Null T‐cell phenotype mycosis fungoides with aberrant CD20 and CD56 expression: A diagnostic dilemma

Mycosis fungoides (MF) represents the most common type of primary cutaneous T‐cell lymphoma. Recognition of MF variants with divergent immunophenotypes is important for accurate diagnosis and appropriate management, as they can be confused with other lymphoma subtypes. We present a case of a 49‐year‐old male previously diagnosed with a cutaneous lymphoproliferative disorder with an unusual NK/T‐cell phenotype. He presented with a 10‐year history of pelvic girdle rash involving the right hip and upper thigh. The lesions were characterized as atrophic patches concentrated in sun‐protected areas and involving 10% of the body surface area. Shave biopsies revealed an atypical epidermotropic infiltrate composed of hyperchromatic small to medium‐sized lymphocytes with perinuclear halos and “tagging” along the dermal–epidermal junction. The immunophenotype was unusual in that the neoplastic lymphocytes showed complete loss of pan T‐cell antigens along with expression of CD56, cytotoxic markers, and weak CD20. All other B‐cell markers were negative. The combination of clinical findings, in addition to the histopathologic and immunophenotypic profile, were diagnostic of null T‐cell phenotype MF with aberrant expression of CD56 and CD20. Null T‐cell phenotype MF is very uncommon, can be diagnostically challenging, and can mislead the diagnosis of aggressive lymphoma subtypes.