Acute nonlymphocytic leukemia developed in a 57-year-old woman following adjuvant therapy with melphalan for ovarian carcinoma. Maturation of differentiating marrow myeloid and erythroid precursors was megaloblastic. The serum vitamin B12 level was low, and Schilling test revealed vitamin B12 malabsorption correctable with intrinsic factor. Megaloblastic maturation of the marrow cells was converted to normoblastic following treatment with vitamin B12 and folic acid. However, blast cells persisted in the marrow, and cytogenetic analysis revealed aneuploidy and trisomy 18. In contrast to the marrow blast cells, there was a decline in circulating blast cells following vitamin replacement, suggesting that these cells were capable of maturation but required vitamin B12 for this purpose.
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