Background: Mycosis fungoides (MF) typically presents as erythematous scaly patches or plaques that may progress to cutaneous tumors. Although MF may be presented like other dermatoses, initial presentation as psoriasiform plaques simulating psoriasis is rare. Differentiating MF from psoriasis is important because systemic therapies used for psoriasis can worsen MF. We describe a case of psoriasiform MF and we also review the clinicopathological features of similar cases in the literature.

Case: A 46-year-old woman was referred to our clinic with a history of psoriasiform plaques for 13 years. She had multiple, generalized, indurated plaques with thick psoriasiform scales that were unresponsive to topical treatments. The histopathology showed marked psoriasiform epidermal hyperplasia with epidermotropic atypical lymphocytes compatible with MF. Immunohistochemical (IHC) staining showed that atypical lymphocytes were positive for CD3, CD4, CD8, and CD5. Of note, upper dermal and intraepidermal large atypical lymphocytes were CD30 positive. The review of similar psoriasiform MF cases revealed that they had all been treated as psoriasis for many years and finally diagnosed as MF especially after deterioration induced by immunosuppressive therapies.

Conclusions: In presumed cases of psoriasis that are unresponsive to treatment, progressive, or ulcerative, biopsy should be considered to rule out MF, particularly before starting a potent immunosuppressive agent.