Aim: Myeloid/natural killer (NK) cell leukaemia is characterised by coexpression of myeloid with natural killer cell antigens, a high incidence of extramedullary disease and an aggressive clinical course.

Methods: We report a case of a 28-year-old woman with myeloid/NK cell precursor acute leukaemia. Clinical presentation was correlated with leukaemic blast morphology, immunophenotype, cytogenetic analysis, molecular studies for clonal rearrangements and histological review.

Results: The patient had noted skin lesions and a breast infiltrate 4 months prior to the diagnosis. Bone marrow biopsy at the time of presentation revealed characteristic morphological features with a dense infiltrate of bizarre, pleomorphic blast cells with marked nuclear invagination and reniform shapes. Immunophenotypic analysis of the blasts displayed coexpression of myeloid and natural killer cell antigens with a relatively immature phenotype: CD34^-, HLADR⁺, CD33⁺, CD56⁺, CD16^-, CD57^-, MPO^-. Cytogenetic analysis revealed a complex karyotype: del(6)(q21);-12 and add(19)(p13). Histological review of the previous breast biopsy was consistent with granulocytic sarcoma of the breast with a phenotype corresponding to the circulating blasts (positive cytoplasmic staining for myeloid markers, CD68 and CD31, and the NK cell marker CD56, with negative staining for MPO). Skin biopsy confirmed leukaemia cutis.

Conclusion: Although nodal extramedullary disease is common in the myeloid/NK cell leukaemias, this is the first description of myeloid/NK cell leukemia primarily involving breast and skin. We speculate that CD56 may predispose to extramedullary localisation of tumour.