Cutaneous lymphomas: an update. Part 1: T-cell and natural killer/t-cell lymphomas and related conditions

Primary cutaneous T-cell lymphomas (CTCL) represent the majority of cutaneous lymphomas (CLs) and are a spectrum of diseases with a wide variety of clinical, histological, and phenotypic features and diverse biologic behavior. This review focuses on the observations on new variants of CTCL and recent developments in deciphering the pathogenetic mechanisms, which have implication for the nosologic concepts and future classification of CLs. Variants of mycosis fungoides (MF) such as the unilesional and the papular form have been characterized in more detail. Studies analyzed the expression of CD30 and PD-1 in MF and other forms of CTCL. New variants in the group of cutaneous CD30⁺ lymphoproliferative disorders include the epidermotropic CD8⁺ variant of lymphomatoid papulosis (type D) and angiocentric lymphomatoid papulosis (type E), which histologically mimic aggressive lymphomas, and therefore may be diagnostically challenging. Cutaneous proliferations of T cell-expressing markers of follicular helper T cells (PD-1, CXCL-13, and bcl-6) display a prognostically heterogeneous group. There is an increasing spectrum of CTCL with the expression of CD8 by tumor cells, including CD8⁺ MF, CD8⁺ forms of cutaneous CD30⁺ lymphoproliferative disorders, and CD8⁺ small/medium-sized lymphoproliferations, which are not included as distinct entities in the World Health Organization-European Organization for Research and Treatment of Cancer for CLs and the World Health Organization classification. Unusual presentations and incomplete phenotypes of blastic neoplasm of plasmacytoid dendritic cells are discussed. Clinicopathologic correlation is mandatory for the diagnosis of primary CLs. Analysis of genetic and epigenetic alterations in CLs revealed new diagnostic markers and putative targets for therapy of aggressive forms of CLs.