Mycosis fungoides with epidermal mucinosis: a variant of mycosis fungoides with a spongiosis-like pattern

Background The histopathologic diagnosis of mycosis fungoides (MF) has classically relied on the presence of atypical epidermotropic T‐lymphocytes predominating over spongiosis. However, in some cases of MF, prominent epidermal mucinosis in a spongiosis‐like pattern mimics a spongiotic dermatitis. To our knowledge, only one series in the literature has thus far recognized the presence of epidermal mucinosis in MF. Methods We evaluated 30 skin biopsies from 18 patients with the clinical diagnosis of MF, which fulfilled all histopathologic criteria for patch‐ or plaque‐stage MF, but also showed epidermal mucinosis in a spongiosis‐like pattern. A total of 15 specimens were studied by immunohistochemistry, and seven were tested for T‐cell receptor (TCR) gene rearrangements. Twenty biopsies of spongiotic dermatitides were included as controls. Results We confirmed the presence of epidermal mucinosis in all 30 cases of MF with a spongiosis‐like pattern based on histopathologic criteria and the colloidal iron stain for mucin. Immunohistochemistry in 15 specimens showed significant loss of pan‐T‐cell antigens CD5 (10/15) and CD7 (14/15); and TCR clonality was detected in 7 specimens from 6 patients, supporting the diagnosis of MF. Conclusions We report helpful histopathologic criteria for distinguishing MF with epidermal mucinosis in a spongiosis‐like pattern from spongiotic dermatitis.