Lymphomatoid papulosis: a practical review for pathologists

Lymphomatoid papulosis (LyP) is a primary cutaneous CD30+ lymphoproliferative disorder characterized by a chronic and self-healing recurrent cluster of erythematous papules or nodules on the skin of the trunk and/or extremities. The disease has an indolent clinical course with spontaneous regression or waxing and waning clinical evolution. The histopathologic spectrum of LyP is vast and may show few to numerous atypical cells immersed in a mild to intense inflammatory background. The backbone for the diagnosis is the positivity for CD30, which is one of the criteria to define this group of lymphoproliferative disorders. The association of these different histological and immunophenotypical findings is used to subclassify this disease in different subtypes from A to E, associated with DUSP22/IRF4 rearrangement, and other rare forms. Although this differentiation is important to raise awareness of different differential diagnosis, it does not impact the prognosis or change the treatment, which is usually centered in symptom relief and faster regression. In this review, we aim to summarize the most updated information of the clinical, histopathological, and molecular characteristics of LyP and provide a practical assessment for the diagnostic features that could help with the main differential diagnosis.