Paucity of leukemic progenitor cells in the bone marrow of pediatric B-lineage acute lymphoblastic leukemia patients with an isolated extramedullary first relapse

Isolated extramedullary relapse in childhood acute lymphoblastic leukemia ( ALL) may be accompanied by occult bone marrow disease. We used a highly sen sitive assay to quantify leukemic progenitor cells (LPCs) in the bone marro w of such patients. Multiparameter flow cytometry and blast colony assays w ere used to detect LPCs in the bone marrow of 31 pediatric B-lineage ALL pa tients with an isolated extramedullary first relapse. Sites of relapse were central nervous system (22 patients), testes (7 patients), and eye (2 pati ents). Bone marrow (BM) LPC counts ranged from 0/10(6) mononuclear cells (M NCs) to 356/10(6) MNCs (mean +/- SE, 27.8 +/- 13.1/10(6) MNCs). LPCs were u ndetectable in 19 patients (61%), The BM LPC burden at the time of extramed ullary relapse was similar, regardless of site (Wilcoxon P = 0.77) or time of relapse (Wilcoxon P = 0.80), Compared with higher risk, standard risk at initial diagnosis showed a trend for increased BM LPC burden (mean a SE, 4 4.6 +/- 17.1 versus 7.5 +/- 3.3; Wilcoxon P = 0.22), After successful postr elapse induction chemotherapy, LPC counts in 21 evaluated patients ranged f rom 0/106 to 175/10(6) MNCs (mean +/- SE, 15.9 +/- 9.6/10(6) MNCs). By comp arison, LPC burden was higher after successful induction chemotherapy among children with an early BM relapse (range, 0 to 3262/ 10(6) MNC; mean +/- S E, 166 +/- 107; Wilcoxon P = 0.11), Thus, not all patients with an extramed ullary relapse have occult systemic failure with substantial involvement of the bone marrow, and after reinduction therapy, LPC counts were lower in t hese patients than in patients treated for an overt BM first relapse.