Cutaneous T-cell lymphoma: epidemiology, etiology, and classification.

The term cutaneous T-cell lymphoma (CTCL) describes a heterogeneous group of neoplasms of skin-homing T-cells that vary considerably in clinical presentation, histologic appearance, immunophenotype, and prognosis. CTCL represent approximately 75-80% of all primary cutaneous lymphomas, whereas primary cutaneous B cell lymphomas account for approximately 20-25%. For many years mycosis fungoides and Sézary's syndrome were the only known types of CTCL. In the last decade, based on a combination of clinical, histological, and immunophenotypical criteria, new types of CTCL have been defined and new classifications for this group of primary cutaneous lymphomas have been formulated. In this overview, characteristic features of the different types of CTCL recognized in the European Organization for Research and Treatment of Cancer (EORTC) classification for primary cutaneous lymphomas and the World Health Organization (WHO) classification will be reviewed. Key conclusions from this brief overview are: (1) that the term CTCL does not refer to a single disease entity, but to a group of diseases with different clinical behaviors, therapeutic requirements, and prognoses; (2) that diagnosis and classification should always be based on a combination of clinical, histologic, and immunologic criteria; and (3) that the WHO and EORTC classification schemes are broadly equivalent for almost 90% of CTCL patients. Key research priorities are to develop effective therapies for peripheral T-cell lymphoma, extranodal NK (natural killer) T-cell lymphoma and so-called blastic NK cell lymphoma, and to determine molecular profiles for all forms of CTCL.