Preleukemic phase of chronic myelogenous leukemia: morphologic and immunohistochemical characterization of 7 cases.

Patients with chronic myelogenous leukemia (CML) present typically with an elevated white blood cell count (WBC) and cytogenetic or molecular genetic evidence of t(9;22)/BCR-ABL1 fusion gene. Rarely, CML patients may present with a normal or mildly elevated WBC and are asymptomatic, and we describe 7 patients in this study. The WBC in these patients ranged from 3.6 to 14.3 K/mm(3) with 50% to 73% granulocytes and 0% blasts. In all patients, t(9;22)(q34;q11.2) was detected by conventional cytogenetics, and BCR-ABL1 fusion was shown, supporting the diagnosis of preleukemic CML (pre-CML). We compared these patients with a group of 5 cases of CML in chronic phase (CML-CP) and 5 bone marrow specimens with a leukemoid reaction (n=5). Reticulin, CD34, and CD61 immunostains were performed on all bone marrow biopsy specimens. Peripheral blood absolute basophilia (≥200/mm(3)) was noted in only 4 of 7 pre-CML cases, whereas it was present in all CML-CP cases and absent in leukemoid reaction cases. The mean ±SD of microvascular density of pre-CML cases (10.0 ± 4.3 vessels/200× field) was twice that of leukemoid reaction cases (5.0 ± 1.0) (P=.02; Student t test) but similar to that of CML-CP cases (12.5 ± 3.6). Microvessels in pre-CML, highlighted by CD34, were tortuous with abnormal branching, although to a lesser extent than those found in CML-CP. Microvessels in leukemoid reaction were generally straight. The percentage of small, hypolobated megakaryocytes, highlighted by CD61 in pre-CML, was 40%, 3 times that found in leukemoid reaction cases (13%) but less than that of CML-CP cases (86%). We conclude that pre-CML should be suspected in patients with a normal to mildly elevated WBC and absolute basophilia. Bone marrow examination can usually distinguish pre-CML from a leukemoid reaction based on the percentage of small, hypolobated megakaryocytes; microvascular density; and morphologic features.