The skin is the second most frequent extranodal site, after the gastrointestinal tract, for lymphoma, with an annual incidence of 0.5–1 per 100 000, although recent Scandinavian studies have suggested an incidence of 4 per 100 000, possibly because of improved diagnosis and registration.
Primary cutaneous T-cell lymphoma (CTCL) comprises a heterogeneous group of non-Hodgkin's lymphoma (NHL), of which mycosis fungoides (MF) is the most common clinicopathologic subtype. MF typically has an indolent course but disease progression may occur in approximately 35% of patients. Sézary syndrome (SS), a leukemic form of CTCL, is very closely related to MF and has a poor prognosis, with a median survival of less than 3 years.
Primary cutaneous B-cell lymphomas (CBCL) are less common, comprising approximately 20% of all primary cutaneous lymphomas. They typically present with cutaneous papules, plaques, or nodules, and can be broadly divided into follicle center cell lymphoma, marginal zone lymphoma, and large B-cell lymphoma.
The WHO-EORTC classification system (Table 15.1) has clarified the classification of primary cutaneous lymphomas, and the distinction of rare CTCL variants from MF/SS is critical as the prognosis is poorer and treatment options are different.
Primary cutaneous T-cell lymphomas (CTCL)
MF is generally associated with an indolent clinical course and is characterized by polymorphic atrophic erythematous patches and scaly plaques. Some patients progress from having limited patches and plaques to extensive thick plaques or tumors and even erythroderma, and a minority (approximately 30%) will present with advanced disease.