Abstract
Abstract
Advances in the biology of lymphoid cells have greatly improved the classification and understanding of the pathogenesis of primary cutaneous lymphomas. Classification is based on clinical, pathological, immunopathological, molecular and cytogenetic findings and recognises that the site of origin of extranodal lymphomas and tumour morphology determines clinical behaviour, which in turn has a critical influence on prognosis and therapeutic approach. Mycosis fungoides and its variants are the most common primary cutaneous T‐cell lymphoma subset, but other subsets with clearly identifiable clinicopathological features and varying prognoses have also been described. A critical observation has been the realisation that lymphomas with a similar pathology arising in different organs have different prognoses and distinct pathogenesis: nodal CD30+ anaplastic large‐cell lymphomas are usually anaplastic lymphoma kinase (ALK) positive but ALK‐negative variants are associated with a poor prognosis, whereas primary cutaneous CD30+ anaplastic large‐cell lymphomas are invariably ALK negative and have a good prognosis. The majority of primary cutaneous B‐cell lymphomas also have an excellent prognosis, and primary cutaneous follicle centre lymphomas are pathogenetically distinct from nodal follicular lymphomas. Furthermore, it is appreciated that cutaneous and systemic T‐cell lymphomas are usually derived from specific tissue‐resident T‐cell subsets.