14th July 2026

Diagnosis

History and Examination

All patients should undergo a full skin examination to assess the body surface area (BSA) and type of lesions, as well as a lymph node examination and assessment for organomegaly/masses. Pruritus is commonly seen, particularly in those with SS.

Heterogeneous cutaneous manifestations include:

• Polymorphic patches/plaques
• Tumours and erythroderma 
• Hypopigmented lesions
• Hyperpigmentation (usually seen in younger children/adolescents)
• Granulomatous slack skin (resembles loose skin) 

SS is a rare leukaemic subtype of CTCL:

• Pruritic erythroderma
• Lymphadenopathy
• Sézary cells (clonally related neoplastic T-cells with cerebriform nuclei)

As both MF and SS may present with similar features, peripheral blood involvement is needed to differentiate the two. In order to definitively diagnose SS, the following is needed alongside Sézary cells:

• Absolute Sézary cell count of >1000/microlitre
• Expanded CD4+ T cell population resulting in CD4/CD8 ratio ≥10, CD4+/CD7- cells ≥30% or CD4+/CD26- cells ≥40%. 

MF typically presents in three stages, namely patch, plaque, and tumour stage. Initial presentation can mimic inflammatory skin conditions such as eczema and psoriasis. These typically progress to tumours and erythrodermic stages. In the few patients who progress straight to tumours, i.e., without patches or plaques, this is termed tumour d’emblee. A biopsy is needed to confirm diagnosis. 

Investigations include:

1. Bloods: FBC (normal or lymphocytosis), LFTs (usually normal), HIV (rule out HIV associated malignancy), LDH – strong predictor of poor prognosis (usually normal), beta 2 microglobulin, lymphocyte subsets CD4:CD8, HTLV serology (to differentiate HTLV-1-associated T-cell leukaemia/lymphoma from other T-cell leukaemias), Sézary cell count on blood film
2. Biopsy: skin, lymph nodes, bone marrow trephine biopsy
3. Immunohistochemistry: CD2, CD3, CD4, CD5, CD7, CD8, CD20, CD30 (tumour cells are typically CD3+, CD4+ and CD8-; CD7 is often lost); TCR gene analysis (if peripheral blood involvement suspected) 
4. Imaging: PET/CT for staging of disease

Clinical staging of CTCL is as follows:

Histology: epidermotropism is typically seen (characterised by T cells in the epidermis), Pautrier microabscesses (cluster of atypical lymphocytes)