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Figure 1: The bone marrow aspirate demonstrates a marked increase in small to medium-sized mature lymphocytes with clumped chromatin (red arrow). Blasts are also increased and appear medium to large in size, with prominent nucleoli and scant to moderate amounts of blue cytoplasm (yellow arrow) (A). The bone marrow core biopsy is markedly hypercellular for age (approximately 90% cellularity) and shows scattered lymphoid aggregates composed predominantly of small, mature lymphocytes with clumped chromatin (red arrow). Immature cells with fine chromatin and prominent nucleoli are present interstitially and in clusters outside the lymphoid infiltrates (yellow arrow) (B). Immunohistochemistry for CD20 highlights aggregates of B-cells, comprising approximately 70% of the bone marrow cellularity (C). CD5 is positive in the B-cells (not shown). CD34 staining reveals an increased population of blasts, accounting for approximately 10–20% of the overall cellularity (D). According to the International Consensus Classification (ICC), the myeloid component is best classified as myelodysplastic syndrome/acute myeloid leukemia (MDS/AML), therapy-related. Per the World Health Organization (WHO) Classification of Hematolymphoid Tumors, it is best categorized as myeloid neoplasm, post-cytotoxic therapy (MN-pCT).