Congenital leukemia is a rare condition diagnosed at birth to 6 weeks of life with an estimated incidence of 4.7 per million live births. In a review, about 25~30% of cases are well documented as leukemia cutis. Leukemia cutis usually develops several months after the diagnosis of systemic leukemia, although it occasionally precedes it. The most common varieties of congenital leukemia are of the myelocytic (M4) or monocytic (M5) FAB subtypes and rarely of lymphoblastic subtypes.

Diagnosis of congenital leukemia cutis may be dif-ficult, because most patients present with asymptomatic clinical manifestations. The commonest skin presentation is multiple papules, nodules or infiltrated plaques. Byrd et al. found that 47% of confirmed cases with extramedullary leukemia were initially misdiagnosed, often as lymphoma.
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The diagnosis is made by suspicion and verified by skin biopsy, immunophenotyping, and B or T cell receptor rearrangement studies. Hematologic studies with complete analysis of bone marrow aspirate and peripheral blood smear are then needed to confirm the diagnosis. Both acute and chronic forms of myeloid progenitors in the bone marrow cells are histochemically positive for myeloperoxidase, Sudan black B, chloroacetate esterase, and naphthol-ASD- chloroacetate esterase (Leder) stains. Myeloid cells do not express B-lineage or T-lineage markers, but may show immunohistochemical staining for the myeloid markers CD13, CD33, CD15, CD117 or megakaryoblastic antigens C41 and CD61.