Acute promyelocytic leukaemia: a study of 39 cases with identification of a hyperbasophilic microgranular variant

Thirty‐nine cases of acute promyelocytic leukaemia (APL) were divided into two morphological subgroups, typical hypergranular APL (31 cases) and microgranular APL (eight cases, 21%). The leukaemic cells in the microgranular APL cases were characterized by striking nuclear folding or lobulation; granulation was present in most of these cells but was less abundant and finer than in typical APL. In three microgranular APL cases a distinctive small leukaemic promyelocyte with unusual nuclear lobulation and deeply basophilic cytoplasm containing few or no visable granules was the predominant leukaemic cell. This small hyperbasophilic promyelocyte was also present as a minor population of cells in the other five microgranular APL cases and in 28 of the 31 typical APL cases. Ultrastructurally the most abundant promyelocytes in microgranular APL had smaller and usually fewer granules than in typical APL; other characteristic ultrastructural features of APL were found with equal frequency. The median blood leucocyte count was significantly higher in microgranular APL, 83·0 × 10⁹/1, than in typical APL, 1 · 8 × 10⁹/1 (P < 0 · 01). The median duration of complete remission (CR) for microgranular APL, 6 · 5 months, was shorter than the 21 + month median CR for typical APL. The morphological characteristics of microgranular APL may mimic those of myelomonocytic leukaemia; however, the presence of cells with multiple Auer rods, large inclusions of Auer‐like material and the small hyperbasophilic promyelocytes are important distinguishing features. In equivocal cases cytochemistry, electron microscopy and cytogenetic studies may verify the diagnosis.