Pustular Mycosis Fungoides Triggered by Systemic Interferon‐γ

ABSTRACT Pustular mycosis fungoides (pMF) is a rare variant of MF characterized by the clinical and histopathological formation of pustules. We report a 71‐year‐old Japanese woman with established MF who developed a generalized pustular eruption temporally triggered by systemic interferon (IFN)‐γ administration. Clinically, the patient presented with generalized erythema with coalescing pustules mimicking generalized pustular psoriasis or acute generalized exanthematous pustulosis. Histological examination revealed the simultaneous presence of spongiform pustules of Kogoj and large transformed CD30 + atypical lymphoid cells. The pustules resolved rapidly upon the discontinuation of IFN‐γ and the administration of systemic antibiotics. Cytokine analysis at the onset of pustulosis revealed marked elevations in serum IL‐8, IL‐18, IL‐23, and IL‐6 compared with healthy controls; these levels normalized during remission. This case suggests that systemic IFN‐γ can act as a trigger for pMF. Furthermore, the distinct cytokine profile highlights that the IL‐23/IL‐17/IL‐8 axis may be a crucial mediator in the pathogenesis of pMF.