Granulomatous mycosis fungoides histologically mimicking tuberculoid leprosy

Summary: A 68-year-old male presented to dermatology clinic with an asymptomatic rash that started as a large red patch on the left lower leg and gradually progressed to red papules and plaques on the chest and back over a couple months. History prior to the eruption was notable only for a recent trip to the northeast and COVID infection. No associated weight loss or systemic symptoms were reported. Examination revealed brown-red papules and plaques on the left lower leg, chest, back and upper arms. Initial punch biopsies revealed a granulomatous dermatitis following neurovascular bundles and an unusually heavy lymphoid infiltrate, concerning for tuberculoid leprosy. Special stains for mycobacteria and syphilis were negative, as was PCR for Myobacterium leprae and lepromatosis. T-cell gamma gene rearrangement (TCR) detected monoclonal peaks. Subsequent punch biopsy of a plaque on the back showed granulomatous dermatitis, with TCR detecting an identical clone as the first specimen. Based on histologic and molecular findings, a diagnosis of granulomatous mycosis fungoides was made. Granulomatous mycosis fungoides is a relatively rare variant of mycosis fungoides and is known to mimic other granulomatous inflammatory and infectious dermatoses. This patient’s presentation highlights a case of granulomatous mycosis fungoides histologically concerning for tuberculoid leprosy.