Classification of Adult Acute Myeloid Leukemia
Most cases with this genetic abnormality have been identified as AMML Eo, but occasional cases have been reported to lack eosinophilia. As is found in rare cases of AML with t(8; 21), the bone marrow blast percentage in this AML is occasionally less than 20%.
Both inv(16)(p13; q22) and t(16; 16)(p13; q22) result in the fusion of the core-binding factor–beta (CBFβ) gene at 16q22 to the smooth muscle myosin heavy chain (MYH11) gene at 16p13, thereby forming the fusion gene CBFβ/MYH11. The use of FISH and RT–PCR methods may be necessary to document this fusion gene because its presence cannot be reliably documented by traditional cytogenetics banding techniques. Patients with this type of AML may achieve higher complete remission rates when treated with high-dose cytarabine in the postremission phase.[16,17,19]
Acute promyelocytic leukemia [AML with t(15; 17)(q22; q12); (PML/RARα) and variants] (FAB Classification M3)
Acute promyelocytic leukemia (APL) AML with t(15; 17)(q22; q12) is an AML in which promyelocytes predominate. APL exists as two types, hypergranular or typical APL and microgranular (hypogranular) APL. APL comprises 5% to 8% of cases of AML and occurs predominately in adults in midlife. Both typical and microgranular APL are commonly associated with disseminated intravascular coagulation (DIC).[24,25] In microgranular APL, unlike typical APL, the leukocyte count is very high with a rapid doubling time.
Common morphologic features of typical APL include the following:
- Kidney-shaped or bilobed nuclei.
- Cytoplasm densely packed with large granules (bright pink, red, or purple in Romanowsky stains).
- Bundles of Auer rods within the cytoplasm (faggot cells).
- Larger Auer rods than in other types of AML.
- Strongly positive myeloperoxidase (MPO) reaction in all leukemic promyelocytes.
- Only occasional leukemic promyelocytes in the blood.
Common morphologic features of microgranular APL include the following:
- Bilobed nuclear shape.
- Apparent scarce or absent granules (submicroscopic azurophilic granules).
- Small number of abnormal promyelocytes with visible granules and/or bundles of Auer rods (faggot cells).
- High leukocyte count in the peripheral blood.
- Strongly positive MPO reaction in all leukemic promyelocytes.