Mycosis fungoides and Sézary syndrome (cutaneous T-cell lymphoma) !

A slow-growing lymphoma of T cells that lives in the skin. It starts as scaly patches that look like eczema and can, over years, spread to blood and nodes.

Updated 2026-11-01

Structures involved

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Why it happens

What it is

Mycosis fungoides (MF) is the most common primary cutaneous T-cell lymphoma: malignant skin-homing CD4+ T cells form patches, plaques and sometimes tumours. Sézary syndrome is the leukaemic form, with generalised red skin (erythroderma), enlarged lymph nodes and circulating malignant T cells (Sézary cells).

Who gets it

Rare, mostly in adults over 50 and more often in men. A Medellín series found that close to one in five MF patients were 18 or younger and that the hypopigmented variant, frequent in darker skin and in children, was the most common form; this variant is easy to mistake for vitiligo or fungal infection.

Causes

Clonal expansion of mature skin-homing (CLA+) CD4+ helper T cells
Cause unknown; chronic antigen stimulation and acquired genetic changes are proposed
Not caused by a fungus despite the historic name

Risk factors

Older age and male sex
Darker skin types for the hypopigmented and some aggressive variants
Immunosuppression (HIV is ruled out in the work-up)

Pathophysiology

Malignant T cells home to the epidermis and collect in small clusters (Pautrier microabscesses), producing patches and then thicker plaques. With progression the clone loses dependence on the skin, forms dermal tumours and enters nodes and blood. In Sézary syndrome the clone circulates and drives a Th2-type immune state with intense itch, high IgE and eosinophilia, and the loss of normal T cells causes immune deficiency and infections.

Classification

MF patch stage (stage IA–IB by skin area involved)
MF plaque and tumour stages (IIA–IIB)
Erythrodermic MF (stage III, at least 80% of the skin red)
Sézary syndrome (stage IV, high blood Sézary count) and visceral spread (IVB)
Variants: folliculotropic, hypopigmented, granulomatous slack skin, pagetoid reticulosis

Symptoms and signs

Fixed, slightly scaly red or brownish patches, often on the buttocks, hips, trunk and other sun-protected areas
Itching, frequently long before the diagnosis is made
Thicker plaques and later raised tumours that may ulcerate
Pale (hypopigmented) patches in children and in darker skin
In Sézary syndrome: red, peeling skin over most of the body, severe itch, thick palms and soles, hair loss and enlarged lymph nodes

Diagnosis

Clinical: Suspected when an itchy patch rash in sun-protected areas persists for months and does not respond to treatment for eczema or psoriasis. Full skin examination estimates the percentage of body surface involved and checks nodes, liver and spleen.
Tests: Skin biopsy (often several, off topical steroids) shows atypical lymphocytes in the epidermis; immunohistochemistry and T-cell receptor gene rearrangement prove a clonal process. Complete blood count with flow cytometry for Sézary cells, LDH, and HIV and HTLV-1 serology (HTLV-1 suggests adult T-cell leukaemia/lymphoma instead).
Imaging: Usually not needed in limited patch disease. CT or PET-CT stages advanced disease; biopsy of enlarged nodes and, when indicated, bone marrow.

Differential diagnosis

Atopic or contact dermatitis (eczema)
Psoriasis
Tinea corporis and other fungal infections
Vitiligo or post-inflammatory hypopigmentation (for the hypopigmented variant)
Erythroderma from drugs or other skin disease
Adult T-cell leukaemia/lymphoma (HTLV-1)

Goal of treatment

Clear or control skin lesions and itch, prevent infection and preserve quality of life. Early disease is managed with skin-directed therapy; advanced disease is treated systemically and is rarely cured except, in selected people, by allogeneic transplant.

Treatment

Medical: Skin-directed therapy for patch and plaque disease: potent topical corticosteroids, topical chemotherapy or retinoids, and emollients.
Procedure: Phototherapy (narrowband UVB or PUVA) for widespread patches; local radiotherapy for tumours and total-skin electron beam therapy for extensive disease.
Medical: Systemic therapy for advanced or refractory disease: retinoids, interferon, histone deacetylase inhibitors, monoclonal antibodies (anti-CD30 or anti-CCR4), low-intensity chemotherapy, and extracorporeal photopheresis for Sézary syndrome.
Procedure: Allogeneic stem cell transplant in selected younger people with advanced disease.
Follow-up: Nursing focus: skin care and itch control, early detection of skin infection, protection of eyes and skin during phototherapy, and follow-up of blood counts and Sézary cells.

Complications

Skin infections with Staphylococcus aureus and sepsis
Large-cell transformation to a more aggressive lymphoma
Spread to lymph nodes, blood, spleen, liver and other organs
Severe itch, sleep loss and psychological distress
Higher risk of second cancers

Prognosis

Patch-stage MF often follows an indolent course with near-normal survival, and many people never progress. Survival falls with tumour stage, erythroderma and Sézary syndrome, where median survival is a few years; large-cell transformation also worsens the outlook.

Test yourself

Think of the answer, then tap the question to see it.

Which cells are malignant in mycosis fungoides?

Mature skin-homing CD4+ helper T lymphocytes.

What triad defines Sézary syndrome?

Erythroderma, enlarged lymph nodes and circulating clonal T cells (Sézary cells) in the blood.

What are the small intraepidermal clusters of atypical lymphocytes seen in MF biopsies called?

Pautrier microabscesses.

Which MF variant is especially common in children and darker skin and mimics vitiligo?

Hypopigmented mycosis fungoides.

What is the main cause of death in advanced cutaneous T-cell lymphoma?

Infection, especially skin-derived Staphylococcus aureus sepsis.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Mycosis Fungoides (Including Sézary Syndrome) Treatment (PDQ®)–Patient Version (National Cancer Institute (NCI), 2025); Mycosis Fungoides in Children and Adolescents: A Series of 23 Cases (Actas Dermo-Sifiliográficas, 2020).

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Representative model, for educational purposes only.

Skin highlighted on the Durgical 3D model