Acute Promyelocyte Leukemia M3

Acute promyelocytic leukemia (APL) accounts for about 10 percent of cases in adult and for 4 to 8 percent in children.83 Typical APL is characterized by the presence of hypergranular blast cells, which have the appearance of abnormal promyelo-cytes. The cytoplasm is dense with coarse dark-staining granules that often obscure the nucleus. The nuclear border is generally irregular and has a folded or reniform appearance (Plate 14-12). Cells contain bundles of delicate needlelike Auer rods, giving a meshwork appearance (so-called faggots), found in some of the leukemic promyelocytes. These abnormal promye-locytes are strongly positive for the MPO, SBB, and CAE reactions. The bundles of Auer rods are recognized easily with the CAE stain. Leukemic blast cells express myeloid antigens with the absence of HLA-DR and CD34.84 In contrast to other AML subtypes, dysplastic features of the myeloid series associated with leukemic proliferation are not seen.

The FAB classification recognizes a variant of APL (FAB M3v),85 characterized by bilobate cells or by cells with reniform nucleus and cytoplasm with minimal or no granulations (Plate 14-13). Typical M3 cells are infrequent in the bone marrow. M3v has the same clinical, cytogenetic, and immunological features as hypergranular APL. Since M3v is easily confused with monocytic leukemia special attention should be given to differentiate these cases from M4 and M5b subtypes.

In the recently developed treatment strategies for patients with APL the detection of the t(15;17) or the PML/RARa rearrangement is critical. In the absence of either the likelihood of a response to all-trans-retinoic acid (ATRA) is remote.86 The t(11;17) and t(5;17) may present with a morphology that can be confused with classical M3 with t(15;17). Recently the FAB group has reviewed 32 cases of APL and variant forms. By utilizing published criteria the ability to make a correct diagnosis by morphology with molecular genetic confirmation and to eliminate cases that did not have the PML/RARa rearrangement was found to be excellent.87 Similar results have also been presented.88

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