Regressing atypical histiocytosis: a review and critical appraisal.

Regressing atypical histiocytosis (RAH) has been defined as a primary cutaneous neoplasm composed of atypical histiocytes. In this study, ten cases of RAH were available for review including the two first reported cases. In addition, one new case was studied immunocytologically, for immunoglobulin and T cell receptor gene rearrangement, and for DNA ploidy analysis. Histologic study of ten cases permitted recognition of microscopic features both common and uncommon to RAH. Clinical follow-up of eight cases suggests an indolent course but with probable substantial long-term risk for development of systemic lymphoma. The histiocytic origin of RAH must now be considered questionable because the results of immunologic phenotyping and the discovery of rearrangement of T cell receptor beta- and gamma-chain genes found in the newly studied case indicate that this primary cutaneous neoplasm, previously considered histiocytic, is most probably of T cell lineage.