A clinical and histologic mycosis fungoides simulant occurring as a T-cell infiltrate coexisting with B-cell leukemia cutis.

One year after the onset of chronic lymphocytic leukemia, an elderly man had scaly cutaneous plaques on the thighs that clinically and histologically resembled the mycosis fungoides type of cutaneous T-cell lymphoma. Two years later the patient had indurated, red dermal nodules on the face that clinically and histologically were characteristic of cutaneous chronic lymphocytic leukemia. Immunophenotyping results from a facial nodule confirmed the presence of a B-cell infiltrate (CD20+). Immunophenotyping of a lesion on the right thigh showed that half the cells were composed of a CD2+, CD45RO+ (UCHL-1+) upper dermal and focally epidermotropic population of T cells consistent with mycosis fungoides; however, these T cells coexisted with an equal number of CD20+ B cells arranged in distinct clusters. DNA from the thigh lesion exhibited a B-cell immunoglobulin gene rearrangement, but the T-cell receptor gene rearrangements were germline. In this case, the evidence favors a mycosis fungoides simulant occurring as a reactive T-cell infiltrate to an underlying B-cell chronic lymphocytic leukemia.