A "phenotypic switch" (PS) is a well-known phenomenon that occurs in hematopoietic neoplasms, often after treatment. However, in cutaneous T cell lymphoma (CTCL), this event has rarely been reported, and thus, very little is known about its relevance to disease prognosis. We report two cases of patients that were diagnosed with a CD4+ mycosis fungoides with positive T cell receptor gene rearrangement studies. Both patients originally responded to treatment, but subsequently, their CTCL came back with a different phenotype of a CD4- CTCL. Gene rearrangement studies were performed on the second occurrence in order to prove that this was the same lymphoma. Both patients died from their CTCL. Additionally, we collected seven cases of primary CTCL from the literature with tissue samples from before and after treatment with molecular studies confirming these neoplasms contained the same T cell clone, providing evidence of a true PS. This too revealed a poor prognosis in the majority of these cases. CTCL should be worked up to determine whether a PS has occurred after therapy since it could confuse management of patients and appears to portend a poor prognosis.
Keywords: Cutaneous T cell lymphoma; Mycosis fungoides; Phenotypic switch; Poor prognosis.