
Response
A measurable improvement in the lymphoma after treatment.
Complete response / complete remission
No detectable evidence of disease using the examinations and tests being used to measure it.
Partial response
The lymphoma has decreased substantially but has not disappeared completely.
Stable disease
The lymphoma is neither clearly improving nor clearly worsening.
Progressive disease / progression
The lymphoma is growing, spreading or otherwise worsening.
Relapse
The lymphoma returns after a period of remission or substantial response.
Refractory disease
The lymphoma does not respond adequately to treatment or stops responding.
Disease compartment
One of the areas in which CTCL can be measured—most commonly skin, blood, lymph nodes and internal organs. A treatment may produce a strong response in one compartment before another.
Disease-directed treatment
Treatment intended to reduce, control or eliminate the lymphoma itself.
Supportive care
Care intended to prevent or reduce symptoms, side effects and other problems associated with cancer or its treatment. Supportive care can be provided at the same time as active cancer treatment.
Palliative care
Care focused on preventing or relieving symptoms and improving quality of life during a serious illness. Palliative care can be given alongside active cancer treatment and does not mean that a patient has entered hospice.
Hospice
A specific form of end-of-life care generally used when treatment intended to cure or control the underlying disease is no longer being pursued. Hospice and palliative care are not interchangeable terms.
Principal Sources
The definitions on this page are written in plain language using established medical references. Principal sources include:
- National Cancer Institute — Dictionary of Cancer Terms
- National Cancer Institute — Mycosis Fungoides (Including Sézary Syndrome) Treatment (PDQ), Patient Version
- National Cancer Institute — Stem Cell Transplants in Cancer Treatment
- Cutaneous Lymphoma Foundation — Sézary Syndrome
- Cutaneous Lymphoma Foundation — Staging Cutaneous T-cell Lymphoma
- Cutaneous Lymphoma Foundation — Treatments
- Cutaneous Lymphoma Foundation — Clinical Practice Guidelines resource
Last reviewed: August 2026. This page should be revisited periodically as terminology, treatment options and clinical guidance evolve.
Please remember: This glossary is for general education and is not medical advice. Your own physicians should interpret your diagnosis, staging, test results and treatment choices.
