Acute promyelocytic leukemia [AML with t(15; 17)(q22; q12); (PML/RARα) and variants] (FAB Classification M3)
(Refer to the Acute promyelocytic leukemia (FAB Classification M3) section of the Acute Myeloid Leukemia With Characteristic Genetic Abnormalities section of this summary for more information.)
Acute myelomonocytic leukemia (FAB Classification M4)
Acute myelomonocytic leukemia (AMML) is characterized by the proliferation of neutrophil and monocyte precursors. Patients usually present with anemia and thrombocytopenia. (Refer to the PDQ summary on Fatigue for more information on anemia.) This classification of AML comprises approximately 15% to 25% of cases of AML, and some patients have a previous history of chronic myelomonocytic leukemia (CMML). (Refer to the PDQ summary on Myelodysplastic/ Myeloproliferative Neoplasms for more information.) This type of AML occurs more commonly in older individuals.
Morphologic and cytochemical features include the following:
- 20% or more blasts in the bone marrow.
- 20% or more neutrophils, monocytes, and their precursors in the bone marrow (to distinguish AMML from AML with or without maturation and to increase monocytes).
- 5 x 109 /L or more monocytes in the blood.
- Large monoblasts with round nuclei, abundant cytoplasm, and prominent nucleoli.
- MPO positivity in at least 3% of blasts.
- Monoblasts, promonocytes, and monocytes typically nonspecific esterase- (NSE) positive.
Immunophenotyping generally reveals monocytic differentiation markers (CD14, CD4, CD11b, CD11c, CD64, and CD36) and lysozyme. The differential diagnosis includes AML with maturation and acute monocytic leukemia.
Most cases of AMML exhibit nonspecific cytogenetic abnormalities. Some cases may have a 11q23 genetic abnormality. Cases with increased abnormal eosinophils in the bone marrow associated with a chromosome 16 abnormality have a favorable prognosis. (Refer to the Acute myeloid leukemia with characteristic genetic abnormalities section of the Classification section of this summary for more information.)
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.