P087 Not so mycosis fungoides

Abstract Mycosis fungoides (MF) and Sézary syndrome (SS) are the most common forms of cutaneous T-cell lymphoma. MF derives its name as the cutaneous findings can mimic dermatophytosis. However, the need to consider alternative diagnoses in cases of treatment-resistant MF is rarely emphasized. We report a case series of five patients who were diagnosed with ‘true mycosis’ while receiving treatment for MF/SS. Case 1 is an 84-year-old man with patch stage IB MF who developed a new annular, scaly rash on his abdomen and groin during the course of narrowband ultraviolet B treatment. Skin biopsy and fungal cultures confirmed tinea cruris and tinea corporis caused by Trichophyton rubrum. He was successfully treated with diflucortolone valerate/isoconazole nitrate cream. Case 2 is a 57-year-old man with a 10-year history of MF who presented with a new itchy annular rash on his buttocks, suspected to be either tinea corporis or a relapse of MF. Biopsy and fungal culture confirmed tinea corporis caused by T. rubrum, which resolved with topical terbinafine treatment. Case 3 is a 75-year-old man with stage IVA2 MF who developed red, itchy patches in flexural folds during psoralen–ultraviolet A and pegylated interferon therapy. Examination revealed erythematous plaques and fungal hyphae in the groin, treated with diflucortolone valerate/isoconazole nitrate with improvement. Case 4 is a 57-year-old man who presented with a chronic, itchy rash since the age of 16 years. Examination revealed parapsoriasis-like patches under the arms and an atypical annular rash on the calf. A skin biopsy from his arms was consistent with parapsoriasis; however, a skin biopsy and scrapings from the calf revealed T. rubrum. The tinea corporis cleared with topical terbinafine. Case 5 is a 66-year-old woman who presented with an scaly erythematous rash and maceration between the toes. She had a diagnosis of nonerythrodermic SS and palmoplantar hyperkeratosis, and had a complete response in blood and a partial response in skin following mogamulizumab–methotrexate combination therapy. Skin scrapings from her foot confirmed T. rubrum, which improved with topical terbinafine cream. In MF/SS, a compromised epidermal barrier and a T helper 2-biased tumour microenvironment predispose individuals to chronic dermatophytosis within the spectrum of skin infections. Additionally, the expression of human beta defensins is dysregulated in patients with MF/SS, further increasing their susceptibility to skin infections. Treatment options for MF, such as topical corticosteroids, phototherapy and systemic therapies, can suppress the immune system, thereby raising the risk of fungal skin infections. Our case series highlights that fungal skin infections can mimic MF in patients with MF/SS and emphasizes the importance of considering a secondary diagnosis when reviewing and treating these patients, particularly if they present with a new atypical rash.