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Case Reports

Abstract

Hypopigmented mycosis fungoides (HMF) is a rare variant of mycosis fungoides. It presents with circular or irregularly shaped hypopigmented patches or thin plaques with a fine scale. It is most often seen in children, adolescents, and young adults. Diagnosis can be challenging when molecular studies are negative. A 50-year-old patient presented with a 6–9 month history of progressive, asymptomatic hypopigmented patches on the bilateral inner arms. Shave biopsy of the left inner arm revealed epidermal atrophy, lichenoid lymphocytic infiltrate at the dermoepidermal junction, papillary dermal edema, upper dermal sclerosis, and marked pigment incontinence. Atypical junctional lymphocytes were moderate-to-large with perinuclear halos. Immunohistochemistry showed a robust dermal and epidermal T-cell infiltrate (CD3⁺) with reversed CD4:CD8 ratio favoring CD8⁺ cells, retained CD5, and markedly decreased CD7. CD20 highlighted rare B lymphocytes. T-cell receptor gene rearrangement studies were negative for clonality, but this finding does not exclude HMF. The combined clinical, histopathologic, and immunophenotypic features supported a diagnosis of CD8⁺-predominant hypopigmented MF. This case illustrates an uncommon presentation of adult-onset HMF with CD8 predominance and negative TCR gene rearrangement, a profile more often associated with pediatric disease. HMF constitutes about 3% of all adult cases of MF. This case underscores the need for high clinical suspicion, even in older patients, and the importance of integrating histopathologic and immunophenotypic features with repeat clinical evaluation when molecular findings are inconclusive.