Introduction
Mycosis fungoides (MF) is the most common type of cutaneous T-cell lymphoma. Although most MF cases show indolent clinical courses, some cases exhibit severe progression. Previous studies have demonstrated that the prognosis for patients with MF depends on several parameters, including age, clinical stage, and specific factors such as large cell transformation (LCT) 1-6. LCT is diagnosed based on large cells (whether CD30 positive or negative) constituting >25% of the infiltrate or forming microscopic nodules within the MF lesion. LCT of MF (MF-LCT) has been observed in 2.3-22.6% of MF patients during the course of the disease 2-11. Notably, previous studies reported the development of LCT in 21.3-31.0% of advanced-stage patients, but in only 1.4% of early-stage patients3,7. Thus, MF-LCT usually is associated with an advanced stage of the disease.
CD30 is a cell membrane protein of the tumor necrosis factor receptor family. CD30 is expressed in activated T-cells and used as a tumor marker. Neoplastic proliferation of CD30+ lymphocytes in the skin is observed in MF-LCT and CD30+ lymphoproliferative disorders (LPDs) such as lymphomatoid papulosis (LyP) and primary cutaneous anaplastic large cell lymphoma (PC-ALCL). Most LyP cases and about 20% of PC-ALCL cases spontaneously regress with excellent prognoses 12-14. However, spontaneous regression of MF-LCT has been reported only rarely. Here, we present a case showing spontaneous regression of MF-LCT in patch-stage MF.