Cutaneous T-cell lymphoma emerging during long-term dupilumab therapy in a patient with longstanding atopic diathesis: diagnostic challenges and mechanistic considerations.

Atopic dermatitis and cutaneous T-cell lymphoma can overlap both clinically and histopathologically, potentially delaying recognition of early disease. We report a case of a woman in her 40s with longstanding atopic diathesis who developed patch-stage mycosis fungoides after more than 5 years of stability on dupilumab. Earlier biopsies showed spongiotic dermatitis without evidence of a lymphoproliferative disorder. She later presented with a rapidly progressive, intensely pruritic eruption of erythematous scaly plaques with near-erythroderma, morphologically distinct from her prior dermatitis. Repeat biopsy revealed an epidermotropic atypical lymphoid infiltrate consistent with mycosis fungoides. This case highlights the importance of reassessment in patients with presumed atopic dermatitis who develop atypical features, rapid progression or diminished response to therapy. It also raises the possibility that dupilumab may unmask previously unrecognised cutaneous T-cell lymphoma in some patients and highlights the need for further study of interleukin-4 and interleukin-13 pathway modulation.