ACUTE PROMYELOCYTIC LEUKEMIA - MORPHOLOGICAL ASPECTS

Among AML with maturation, acute promyelocytic leukemia (APL) represen ts a distinct subtype which accounts for 5-10% of all the FAB variants . APL may be recognized by different cytological pictures: (i) Hypergr anular APL, the most typical form, showing promyelocytes with cytoplas m packed with purple granules. Most of the primary granules may be inc orporated into Auer rods, sometimes stacked in bundles of faggots. (ii ) Microgranular APL, characterized by fine dustlike granulation in the cytoplasm; some promyelocytes may even appear agranular by light micr oscopy. Most of the cells show bilobed or folded nuclei, a picture whi ch may simulate that of acute myelomonocytic leukemia. (iii) Hyperbaso philic form, characterized by cells with high N/C ratio, and strongly basophilic cytoplasm with either sparse or no granules. Conspicuous cy toplasmatic budding is usually present, recalling the feature of micro megakaryocytes. Strong positivity for myeloperoxidase, Sudan black B a nd chloroacetate esterase represents the typical cytochemical pattern of M(3); usually a weaker reactivity may be observed in M(3)v. However , sometimes a degree of cytochemical heterogeneity of APL cells may be observed, as suggested by cases displaying a strong sodium fluoride-s ensitive nonspecific esterase reaction. Recently a distinct entity ass ociated with basophilic differentiation has been described. Differenti al diagnosis of this form with M(2)-baso subtype and with cases of MDS or AML with basophilia (M(2), M(4) with t(6;9) translocation) may be obtained by the use of cytochemistry, cytogenetic investigations, and electron microscopy.