Primary cutaneous lymphomas may be either of T- or B-cell origin. Cutaneous T-cell lymphomas (CTCLs) account for 75%-80% of these lymphomas and are a heterogeneous group of neoplasms that vary considerably in their clinical presentation, histology, immunophenotype, genetics, and prognosis. CTCLs are seen most often in the elderly, but they can occur in patients of all ages. The definitive diagnosis of CTCL may require large or multiple biopsies as well as specialized testing of the biopsy specimens. Typing of the CTCL and staging are important to determine the extent of disease and treatment strategy.

Mycosis fungoides and its variants (folliculotropic mycosis fungoides, pagetoid reticulosis, and granulomatous slack skin), Sézary syndrome, lymphomatoid papulosis, and cutaneous anaplastic large cell lymphoma make up 90% of all CTCL cases.

This summary focuses on mycosis fungoides and variants and Sézary syndrome.

Other types of CTCL discussed separately include subcutaneous panniculitis-like T-cell lymphoma, adult T-cell leukemia / lymphoma, and nasal type extranodal NK/T-cell lymphoma.

Mycosis Fungoides
Mycosis fungoides (MF) is the most common type of CTCL, accounting for 50% of all primary CTCL cases. Erythematous patches and plaques with fine scale and tumors that anatomically favor the buttocks and sun-protected areas of the trunk and limbs characterize this subtype. The etiology remains unclear. Current hypotheses propose that persistent antigenic stimulation occurs and that CD8+ T-cells play a critical role. MF takes on an indolent course over years to decades. Associated features include pruritus, poikiloderma, and ulceration of tumors. Extracutaneous involvement correlates with generalized skin involvement and erythroderma. Neoplastic T-cells demonstrate a memory T-cell phenotype: CD3+, CD4+, CD45RO+. MF staging requires CTCL physician specialists. Some consider large plaque parapsoriasis to be early patch stage MF.

MF is approximately twice as common in men as in women, and sex differences in incidence increase with age. In the United States, individuals of African descent have twice the incidence of individuals of Northern European descent, though this difference decreases with age. MF is mostly seen in older adults (median age at presentation is 50-55 years), although any age group may be affected. Onset of disease may occur at an earlier age (before age 40), and this has especially been reported in people of African descent and US women of Hispanic descent. Although the clinical course of MF usually progresses slowly, US patients of African descent, especially women with early-onset disease, have a worse prognosis than other groups. Development or unmasking of CTCL, including MF, has been documented in patients receiving dupilumab for atopic dermatitis and other biologic therapies. 

Hypopigmented MF – A hypopigmented variant of MF is seen primarily in individuals of African descent but has also been reported in those of Asian, Indian, and Latin American descent. This is the most common variant in children.

Folliculotropic MF – Folliculotropic MF (FMF, also known as follicular MF or pilotropic MF) is the most common variant of MF (6% of all CTCL), characterized by lesions on the head and neck clinically and follicular involvement ("folliculotropism") microscopically. Most cases are seen in adult males, but children and adolescents may also be affected. Early stage (IA, IB) disease with patches and plaques with follicular accentuation and keratosis pilaris-like or acneiform lesions may exhibit better prognosis with an indolent course. These lesions have a predilection for the trunk and extremities. On the other hand, advanced stage (IIB) disease with indurated or infiltrative alopecic plaques and tumors may be more aggressive with poorer prognosis. These lesions favor the head and neck and are more pruritic. 5-year survival rate approaches 80%.

Pagetoid reticulosis – Pagetoid reticulosis, also known as Woringer-Kolopp disease, represents approximately 1% of all cases of CTCL. It is a localized variant of MF, characterized by a solitary, slow-growing patch or plaque on a distal extremity clinically and striking epidermotropism microscopically. Pagetoid reticulosis has a predilection for middle-aged males but it can be seen in any age group. Prognosis is excellent with a 100% 5-year survival rate.

Ketron-Goodman disease is a disseminated and aggressive variant of pagetoid reticulosis with a potential for systemic involvement.

Granulomatous slack skin – Granulomatous slack skin, also known as granulomatous dermohypodermitis, is a rare variant of CTCL (<1% of all cases). It is characterized by asymptomatic pendulous loose skin masses in the body folds clinically, and granulomatous T-cell infiltration microscopically. The disease may be associated with a lymphoproliferative disorder, such as MF, Hodgkin lymphoma, and non-Hodgkin lymphoma. Males in the third and fourth decades are more commonly affected. Overall, prognosis is good with a 100% 5-year survival rate.

Sézary Syndrome
Sézary syndrome accounts for less than 5% of all primary CTCL cases. Erythroderma, generalized lymphadenopathy, and neoplastic T-cells (Sézary cells) in the skin and peripheral lymph nodes classically characterize Sézary syndrome. Associated features include severe pruritus, palmoplantar hyperkeratosis, lichenification, edema, and exfoliation. The etiology remains largely unknown. Immunophenotyping demonstrating a CD4:CD8 ratio of >10, an absolute count of >1000 Sézary cells per µL, and demonstration of T-cell clonality in the peripheral blood are diagnostic criteria. The prognosis is poor, with a 24% 5-year survival.