Acute monoblastic leukemia and acute monocytic leukemia (FAB classifications M5a and M5b)
Acute monoblastic and acute monocytic leukemia are AMLs in which 80% or more of the leukemic cells are of a monocytic lineage. These cells include monoblasts, promonocytes, and monocytes. These two leukemias are distinguished by the relative proportions of monoblasts and promonocytes. In acute monoblastic leukemia, most monocytic cells are monoblasts (usually ≥80%). In acute monocytic leukemia, most of the monocytic cells are promonocytes. Acute monoblastic leukemia comprises 5% to 8% of cases of AML and occurs most commonly in young individuals. Acute monocytic leukemia comprises 3% to 6% of cases and is more common in adults. Common clinical features for both acute leukemias include bleeding disorders, extramedullary masses, cutaneous and gingival infiltration, and central nervous system involvement.
Morphologic and cytochemical features of acute monoblastic leukemia include the following:
- Large basophilic monoblasts with abundant cytoplasm, pseudopod formation, round nuclei, and one or more prominent nucleoli.
- Rare Auer rods.
- Typically intensely NSE-positive and MPO-negative.
- Hypercellular marrow with large numbers of monoblasts.
- Lysozyme positive.
Morphologic and cytochemical features of acute monocytic leukemia include the following:
- Promonocytes with an irregular nuclear configuration with a moderately basophilic cytoplasm and cytoplasmic azurophilic granules.
- Typically intensely NSE-positive.
- Occasional MPO positivity.
- Hemophagocytosis (erythrophagocytosis).
The extramedullary lesions of these leukemias may be predominantly monoblastic or monocytic or an admixture of the two cell types. Immunophenotyping of these leukemias may reveal expression of the myeloid antigens CD13, CD33, CD117, CD14 ( + ), CD4, CD36, CD 11b, CD11c, CD64, and CD68. The differential diagnosis of acute monoblastic leukemia includes AML without maturation, minimally differentiated AML, and acute megakaryoblastic leukemia. The differential diagnosis of acute monocytic leukemia includes AMML and microgranular APL.
An abnormal karyotype has been observed in approximately 75% of cases of acute monoblastic leukemia while approximately 30% of cases of acute monocytic leukemia are associated with an abnormal karyotype. Almost 30% of cases of acute monoblastic leukemia and 12% of cases of acute monocytic leukemia are associated with 11q23 genetic abnormalities involving the MLL gene. (Refer to the Acute myeloid leukemia with characteristic genetic abnormalities section of the Classification section of this summary for more information.) Mutation of FLT3, a receptor tyrosine kinase gene, has been observed in about 30% of cases of acute monocytic leukemia (approximately 7% in acute monoblastic leukemia). The translocation t(8;16)(p11; p13) (strongly associated with acute monocytic leukemia, hemophagocytosis by leukemic cells, and a poor response to chemotherapy) fuses the MOZ gene (8p11) with the CBP gene (16p13). Median actuarial DFS for acute monocytic leukemia has been reported to be approximately 21 months.