This 9-year-old boy has a chronic, eczematous eruption over his thighs and buttocks. It has not responded well to topical steroids but diminishes with light exposure. The clinical impression includes pityriasis lichenoides chronica and parapsoriasis, but the biopsy was taken to rule out mycosis fungoides. Mycosis fungoides is unusual in the pediatric population, and when it occurs, it is often CD8+. Unfortunately, just as the clinical diagnosis is challenging, often the histologic diagnosis is equally challenging. Patients with contact dermatitis or PLEVA can, on occasion, have cytologic atypia in their infiltrate. Also, clonal populations using molecular assays have been demonstrated in benign inflammatory conditions. Therefore, serial biopsies and clinical monitoring are often necessary to come to the correct diagnosis. The histology in this case is very worrisome for mycosis fungoides. The TCR gene rearrangement analysis came back polyclonal.
Histologic sections of skin show regular (psoriasiform) epidermal acanthosis with overlying orthokeratosis. Within the epidermis, there is a population of mononuclear cells, tagging along the dermal/epidermal junction and extending into the epidermis in a pattern of epidermotropism. These cells appear enlarged with perinuclear halos. The dermis shows mild fibrosis and has an interstitial infiltrate with similar mononuclear cells.