Cutaneous Richter Syndrome Mimicking Primary Cutaneous CD4-Positive Small/Medium T-cell Lymphoma: Case Report and Review of the Literature.

Richter syndrome represents the transformation of chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma into an aggressive large B-cell lymphoma. Skin involvement is exceptionally rare, with <20 cases reported and its presence as the first presentation of CLL/small lymphocytic lymphoma, as an isolated skin lesion has never been described. Primary cutaneous CD4-positive small/medium T-cell lymphoma (CD4 PCSM-TCL) characteristically presents with a dense dermal infiltrate consisting primarily of small-/medium-sized pleomorphic T-cells and less than 30% large pleomorphic cells. A polymorphous reactive infiltrate composed of B-cells, histiocytes, plasma cells, and eosinophils is also found in differing proportions. Because of these morphological characteristics, the differential diagnosis includes not only other forms of cutaneous T-cell lymphomas but also B-cell lymphomas and reactive lymphoid infiltrates. We report a case of a cutaneous Richter syndrome as the initial manifestation of CLL, mimicking CD4 PCSM-TCL, in a 65-year-old apparently healthy asymptomatic man who presented with a solitary nodule in his arm. The objective of this study is to draw attention to this potential pitfall and describe this rare presentation.