Cutaneous T-cell Lymphoma

Mycosis Fungoides, Sézary Syndrome, and primary cutaneous ALCL — skin-homing T-cell malignancies ranging from indolent patches to aggressive erythroderma

Key Points

• Cutaneous T-cell lymphomas (CTCL) are a heterogeneous group of extranodal non-Hodgkin lymphomas arising from skin-homing T cells; Mycosis Fungoides (MF) is the most common subtype (~50% of CTCL), followed by Sézary Syndrome (SS) and primary cutaneous ALCL (pcALCL).
• MF follows an indolent course through three clinical stages — patches, plaques, and tumors — with progression to erythroderma or systemic dissemination in a minority of patients; early-stage MF has a near-normal life expectancy.
• Sézary Syndrome is the leukemic, erythrodermic form of CTCL defined by the triad of erythroderma, lymphadenopathy, and circulating Sézary cells (≥1000/µL or ≥20% of lymphocytes); it carries a significantly worse prognosis than MF (median OS 2–4 years).
• Primary cutaneous ALCL (pcALCL) is a CD30+ indolent skin lymphoma with frequent spontaneous regression; it is managed with skin-directed therapy, radiation, or brentuximab vedotin — systemic chemotherapy is rarely needed.
• Skin-directed therapies (topical corticosteroids, nitrogen mustard, bexarotene, phototherapy — PUVA or NB-UVB, and radiation) form the backbone of early-stage MF management; systemic agents (brentuximab vedotin, mogamulizumab, romidepsin, bexarotene) are used for advanced or refractory disease.
• Total skin electron beam therapy (TSEBT) is a highly effective skin-directed modality for widespread disease; allogeneic stem cell transplantation is the only potentially curative option for advanced MF/SS.

Classification and Pathogenesis

CTCL encompasses more than 20 distinct entities in the 2022 WHO-HAEM5 and ICC classifications, the most clinically important of which are Mycosis Fungoides, Sézary Syndrome, and primary cutaneous CD30+ lymphoproliferative disorders (including pcALCL and lymphomatoid papulosis). **Mycosis Fungoides** arises from skin-homing, memory CD4+ T cells (CD4+, CD45RO+, CCR4+, CLA+) that have undergone malignant transformation. Pathogenic mechanisms include chromosomal copy number alterations, mutations in TP53, CDKN2A, CARD11, PLCG1, and DNMT3A, and constitutive activation of NF-κB, JAK/STAT, and…

Clinical Features and Diagnosis

**Mycosis Fungoides** presents as a slowly progressive skin disease. The classic stages are: - *Patch stage:* Flat, erythematous, scaly lesions often in sun-protected areas (bathing trunk distribution). May persist for years or decades before progression. Histology shows epidermotropism of atypical lymphocytes with Pautrier microabscesses. - *Plaque stage:* Indurated, raised plaques with more pronounced erythema and scaling. Dermal infiltration increases; epidermotropism remains prominent. - *Tumor stage:* Dome-shaped or ulcerated tumors arising on or independent of patches/plaques; loss of…

Staging and Risk Stratification

MF and SS are staged using the TNMB (tumor, node, metastasis, blood) classification established by the ISCL/EORTC: - **Stage IA:** Patches/plaques <10% BSA (T1), no nodal/blood/visceral involvement — near-normal life expectancy - **Stage IB:** Patches/plaques ≥10% BSA (T2) — 10-year DSS ~80% - **Stage IIA:** Any T with dermatopathic lymphadenopathy (N1/N2) - **Stage IIB:** Skin tumors (T3) — 10-year DSS ~42% - **Stage III:** Erythroderma (T4) — Stage IIIA (B0/B1) vs IIIB (B2, Sézary) - **Stage IVA:** Lymph node involvement with effacement (N3) or B2 blood involvement — 5-year DSS ~40% -…

Treatment

Treatment is stage-guided and aims to achieve disease control, relieve symptoms (especially pruritus), and preserve quality of life. Cure with systemic therapy is not achievable in most patients with MF/SS; treatment is chronic and sequential. **Early-Stage MF (IA–IIA):** Skin-directed therapies (SDTs) are first-line: - *Topical corticosteroids:* First-line for limited patch/plaque disease; high-potency agents achieve response in >90% of IA disease - *Topical nitrogen mustard (mechlorethamine gel):* FDA-approved for MF; effective for widespread patch/plaque disease - *Phototherapy:*…

Prognosis and Quality of Life

Prognosis in CTCL is heavily stage-dependent. Stage IA MF has a life expectancy indistinguishable from age-matched controls; most patients with early-stage disease will never progress to advanced stages. However, once tumor stage, large cell transformation, or significant blood involvement develops, outcomes worsen substantially. Sézary Syndrome carries a median OS of 2–4 years; aggressive infections and cardiovascular events are major causes of death. Multidrug sequencing is the norm; no single agent produces durable responses in most patients, and the disease typically progresses through…