What are the differences between Sézary syndrome and mycosis fungoides?

Difference Between Sézary Syndrome and Mycosis Fungoides

Sézary syndrome is a distinct, aggressive leukemic variant of cutaneous T-cell lymphoma characterized by erythroderma, significant blood involvement (≥1000 Sézary cells/µL or B2 criteria), and poor prognosis (median survival 32 months), while mycosis fungoides typically presents with localized patches, plaques, or tumors that progress slowly over years with generally favorable prognosis in early stages. 1, 2

Key Clinical Distinctions

Skin Presentation

• Mycosis Fungoides: Presents with discrete patches, plaques, or tumors that evolve gradually. Early disease often involves limited body surface area with characteristic distribution patterns. May remain localized for years 1, 2
• Sézary Syndrome: Presents with generalized erythroderma (>80% body surface area involvement) from onset. The skin involvement is diffuse rather than patchy 3, 2, 4

Blood Involvement

• Mycosis Fungoides: Blood involvement is typically absent in early stages. When present in advanced disease, it's usually limited (B0-B1) 5
• Sézary Syndrome: Defining feature is significant blood involvement meeting B2 criteria: either Sézary cell count >1000 cells/µL, CD4:CD8 ratio ≥10, or abnormal CD4+ T-cell population with loss of CD7 (>30%) or CD26 (>40%) 5, 2

Staging and Classification

• Mycosis Fungoides: Staged IA through IVB based on extent of skin involvement (T1-T4), with early stages (IA-IIA) being most common 1, 5
• Sézary Syndrome: By definition staged as T4 (erythroderma) with B2 blood involvement, automatically placing patients in stage IIIB or IVA 5, 1, 5

Prognostic Differences

Mycosis Fungoides Prognosis

• Stage IA: Life expectancy not adversely affected; 5-year survival approaches normal population 1
• Stage IB/IIA: 73-86% and 49-73% 5-year survival respectively 1
• Stage IIB: 40-65% 5-year survival 1
• Prognosis worsens with age >60 years, thick plaques, and peripheral blood T-cell clone detection 1

Sézary Syndrome Prognosis

• Median survival: 32 months from diagnosis 1
• 5-year survival: 45-57% for erythrodermic stage III disease 1
• Peripheral nodal disease is the most important negative prognostic factor 1
• Degree of hematological involvement also significantly impacts prognosis 1

Diagnostic Approach

Mycosis Fungoides

• Skin biopsy showing epidermotropic infiltrate with atypical lymphocytes
• Clinical-pathological correlation essential, especially in early disease where histology may be subtle 6, 2, 7
• T-cell receptor gene rearrangement in skin 2

Sézary Syndrome

• Requires T4 (erythroderma) plus B2 criteria 5
• Identical clonal T-cell receptor gene rearrangement in both skin AND peripheral blood 2
• Flow cytometry demonstrating abnormal CD4+ T-cell population with loss of pan-T-cell markers (CD7, CD26) 2
• Skin biopsy may show only suggestive features; diagnosis can be based on node biopsy or fulfillment of B2 criteria with matching clone 5

Treatment Strategy Differences

Mycosis Fungoides (Early Stage)

• Skin-directed therapies are primary: topical corticosteroids, mechlorethamine, PUVA, superficial radiotherapy 1
• Response rates with PUVA: 79-88% in stage IA, 52-59% in stage IB 1
• Systemic therapy reserved for stage IIB or higher 1

Sézary Syndrome

• Requires systemic therapy from diagnosis 1, 3
• Primary treatments: Extracorporeal photopheresis (ECP), interferon alfa, bexarotene, low-dose methotrexate, denileukin diftitox 3
• Chemotherapy generally produces poor responses in erythrodermic CTCL; immunotherapy and ECP preferred 1
• Allogeneic stem cell transplantation is the only potentially curative option for selected patients 7, 8

Critical Pitfalls to Avoid

1. Don't confuse erythrodermic mycosis fungoides with Sézary syndrome: Both present with erythroderma, but Sézary syndrome requires B2 blood criteria with matching clonal population 5, 2

2. Don't miss blood involvement assessment: Flow cytometry and T-cell receptor gene rearrangement in blood are mandatory to distinguish these entities 2

3. Don't apply mycosis fungoides treatment algorithms to Sézary syndrome: SS requires systemic therapy from diagnosis, not skin-directed therapy alone 1, 3

4. Don't overlook mimics: Sézary syndrome can present with ulcerative or hemorrhagic bullous lesions mimicking Stevens-Johnson syndrome, but lacks mucosal involvement 9