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 clas sification 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 prec ursor acute lymphoblastic leukemia (BP-ALL), may confound such evaluation. In this study the value of more specific genotypic markers (polymerase chai n reaction evaluation of 2 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 1 or more pan-B cell antigens in addition to CD10, CD34, and terminal deoxynucleotidyl tran sferase in all 14 patients studied. Inasmuch as this mimicked the immunophe notype of the original leukemic clone, PBCs, in isolation, were considered suspicious for minimal residual disease. However; II of the 14 posttherapy specimens (79%) revealed no monoclonally rearranged antigen receptor genes, and 7 of these II patients had trackable genotypic markers at presentation . Accordingly, by PCR these 7 patients had complete molecular remission, su pported by clinical follow up of 16 to 73 months. Among the remaining 4 pat ients with PCR-negative disease, 3 continue in remission, confirming the in terpretation of false-positive flow cytometric analysis. In conclusion, flo w cytometric monitoring of posttherapy bone marrow specimens from patients with BP-ALL may be misleading, if considered in isolation, in falsely sugge sting the presence of minimal residual disease. Rather PCR for antigen rece ptor gene rearrangements is a valuable and specific tool, helpful in differ entiating hematogones from minimal residual disease in patients with treate d BP-ALL whose bone marrow harbors increased PBCs.