Mycosis fungoides is a primary cutaneous T-cell lymphoma characterized by epidermotropism. It is the most common type of cutaneous T-cell lymphoma. It typically presents in adults and has an indolent clinical course.

Macroscopically, it has a stepwise evolution of appearance. Typically, it begins as pruritic, rash-like patches on the trunk, but can arise anywhere on the body. Next, it progresses to a palpable plaque and finally a tumour. Microscopically, it is characterized by small-to medium-sized lymphoid cells with cerebriform nuclei and a clear perinuclear halos. They classically reside in the basal layer of the epidermis. As well, intraepidermal collections of atypical cells (Pautrier microabscesses) are a classic feature.

This is a case of a patient with hypopigmented patch of mycosis fungoides. The immunohistochemical profile for this case shows positivity in T cell markers, including CD3, CD4, CDS, and CD8. CD7 expression is lost. Further immunohistochemical stains performed include CD20, CD56, CD10, CD30 and Ki-67. CD20 is expressed in background B cells. CD30, CD10, and CD56 are negative. Ki-67 stains approximately 10% of cells. In this case, the ratio of CD4 to CD8 positivity is approximately 1:6 with CD8 as the predominant neoplastic cell. It is more common to have a predominant expression of CD4.

This slide shows CD3 stain. See Related Content for H&E, CD20, CD7, CD4, and CD8 stains.