Clinical features

Patch stage — well-defined erythematous, scaly, often atrophic or wrinkled patches on covered ("double-covered") areas (buttocks, breasts, medial thighs).
Plaque stage — infiltrated, indurated red-brown plaques, often annular or polycyclic, with or without scale.
Tumour stage — dome-shaped or ulcerated nodules > 1 cm; may coexist with patches and plaques.
Erythroderma — generalised > 80% body-surface involvement; if accompanied by blood involvement → Sézary syndrome.
Median age at diagnosis 55–60; male predominance ~2:1; commoner in Fitzpatrick IV–VI in some series, with hypopigmented MF a recognised variant in skin of colour.
Pruritus is common and a major contributor to morbidity.

Histology and immunophenotype

Atypical lymphocytes with cerebriform nuclei within the epidermis (epidermotropism) — single cells along the basal layer and as Pautrier microabscesses (intraepidermal collections).
Band-like superficial dermal infiltrate.
Immunophenotype — CD2+ CD3+ CD4+ CD5+, with characteristic loss of CD7 and CD26; CD8+ variants exist.
T-cell receptor gene rearrangement studies confirm a clonal T-cell population (PCR or NGS); identical clones in skin and blood support Sézary syndrome.
Large-cell transformation = ≥ 25% large cells in the infiltrate; worse prognosis; CD30 expression supports brentuximab eligibility.
Early patch MF is histologically subtle; repeat biopsies over time are often required.

T — T1 patches/plaques < 10% BSA (T1a patches only, T1b plaques ± patches); T2 patches/plaques ≥ 10% BSA (T2a patches only, T2b plaques ± patches); T3 ≥ 1 tumour ≥ 1 cm; T4 erythroderma ≥ 80% BSA.
N — N0 no clinically abnormal nodes; N1–N3 by histology/molecular involvement; Nx clinically abnormal, no biopsy.
M — M0 no visceral; M1 visceral involvement (histology required).
B — by absolute Sézary-cell count (ISCL/EORTC): B0 <250/µL (no significant blood involvement); B1 250–999/µL (low blood tumour burden); B2 ≥1000 Sézary cells/µL (high blood tumour burden), or equivalent aberrant CD4 population thresholds, with a positive blood clone matching the skin.
Stage groups — IA T1 N0 M0 B0/1; IB T2 N0 M0 B0/1; IIA T1/2 N1/2 M0 B0/1; IIB T3 any N M0 B0/1; III T4 N0–2 M0 B0/1; IVA1 any T N0–2 B2; IVA2 any T N3; IVB any M1.

Chronic dermatitis (eczema, contact dermatitis) — overlapping clinical and early histological features.
Psoriasis — plaques but with characteristic silvery scale and distribution.
Large-plaque parapsoriasis — considered by some a precursor / early form of MF.
Drug-induced pseudolymphoma (anticonvulsants, ACE inhibitors).
Atopic dermatitis — particularly in skin of colour where hypopigmented MF is misdiagnosed for years.
Tinea corporis, secondary syphilis, lupus erythematosus.

Skin-directed therapy first-line:
- Topical potent / superpotent corticosteroids (clobetasol propionate).
- Topical mechlorethamine (nitrogen mustard) gel — restricted UK availability.
- Topical bexarotene gel.
- Phototherapy — narrowband UVB for patches; PUVA for plaques and thicker disease.
- Localised radiotherapy 20–30 Gy for unilesional MF and resistant plaques/tumours (highly effective; melanoma-style hypofractionation acceptable).
Total skin electron beam therapy (TSEB) for extensive stage IB / IIA refractory to skin-directed therapy.
Patient education — photoprotection, pruritus management, lifelong surveillance.

Oral bexarotene — retinoid X receptor agonist; monitor lipids and thyroid function.
Interferon-α — often combined with PUVA or bexarotene.
Methotrexate at low dose (10–25 mg weekly).
Histone deacetylase inhibitors — vorinostat, romidepsin (less commonly used UK).
Brentuximab vedotin — anti-CD30 ADC; NICE TA577 for CD30+ CTCL after at least one prior systemic therapy.
Mogamulizumab — anti-CCR4; NICE TA754 for MF/Sézary after at least one prior systemic therapy.
Allogeneic stem-cell transplantation — selected younger patients with refractory advanced MF or Sézary syndrome.
Extracorporeal photopheresis primarily for Sézary syndrome / erythrodermic MF.
Palliative radiotherapy for symptomatic tumours.

Stage IA — 10-year disease-specific survival > 95%; near-normal life expectancy.
Stage IB–IIA — 10-year DSS 75–85%.
Stage IIB (tumour) — 10-year DSS ~ 40%.
Stage IVA (B2 / N3) — median survival 3–4 years.
Stage IVB (visceral) — median survival 1–2 years.
Adverse features — large-cell transformation, folliculotropic variant in advanced stage, raised LDH, age > 60.
Follow-up — every 3–6 months in early stage and indefinitely; bloods, skin examination, lymph-node examination, low threshold for imaging if change in disease pattern.