Contralateral Cutaneous T-cell Lymphoma as a Mimicker of Multi-decade Atopic Dermatitis.

Cutaneous T-cell lymphoma (CTCL) encompasses a group of low-grade, non-Hodgkin lymphomas that pose significant diagnostic challenges owing to variable clinical presentations and an indolent natural history. The disease is well established as a mimicker of benign inflammatory dermatoses, including atopic dermatitis (AD) and psoriasis, and diagnosis is further complicated by transient clinical improvement with topical corticosteroids, which may concurrently obscure histopathological findings on biopsy. We report the case of a 75-year-old male with a 25-to-30-year history of treatment-refractory pruritic eruptions confined to a single palm and single foot, initially attributed to AD. Despite multiple prior nondiagnostic skin biopsies, a repeat broad shave biopsy of the right medial Achilles region performed five months after an initial nonconclusive punch biopsy of the left hypothenar eminence revealed epidermotropic CD4+ T-cell infiltrates with significant loss of CD7 expression and a CD4:CD8 ratio of 5:1, establishing a diagnosis of CTCL consistent with mycosis fungoides (MF). The patient was subsequently referred to a tertiary oncology center for formal disease staging and management. This case underscores the importance of maintaining a high index of suspicion for CTCL in patients with chronic, recalcitrant dermatitis, the critical value of topical steroid cessation prior to tissue sampling, and the utility of repeat biopsy with comprehensive immunohistochemical profiling in arriving at a timely and accurate diagnosis.