Korean J Pediatr 2012;55(3):100-106 • http://dx.doi.org/10.3345/kjp.2012.55.3.100 101 Introduction Despite major improvements in the overall survival of children with acute lymphoblastic leukemia (ALL), for the 20 to 25% of patients who relapse, the prognosis remains poor1). Notwithstanding its potential complications, allogeneic hematopoietic stem cell transplantation (HSCT), preferably from a matched sibling donor (MSD), is known to result in better outcome rather than chemotherapy only in second complete remission (CR)2-4). With improvements in human leukocyte antigen (HLA) typing and supportive care of transplant-related complications, recent evidence suggests similar outcomes when comparing the results of MSD HSCT and matched unrelated (MUD) HSCT5-7). However, most of the pa- tients undergoing transplantation will fail to be cured of their disease, emphasizing the need to identify important prognostic factors in children who receive HSCT, as well as novel therapeutic modalities. Previous analyses indicate that the duration of first remission has overall prognostic significance for children with ALL who receive allogeneic HSCT after first relapse6,7). Other important factors include the conditioning regimen utilized as well as the disease status at the time of transplantation8,9). Efforts to identify other significant prognostic variables are key to maximizing the efficacy of allogeneic HSCT in childhood ALL, and improve upon its current shortcomings. In this study, we retrospectively reviewed the results of allogeneic HSCT undertaken in children with ALL in second CR, during a period spanning more than 17 years at a single institution, with the aim of summarizing overall therapeutic outcome for these children, identifying major prognostic variables, and comparing our institution results for MSD HSCT and MUD HSCT. Materials and methods 1. Patient cohort 1) Pre-transplantation Patients diagnosed with ALL, who received allogeneic HSCT in second CR from August, 1991 to February, 2009 at the Department of Pediatrics, The Catholic University of Korea, were included. Recipients of cord blood transplantation or familial donor transplantation with 1 antigen or more mismatches were excluded from the study group. Overall, 53 patients (42 male, 79%) formed the study cohort (Table 1). Median age at diagnosis was 6.5 years (range, 0.3 to 15 years). Risk group classification before 2001 derived from National Cancer Institute criteria, and then, subsequent to 2001, was based on previously published institutional criteria10). Twenty patients (38%) were classified as low or standard risk ALL, and 33 (62%) were classified as high or very high risk ALL. Of 41 patients with available cytogenetic studies at diagnosis, 8 had high risk features including 4 patients withBCR-ABL1 , 3 withE2A-PBX1 , and 1 patient withMLL gene rearrangement. All patients in the study cohort were diagnosed with first bone marrow relapse at a median of 32.6 months since achieving CR (range 1.1 to 118) (Table 2). For the purposes of risk stratification according to duration of first CR, patients were categorized into early relapse (<12 months since CR, n=11, 21%), intermediate relapse (≥12 months and <36 months since CR, n=18, 34%), and late relapse groups (≥36 months since CR, n=24, 45%)7). 2) Transplantation All patients underwent allogeneic HSCT after having achieved second CR (Table 3). Median age at transplantation was 9.7 years (range, 0.9 to 20.7 years). Similar numbers of sibling donor and un- related donor transplantations were included (26 sibling, 49%; 27 unrelated HSCTs, 51%), but with regards to cell source, a far greater number of bone marrow transplantations (BMT) were included than granulocyte-colony stimulating factor mobilized peripheral blood stem cell transplantations (PBSCT) (42 BMTs, 79%; 11 PBSCTs, 21%). As the study covered HSCTs undertaken during a lengthy period in time, HLA typing for the study cohort was also done with varying Table 1. Patient Characteristics at Initial Diagnosis (n=53) Characteristic Value Median age (yr) 6.5 (0.3-15) Gender Male 42 (79) Female 11 (21) Median WBC count (/mm3) 11,500 (880-306,500) Risk group Low or standard 20 (38) High or very high 33 (62) Values are presented as median (range) or number (%). WBC, white blood cell Table 2. Patient Characteristics at First Relapse (n=53) Characteristic Value Age (yr) 9.2 (0.6-20.3) WBC count (/mm3) 5,080 (900-242,000) Time from CR to relapse (mo) 32.6 (1.1-118) Early (<12) 11 (21) Intermediate (≥12, <36) 18 (34) Late (≥36) 24 (45) Values are presented as median (range) or number (%). CR, complete remission; WBC, white blood cell.