CD3/CD20/CD45 negative leukemia cutis.

A 56-year-old male presented to the clinic with complaints of multiple skin lesions. A complete blood count (CBC) was not available. No constitutional symptoms were present, and physical examination revealed tender skin lesions of the back, arms, legs, and scalp. A skin punch biopsy showed fragments of skin with extensive lymphoid infiltrates. The initial lymphoma workup by immunohistochemistry demonstrated negative staining for CD45, CD3, and CD20. Additional stains were performed which revealed the atypical lymphoid infiltrate to be positive for PAX5, TdT, CD10, CD34, CD79a, and CD99 and negative for CD4, CD8, Keratin, S100, CD56, CD138, and EMA. These histologic and immunophenotypic findings supported the diagnosis of skin involvement by B-lymphoblastic leukemia/lymphoma (B-ALL/LBL). Consequent peripheral blood and bone marrow biopsy evaluations supported this diagnosis. To avoid misdiagnosis, it is important to remember that B-ALL/LBL can rarely present as a skin lesion and can be negative for the most commonly used lymphoma immunohistochemical markers: CD45, CD3, and CD20. Additionally, skin involvement by B-ALL/LBL, although very uncommon, is most often reported in children or young adults, unlike this unique case occurring in an adult.