Posttherapy surveillance of B-cell precursor acute lymphoblastic leukemia. Value of polymerase chain reaction and limitations of flow cytometry.

Flow cytometric immunophenotypic analysis is critical in diagnosis and classification of acute leukemia and has been used after therapy to monitor for minimal residual disease. However, the presence of normal B-cell precursors, hematogones, particularly in the context of treated pediatric B-cell precursor acute lymphoblastic leukemia (BP-ALL), may confound such evaluation. In this study, the value of more specific genotypic markers (polymerase chain reaction evaluation of2 antigen receptor genes) was assessed to resolve this issue. Flow cytometric analysis of enriched mononuclear cells revealed 1% to 20% precursor B cells (PBCs), based on expression of I or more pan-B cell antigens in addition to CD10, CD34, and terminal deoxynucleotidyl transferase in all 14 patients studied. Inasmuch as this mimicked the immunophenotype of the original leukemic clone, PBCs, in isolation, were considered suspicious for minimal residual disease. However 11 of the 14 posttherapy specimens (79%) revealed no monoclonally rearranged antigen receptor genes, and 7 of these 11 patients had trackable genotypic markers at presentation. Accordingly, by PCR these 7 patients had complete molecular remission, supported by clinical follow up of 16 to 73 months. Among the remaining 4 patients with PCR-negative disease, 3 continue in remission, confirming the interpretation of false-positive flow cytometric analysis. In conclusion, flow cytometric monitoring of posttherapy bone marrow specimens from patients with BP-ALL may be misleading, if considered in isolation, in falsely suggesting the presence ofminimal residual disease. Rather, PCR for antigen receptor gene rearrangements is a valuable and specific tool, helpful in differentiating hematogones from minimal residual disease in patients with treated BP-ALL whose bone marrow harbors increased PBCs.