The skin is the second most frequent extranodal site, after the gastrointestinal tract, for lymphoma. Cutaneous lymphomas have an annual incidence of 0.5–1.0 per 100 000, although recent Scandinavian studies have suggested an incidence of 4 per 100 000, possibly due to improved diagnosis and registration.
Primary cutaneous T-cell lymphoma (CTCL) comprises a heterogeneous group of non-Hodgkin's lymphomas, of which mycosis fungoides (MF) is the most common clinicopathologic subtype. Mycosis fungoides typically has an indolent course, but disease progression may occur in approximately 25% 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 three 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 recent publication of the WHO EORTC classification system (Table 16.1) has clarified the classification of primary cutaneous lymphomas. 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)
Mycosis fungoides (MF) is the commonest variant of primary CTCL, and it is generally associated with an indolent clinical course. The disease is characterized by cutaneous polymorphic atrophic erythematous patches and scaly plaques.