Long-term outcome and prognostic factors of unrelated cord ... - Nature

Bone Marrow Transplantation (2014) 49, 767–772 © 2014 Macmillan Publishers Limited All rights reserved 0268-3369/14 www.nature.com/bmt

ORIGINAL ARTICLE

Long-term outcome and prognostic factors of unrelated cord blood transplantation in children with haematological malignancies: a retrospective study using the Spanish Working Party for BMT in Children (GETMON) database C Díaz de Heredia1, M González2, A Verdeguer3, I Elorza1, A Rodriguez4, A Martinez5, JM Pérez6, I Badell7, ME Gonzalez8, T Olivé1, JM Fernández3, MS Maldonado9, MA Díaz2 and J Sánchez de Toledo1 Outcomes of unrelated cord blood transplants (UCBT) were assessed in 172 consecutive children, median age 5 years (range: 0.5–18), with haematological malignancies treated at nine Spanish hospitals between February 1996 and April 2009. Data were collected from the Spanish Working Party for Blood and Marrow Transplantation in Children (GETMON) database. ALL was diagnosed in 125 patients, AML in 43 and myelodysplastic syndrome in 4. Myeloid engraftment (ANC ⩾ 0.5 × 109/L) occurred in 87.2% at a median of 22 days and was associated with the total nucleated cell (TNC) dose infused and use of a TT-containing conditioning regimen. Cumulative incidence of relapse was 20% at 1 year post transplant and 29% at 3 years, being higher in patients with a diagnosis of ALL, very high risk disease and GVHD grades 0–1. Cumulative incidence of non-relapse mortality (NRM) was 19% at 100 days post transplant and 39% at 1 year. BU-FLU-TT-ATG-conditioned patients had lower NRM. Disease-free survival (DFS) was 40% at 2 years post transplant (for patients transplanted since 2006). On multivariate analysis, TNC dose infused, AML and BU-FLU-TT-ATG-conditioning regimen increased the probability of DFS. It is of paramount importance to select cord blood units with the highest cell dose. As the BU-FLU-TT-ATG-conditioning regimen was associated with better DFS owing to lower NRM, further prospective studies testing this regimen are warranted. Bone Marrow Transplantation (2014) 49, 767–772; doi:10.1038/bmt.2014.30; published online 10 March 2014

INTRODUCTION Cord blood has rapidly become a valuable alternative stem cell source for allogeneic haematopoietic SCT. Extensive research over the last 20 years has established the safety and efficacy of umbilical cord blood transplantation (CBT) in both children and adults with a variety of malignant and non-malignant diseases.1–5 Unrelated CBT (UCBT) overcomes some limitations of BMT including HLA-matching requirements, donor availability, complications of GVHD associated with HLA disparities or the likelihood of infection transmission.1 Numerous clinical studies have shown that UCBT in children with haematological malignancies is able to reconstitute haematopoiesis and achieve sustained engraftment in most cases, with a low incidence of GVHD and without an increase in relapse risk.6–14 Furthermore, patient-, disease- and transplant-related factors influencing outcome have been identified; among these, non-advanced stage of the disease at transplantation, high cell dose infused, ⩽ 2 HLA disparities, negative CMV serology before UCBT, non-use of methotrexate for GVHD prophylaxis and the use of fludarabine (FLU) in conditioning regimens have been associated with better

outcomes.2,3,12–14 The optimal conditioning regimen remains unclear. Non-TBI- containing regimens may reduce treatmentrelated mortality. Recently, regimens with BU or melphalan in combination with FLU, thiotepa (TT) and antithymocyte globulin (ATG) have been described for children and adults with haematological malignancies, with encouraging results.5,15 A retrospective study using the database of the Spanish Working Party for BMT in Children (GETMON, Grupo Español de Trasplante de Médula Ósea en Niños) was conducted to evaluate the long-term outcome and prognostic factors of UCBT in children with haematological malignancies.

PATIENTS AND METHODS Eligibility criteria From February 1996 to April 2009, 172 consecutive children with haematological malignancies underwent UCBT at nine different hospitals of the Spanish Working Party for BMT in Children (GETMON). Patients fulfilled the following eligibility criteria: (1) high-risk haematological malignancy requiring urgent haematopoietic cell transplantation; (2)

1 Servicio de Oncología y Hematología Pediátricas, Hospital Universitario Vall d'Hebron, Universitat Autónoma de Barcelona, Barcelona, Spain; 2Unidad de Trasplante, Hospital Niño Jesús, Universidad Autónoma de Madrid, Madrid, Spain; 3Unidad de Oncología y TPH Pediátrico, Hospital Universitario Politécnico La Fe, Universidad de Valencia, Valencia, Spain; 4Servicio de Hematología, Unidad de Hematología Infantil, Hospital Universitario Reina Sofía, Universidad de Córdoba, Córdoba, Spain; 5Unidad de Hematología y Oncología Infantil, Hospital la Paz, Universidad Autónoma de Madrid, Madrid, Spain; 6Servicio de Hematología, Hospital Virgen del Rocío, Universidad de Sevilla, Sevilla, Spain; 7 Unidad Pediátrica de Hematología, Oncología y TPH, Hospital de Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain; 8Servicio de Hematología, Hospital Universitario Regional Carlos Haya, Universidad de Málaga, Málaga, Spain and 9Unidad de Hematología Pediátrica, Servicio de Pediatría, Hospital Universitario Ramón y Cajal, Universidad de Alcalá, Madrid, Spain. Correspondence: Dr C Díaz de Heredia, Servicio de Oncología y Hematología Pediátricas, Hospital Universitario Vall d’Hebron, Passeig Vall d’Hebron 119-129, Barcelona 08035, Spain. E-mail: [email protected] Received 1 July 2013; revised 16 January 2014; accepted 16 January 2014; published online 10 March 2014

UCBT in children with haematological malignancies C Díaz de Heredia et al

768 absence of a suitable-related donor (HLA-identical or one-Ag mismatch); and (3) lack of a matched unrelated donor after an international registry search. Informed consent was provided by patients, parents or legal guardians for patient data to be entered in the GETMON database.

HLA typing and CBU selection CBU were required to be ⩾ 4/6 HLA-matched Ags. HLA typing was performed differently according to the period of transplantation. Before 2005, HLA typing was not uniformly performed by the different centres. Since 2005, HLA class I Ags (A and B) have been determined considering the Ag level using serologic or low-resolution DNA typing, and class II Ags (DRB1) considering the allele level using high-resolution DNA typing. All cord blood units were evaluated for HLA-A, HLA-B and HLA-DRB1 typing. A minimum of total nucleated cells (TNC) ⩾ 2 × 107/kg recipient body weight was required until 2005, and TNC ⩾ 2.7 × 107/kg and CD34+ cells ⩾ 1 × 105/kg since 2005.

Conditioning regimen and GVHD prophylaxis All patients received myeloablative conditioning regimens. The main regimens used were: (a) TBI-containing regimens: TBI 12–14 Gy +/ − CY 120 mg/kg total dose +/ − etoposide (VP-16) 30–60 mg/kg; (b) TT 10 mg/kg total dose+CY 120 mg/kg total dose+BU (oral BU for 3 days at 12 mg/kg total dose); (c) FLU 150 mg/m2 total dose+TT 10 mg/kg total dose+i.v. BU in a single dose for 3 days. In all cases in which i.v. BU was given, the dose was adjusted in patients weighing o34 kg to achieve a total dose of 12, 14.4, 13.2 and 11.4 mg/kg in patients weighing o9 kg, 9–16 kg, 17–23 kg and 24–34 kg, respectively. Rabbit ATG (Thymoglobulin, Sangstat/Genzyme, Cambridge, MA, USA) at a dose of 6–10 mg/kg was given to promote engraftment and for GVHD prophylaxis in the majority of patients. GVHD prophylaxis consisted of a combination of two drugs: CYA with either prednisone or mycophenolate mofetil (MMF).

Definitions Myeloid engraftment was defined as an ANC ⩾ 0.5 × 109/L for three or more consecutive days. Platelet engraftment was defined as a platelet count ⩾ 20 × 109/L, without transfusion support, for seven consecutive days. Acute and chronic GVHD were defined and graded according to the standard criteria.16,17. Disease status at the time of transplantation was classified as (a) ‘high risk’, which included high-risk ALL in first CR (CR1), ALL patients in second CR (CR2), high-risk AML in CR1 and patients with myelodysplastic syndrome; and (b) ‘very high risk’, which included ALL patients in ⩾ third complete remission (CR3), ALL patients not in CR, AML patients in CR2, AML patients not in CR and patients who received a second transplant. Non-relapse mortality (NRM) was defined as any cause of death other than disease.

RESULTS Patient characteristics The main characteristics of the 172 patients included in the study (94 boys, 78 girls, median age: 5 years (range: 0.5–18) and median weight: 21 kg (range: 6–89) ) are shown in Table 1. ALL was diagnosed in 72.7% of patients, AML in 25% and myelodysplastic syndrome in 2.3%. The disease status was ‘high risk’ in 96 (55.8%) and ‘very high risk’ in 76 (44.2%). Thirty-seven patients had received a previous transplant. Patients were transplanted at nine different centres, with the number of patients transplanted per centre being 51, 41, 25, 17, 12, 11, 8, 5 and 2, respectively. Statistical analysis stratified by hospital showed no centre-effect influencing outcomes. Transplant characteristics Thirty patients underwent UCBT before 2000. A myeloablative TBIbased regimen was used in 33.7% of patients, TT-containing regimens in 59.3% and a BU-FLU-TT-ATG-conditioning regimen in 30.2%. ATG was used as part of the conditioning regimen in 87.2%. A combination of CyA and prednisone was used as GVHD prophylaxis in 131 patients, CyA and MMF in 26 and other drug combinations in 8.7% (Table 2). Characteristics of the cord blood unit Twenty-six (15.1%) patients received a 6/6 umbilical cord blood unit, 86 (50%) a 5/6, 52 (30.2%) a 4/6 and 8(4.6%) a 3/6. Median number of nucleated cells infused x 107/kg was 4.9 (IQR 3.1–7) and median number of CD34+ cells x 105/kg was 3.0 (IQR 2.4–3.7). Engraftment One hundred and fifty patients (87.2%) achieved myeloid engraftment (ANC ⩾ 0.5 × 109/L) at a median of 22 days. The myeloid engraftment rate increased over time (Figure 1) and, after 2006, 94% of patients engrafted. On multivariate Cox regression analysis, the only variables related to myeloid engraftment were TNC dose infused (HR = 1.60, 95% CI: 1.03–2.49; P = 0.033) and the use of a TT-containing conditioning regimen (HR = 1.45, 95% CI: 0.97–2.15; P = 0.066) (Table 3). Table 1.

Statistical methods Data were expressed as frequencies and percentages for categorical variables and as median and range (minimum–maximum) or interquartile range (IQR) (25th–75th percentile) for quantitative variables. The probabilities of engraftment (myeloid and platelet), OS and disease-free survival (DFS) were analysed using Kaplan–Meier methods. The log-rank test and Cox proportional hazard models were constructed to test differences in cumulative probabilities. Variables included were age, weight, TNC, CD34+ cell dose, HLA compatibility, use of FLU, TT or TBI as conditioning, use of MMF in GVHD prophylaxis, baseline diagnosis, disease status at transplantation and acute and chronic GVHD development. For multivariate models, one variable at a time was added chosen by the greatest difference between the likelihood of the model with the variable and the previous one. We stopped when no likelihood ratio test was statistically significant using a conservative approach of P = 0.1. Data were expressed as hazard ratio (HR) and 95% confidence interval (CI). Time to relapse or death was studied in a competitive risk framework. Cumulative incidence of relapse and NRM were calculated. A competing risk regression model for the subhazard function of relapse or death was constructed in the same way as the previous Cox model. Collected data were analysed in July 2011 using the Stata software package (version 11.2; StataCorp L.P., College Station, TX, USA) on the Windows platform. Bone Marrow Transplantation (2014) 767 – 772

Patient characteristics

No. of patients Sex, no. (%) Male Female

172 94 (54.7) 78 (45.3)

Age, years Median Rangea 0–2 years, no. (%) 3–10 years, no. (%) >10 years, no. (%) Weight, kg Median Rangea

5 0.5–18 31 (18) 109 (63.4) 32 (18.6) 6–89 21 6–89

Diagnosis, no. (%) ALL AML Myelodysplastic syndrome

125 (72.7) 43 (25) 4 (2.3)

Disease status at transplant, no. (%) High risk Very high risk Previous transplant, no. (%) CMV serology, no. (%)

96 (55.8) 76 (44.2) 37 (21.5) 109 (63.4)

a

Minimum–maximum.

© 2014 Macmillan Publishers Limited


769 Table 2.

Transplant characteristics

No. of patients Year of transplant, no. (%) 2000 or before 2001–2005 2006 or later

172 30 (17.4) 72 (41.8) 70 (40.7)

HLA compatibility, no. (%) 6/6 5/6 4/6 3/6

26 86 52 8

(15.1) (50) (30.2) (4.6)

Myeloablative conditioning regimen, no. (%) TBI-containing regimen TT-containing regimen BU-FLU-TT-ATG ATG, no. (%)

58 102 52 150

(33.7) (59.3) (30.2) (87.2)

GVHD prophylaxis, no. (%) CyA+prednisone CyA+MMF Other TNC infused x 107/kg, median (range)a CD34a, cells infused x 105/kg, median (range)a

131 26 15 4.9 3.0

(76.2) (15.1) (8.7) (0.8–44.0) (0.1–31.0)

Abbreviations: ATG = antithymocyte globulin; FLU = fludarabine; MMF = mycophenolate mofetil; TNC = total nucleated cells; TT = thiotepa. a Minimum–maximum.

1.00

transplantation raised the risk of NRM, whereas BU-FLU-TT-ATGconditioned patients had a lower risk. HLA disparity did not influence NRM. Relapse Forty-seven patients relapsed (27.3%). The cumulative incidence of relapse was 20% at 1 year post transplant and 29% at 3 years. The cumulative incidence of relapse in a competing risk framework is shown in Figure 2. The effects of significant variables observed in the multivariate competing risk regression model for relapse are shown in Table 4. AML and acute GVHD stages II-IV were associated with a lower risk of relapse, whereas BU-FLU-TTATG-conditioned patients and patients classified as having very high disease risk at transplantation had a higher risk. Complications and causes of death In total, 38% of patients developed acute GVHD II-IV and 21% grades III-IV. Mild chronic GVHD was observed in 18%. No extensive chronic GVHD was observed. Infection and viral reactivations were frequent: bacterial infections (73% of cases) followed by viral reactivations (CMV 30%, adenovirus 18%) and fungal infections (11%). Other complications included veno-occlusive liver disease in 4% and late-onset haemorrhagic cystitis in 20%. Post-transplant lymphoproliferative syndrome associated with EBV was diagnosed in three patients. At the time of analysis, 101 patients (58.7%) had died. Causes of death included disease progression in 33 patients and UCBTrelated complications in 68 (infection 39%, multiorgan failure 25.5%, GVHD 17%, graft failure 10% and complications of a further stem cell transplant 8.5%).

0.75

0.50

0.25

0.00 0

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Days after transplantation Prior to 2000

2001–2005

2006+ P value = 0.0174

Figure 1. Cumulative probability of myeloid engraftment according to period of transplantation.

One hundred and thirty-four patients (78%) achieved platelet engraftment (platelet count ⩾ 20 × 109/L) at a median of 49 days. On multivariate Cox regression analysis, only TT-containing conditioning regimens were independent predictors of platelet engraftment (HR = 2.75, 95% CI: 1.37–5.51, P = 0.004) (Table 3). HLA disparity (⩽ 2 disparities) did not influence myeloid and platelet engraftment. Non-relapse mortality Median follow-up of surviving patients was 36.8 months (range: 2–172.7) (IQR: 19.9–74.3). The cumulative incidence of NRM was 19% at 100 days post transplant and 39% at 1 year. The cumulative incidence of NRM in a competing risk framework is shown in Figure 2. The effects of significant variables observed in the multivariate competing risk regression model for NRM are shown in Table 4. Older age at © 2014 Macmillan Publishers Limited

DFS and OS DFS was 40% at 2 years post transplant (for patients transplanted after 2006). A trend towards better DFS was observed after 2006, although differences were not statistically significant. DFS was significantly better for AML patients compared with those with ALL (P = 0.007). When ALL and AML patients were taken together, no significant differences in DFS were found between BU-FLU-TTATG-conditioned patients and those with other conditioning regimens, although a trend towards better DFS was observed in BU-FLU-TT-ATG-conditioned patients (Figure 2). On multivariate analysis, TNC dose infused (HR = 0.47, 95% CI: 0.29–0.74; P = 0.001), AML (HR = 0.40, 95% CI: 0.23–0.70; P = 0.001) and conditioning with BU-FLU-TT-ATG (HR = 0.60, 95% CI: 0.37–0.98; P = 0.04) raised the probability of DFS. HLA disparity ( ⩽ 2 disparities) did not influence DFS (Table 4) OS was 37.7% (95% CI: 30.22–45.11) at 2 years and 30.8% (95% CI: 23.26–38.63) at 5 years post transplant. OS survival at 5 years post transplant was 54.1% (95% CI: 36.68–68.97) for AML and 23.3% (95% CI: 15.45–32.08) for ALL (P = 0.013) patients. DISCUSSION This study, based on data retrospectively collected from the nationwide database, confirms that UCBT is an alternative in children with haematopoietic malignancies for which haematopoietic SCT is indicated. Different studies have shown UCBT in children and adults to be associated with consistently equivalent survival outcomes compared with unrelated donor BMT despite a greater donor-recipient HLA disparity with UCBT.18–22 However, whether engraftment is optimum and which conditioning regimen is the best remain unclear. The probability of engraftment at day 100 was 88% (median 22 days). In a single Spanish institutional study of 42 paediatric patients with high-risk leukaemia treated with UCBT, the probability of myeloid Bone Marrow Transplantation (2014) 767 – 772


770 Table 3.

Univariate and multivariate analyses to identify variables that influenced myeloid and platelet engraftment Myeloid engraftment (ANC ⩾ 0.5 × 109/L)

Variables

Univariate analysis

HR (95% CI) Age Weight AML vs ALL High vs very high disease risk TNC cells infused>3.4 × 107/kg CD34+ cells infused>1.5 × 105/kg HLA disparity 3/6 or 4/6 vs 5/6 or 6/6 TBI-containing conditioning regimen BU-FLU-TT-ATG conditioning regimen TT- containing conditioning regimen ATG GVHD prophylaxis with MMF Period of transplantation 2001–2005 Period of transplantation ⩾ 2006 Acute GVHD stage II/IV vs 0/I

0.98 1.00 1.15 0.91

Platelet engraftment (platelet count ⩾ 20 × 109/L)

Multivariate analysis

P-value

(0.94–1.02) (0.99–1.01) (0.79–1.66) (0.66–1.26)

0.277 0.672 0.474 0.583

1.64 (1.13–2.37) 1.58 (1.14–2.19) 0.76 (0.53–1.07)

0.010 0.006 0.118

0.80 (0.57–1.12)

Univariate analysis

P-value

HR (95% CI)

HR (95% CI 0.98 1.00 1.08 1.07

Multivariate analysis P-value

(0.94–1.02) (0.99–1.01) (0.71–1.66) (0.74–1.55)

0.269 0.680 0.720 0.719

0.64 (0.34–1.19) 1.28 (0.87–1.88) 0.81 (0.54–1.22)

0.160 0.210 0.316

0.189

0.79 (0.53–1.18)

0.252

1.43 (1.01–2.02)

0.045

1.58 (1.08–2.31)

0.019

1.51 (1.09–2.10)

0.014

1.56 (1.07–2.28)

0.020

1.10 (0.67–1.80) 1.07 (0.67–1.69)

0.707 0.788

1.59 (0.87–2.89) 1.18 (0.71–1.96)

0.131 0.517

1.48 (0.93–2.36)

0.024

1.28 (0.75–2.19)

0.337

1.91 (1.20–3.06) 1.39 (0.99–1.96)

0.057

1.48 (0.87–2.52) 1.04 (0.71–1.53)

0.833

1.60 (1.03–2.49)

0.033

1.45 (0.97–2.15)

0.066

HR (95% CI)

P-value

2.75 (1.37–5.51)

0.004

Abbreviations: ATG = antithymocyte globulin; CI = confidence interval; FLU = fludarabine; HR = hazards ratio; MMF = mycophenolate mofetil; TT = thiotepa.

1

1.00

0.8

0.75

0.6 0.50 0.4 0.25

0.2

P = 0.0921

P = 0.046 0

0.00 0

1

2

3

4

5

6

7 8 Years

9 10 11 12 13 14 15

0

1

2

3

4

5

6

7 8 9 10 11 12 13 14 15 Years

1 0.8

BU-FLU-TT-ATG

0.6

Other regimens

0.4 0.2 P = 0.040 0 0

1

2

3

4

5

6

7 8 9 10 11 12 13 14 15 Years

Figure 2. Cumulative incidence of relapse (a) and non-relapse mortality (b), in a competing risk framework, and disease-free survival (c) according to conditioning regimen.

engraftment was 95 ±5% and median time to neutrophil count >500/μL was 20 days.11 In a group of 18 children and 70 adults with haematological malignancies reported by different Spanish institutions, the overall cumulative incidence of myeloid engraftment was 94% at a median of 19 days.5 The cell dose infused in Bone Marrow Transplantation (2014) 767 – 772

the present study was the main variable that influenced engraftment, which is in line with other published studies.2,3,5,6 Other variables have been associated with better engraftment in several studies: avoidance of cord blood units with rejection direction mismatches and presence of anti-HLA antibodies against © 2014 Macmillan Publishers Limited


771 Table 4. Risk factors in the multivariate competing risk regression analysis for relapse and non-relapse mortality (NRM) and disease-free survival (DFS) Risk factors

Relapse Subhazard ratio (95% CI)

Age TNC infused AML vs ALL Very high vs high disease risk BU-FLU-TT-ATG conditioning Acute GVHD stage II/IV vs 0/I

0.90 (0.82–0.99) 0.30 5.55 2.84 0.34

(0.11–0.78) (2.50–11.10) (1.44–5.63) (0.15–0.78)

NRM P-value

DFS

Subhazard ratio (95% CI)

P-value

0.033

1.11 (1.04–1.18)

o 0.001

0.014 o0.001 0.003 0.01

0.46 (0.20–1) 0.56 (0.29–0.98) 0.23 (0.10–0.52)

0.057 0.042 o 0.001

Subhazard ratio (95% CI)

P-value

0.47 (0.29–0.74) 0.40 (0.23–0.76)

0.001 0.001

0.60 (0.37–0.98)

0.04

Abbreviations: ATG = antithymocyte globulin; CI = confidence interval; FLU = fludarabine; HR = hazards ratio; MMF = mycophenolate mofetil; TT = thiotepa.

Ag cells of the cord blood unit; however, these factors were not taken into account in this study.23,24 Moreover, different procedures have been studied to overcome the problem of insufficient cell number for transplantation and shorten the neutropenic period, mostly in adults, such as ex vivo expansion of cord blood cells, the use of double cords, the co-infusion of mobilised haematopoietic stem cells from a third party donor and the cord blood cell direct intrabone injection.25–29 The use of a TT-containing conditioning regimen also had a favourable effect on myeloid and platelet engraftment in this study. TT is a welltolerated agent that, in combination with other drugs, has been associated with a high probability of engraftment in UCBT and in unrelated transplants for non-malignant diseases.5,30–34 However, these observations should be confirmed in future studies.35 Furthermore, the role of TT in the treatment of some haematological malignancies is under investigation. HLA disparity (⩽ 2 disparities) did not influence myeloid or platelet engraftment in this study. It has been suggested that cell dose and number of HLA mismatches interact mutually on engraftment and outcome. Thus, a higher cell dose in the graft could partially overcome the negative impact of HLA disparity.36 Consequently, cord blood grafts with no more than two HLA disparities have been recommended.37 NRM was the principal obstacle to successful outcome in this study. The cumulative incidence of NRM was 19% at 100 days post transplant and 39% at 1 year. Several series with predominantly paediatric patients receiving UCBT grafts have reported 100-day and 1-year NRM of 27–39% and 30–40%, respectively,4,19,20 which concur with our findings. However, the NRM of UCBT in our series and others was higher than that reported for MUD transplants.22 Patients conditioned with the i.v. BU-FLU-TT-ATG regimen had a lower risk of NRM. FLU and TT are well-tolerated agents with a low toxicity profile. Low 100-day NRM with this regimen has been reported in adults, however, NRM occurred later, most between 3 and 18 months post transplant.5 Infection was the main cause of death in our series as previously reported by others,5 followed by multiorgan failure and acute GVHD. Furthermore, different strategies have been attempted to lower NRM and proved to have a role in other studies: omitting ATG from the conditioning regimen with the aim of achieving early immune reconstitution and the use of treosulfan in children with high risk for regimenrelated toxicity.38,39 However, these factors could not be studied in our patients as the majority (87%) received ATG and non-TBIconditioned patients received oral or i.v. BU instead of treosulfan. Transplants with 6/6 compatibility had lower NRM and higher DFS and OS than transplants with 5/6 or 4/6 compatibility; however, the differences were not statiscally significant, unlike in other studies that reported better results in transplants with higher compatibility.21,34,40 Other factors have been implicated in higher NRM such as NIMA-mismatched units, additional HLA-C disparity in patients matched at HLA-A, B and © 2014 Macmillan Publishers Limited

DRB1 levels or in those with a single-locus mismatch.41,42 However, these factors were not studied in our series. The cumulative incidence of relapse was 29% at 3 years post transplant. Young age, ALL, very high disease risk and GVHD grades 0-I were factors for high risk of relapse. The relapse rate was higher in patients with very high disease risk in whom, consequently, UCBT was not a good option and should not be indicated. Moreover, a drawback of UCBT is that donor lymphocytes cannot be infused post transplant. Favourable factors for DFS were TNC cells infused >3.4 ×107/kg, AML diagnosis and the use of the BU-FLU-TT-ATG-conditioning regimen. If this combination replaced TBI in conditioning treatment, early and late TBI-related effects in children would be avoided. In conclusion, our results provide further evidence of the role of UCBT treatment in children with haematological malignancies. The cord blood unit chosen for transplantation should be based on cell dose. It is paramount to select units with the highest cell dose, as this was one of the main factors associated with engraftment and DFS. TT-containing regimens also favoured myeloid and platelet engraftment. The cumulative incidence of relapse and NRM in UCBT was higher compared with related and unrelated well-matched donor transplants. However, as the BUFLU-TT-ATG regimen was associated with better DFS owing to lower NRM, prospective studies testing this regimen are warranted.

CONFLICT OF INTEREST The authors declare no conflict of interest.

ACKNOWLEDGEMENTS We thank Santiago Perez Hoyos for statistical analyses, Marta Pulido, MD, for manuscript editing and editorial assistance and Christine O’Hara for the English revision.

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