Mycosis Fungoides (MF) and Its Variants — Pathology (10 Marks)

Definition

Mycosis fungoides is the most common type of cutaneous T-cell lymphoma (CTCL), a malignant neoplasm derived from mature, skin-homing CD4+ helper/memory T cells that infiltrate the epidermis and superficial dermis (epidermotropism) - Robbins, Cotran & Kumar Pathologic Basis of Disease. The tumor cells express cutaneous lymphocyte antigen and CCR4, the same homing molecules used by normal skin-seeking CD4+ T cells - Robbins, Cotran & Kumar Pathologic Basis of Disease.

Epidemiology

Incidence ~1 in 300,000/year, rising over recent decades.
Affects all races; in the US, Black patients are affected more often than White patients.
Male : female ratio approximately 2:1.
Typically a disease of older adults with a long, indolent evolution - Andrews' Diseases of the Skin.

Natural History / Clinical Stages

MF classically evolves through three stages over years:

Patch stage ("premycotic" phase) - erythematous, scaly patches favoring the "bathing suit" distribution (trunk, buttocks, thighs); pruritus is often marked; biopsy may be non-diagnostic at this point.
Plaque stage - patches become infiltrated, indurated plaques.
Tumor stage - nodules/tumors develop, sometimes with ulceration.

Variants include erythrodermic MF and, rarely, tumors appearing without preceding patches/plaques ("MF d'emblee"). Lymph node involvement usually occurs first among extracutaneous sites, followed by peripheral blood and visceral spread - Andrews' Diseases of the Skin.

Histopathology

Band-like ("lichenoid") infiltrate of neoplastic T cells in the superficial dermis with epidermotropism.
Neoplastic lymphocytes show a characteristic cerebriform nucleus due to marked infolding of the nuclear membrane (best appreciated on electron microscopy; in blood these are called Lutzner cells).
Pautrier microabscesses: small, well-circumscribed intraepidermal collections of atypical lymphocytes - a classic but not universally present finding.
Despite early epidermal/dermal infiltration, sensitive molecular (clonality) studies show tumor cells already circulate in blood, marrow, and nodes early in disease, though overt extracutaneous disease appears late - Robbins, Cotran & Kumar Pathologic Basis of Disease.

Immunophenotype

CD2+, CD3+, CD4+, CD45RO+ (memory phenotype), CCR4+, cutaneous lymphocyte antigen+; typically CD8-. Loss of CD7 and/or CD26 is often used to support a diagnosis of neoplastic circulating cells (relevant to Sezary syndrome criteria: CD4/CD8 ratio >10, CD4+CD7- ≥40%, or CD4+CD26- ≥30%) - Dermatology, 2-Volume Set.

Staging (TNMB system)

T: skin involvement - T1 (<10% patches/plaques), T2 (>10%), T3 (tumors), T4 (erythroderma)
N: node involvement - N0 to N3
M: visceral involvement (M0/M1); B: blood involvement (B0/B1)
Stage IA (T1N0M0) has a 10-year disease-specific survival of 96%; Stage IB 77-83%; Stage IIB 42%; Stage IV only 20% - Dermatology, 2-Volume Set; Andrews' Diseases of the Skin.

Prognosis

Mycosis fungoides is an indolent lymphoma with median survival around 10 years. It can transform, occasionally, into a more aggressive large-cell lymphoma as a terminal event. Death is usually from systemic disease or infection/septicemia - Robbins, Cotran & Kumar Pathologic Basis of Disease; Andrews' Diseases of the Skin.

Variants of Mycosis Fungoides (WHO-EORTC classification)

Apart from classic Alibert-Bazin type MF, variants are divided into:

Clinical variants (behave like classic MF, not separately classified):

Hyperpigmented and hypopigmented MF (the latter more common in darker skin, presents as hypopigmented patches, often in children/young adults)
Bullous MF

Distinct histologic/clinicopathologic variants (recognized separately due to distinctive behavior/prognosis):

Folliculotropic MF (adnexotropic MF) - neoplastic infiltrate concentrates around hair follicles, often sparing the epidermis; predilection for head and neck; frequently associated with follicular mucinosis; tends to have a worse prognosis than classic patch/plaque MF because of deeper follicular/dermal involvement that responds poorly to skin-directed therapy.
Pagetoid reticulosis (Woringer-Kolopp disease) - localized patches/plaques, usually on distal extremities, with intraepidermal proliferation of atypical (sometimes CD8+) T cells resembling Paget cells; excellent prognosis, treated with radiotherapy or excision.
Granulomatous slack skin - extremely rare; presents as pendulous folds of lax skin, favoring axilla/groin, due to a granulomatous infiltrate of clonal CD3+CD4+CD8- T cells with cerebriform nuclei, macrophages, and multinucleated giant cells causing elastolysis/elastophagocytosis; ~1/3 of cases associated with Hodgkin lymphoma; indolent course, radiotherapy is the preferred treatment - Dermatology, 2-Volume Set.

Relationship to Sezary Syndrome

Sezary syndrome is closely related but clinically and now genomically distinct: it presents with the triad of generalized exfoliative erythroderma, generalized lymphadenopathy, and a leukemic phase with circulating Sezary cells (cerebriform nuclei) in blood. Unlike MF, skin lesions in Sezary syndrome rarely progress to tumors. Prognosis is worse than MF - only a minority survive 5 years - Robbins, Cotran & Kumar Pathologic Basis of Disease.

Note on recent evidence: A 2024 systematic review (PMID: 38422999) on granulomatous MF found early organ metastasis can occur in this variant, and a 2025 systematic review (PMID: 40248965) described dermoscopic/trichoscopic features specific to folliculotropic MF for earlier diagnosis - these support and refine, but do not contradict, the textbook classification above.

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