The radio-labeled anti-CD66 monoclonal antibody (with 111In for dosimetry and 90Y for therapy) will be administered in the T11 North room, UCLH, while the reduced intensity conditioning regimen and the allogeneic hematopoietic stem cell transplant will be performed in 2 centers, according to the age of the patient: A) patients aged < 13 years will be transplanted at the Bone Marrow Transplantation Department, Great Ormond Street Hospital (GOSH), and B) patients aged 13-18 years will be transplanted at the Bone Marrow Transplantation Department, University College London Hospitals (UCLH).

Inclusion criteria:

1. An underlying haematological malignancy including:

   1. isolated bone marrow relapse of AML after allogeneic haematopoietic stem cell transplantation;
   2. isolated bone marrow relapse of ALL after allogeneic haematopoietic stem cell transplantation;
   3. bone marrow relapse of JMML after allogeneic haematopoietic stem cell transplantation;
   4. refractory AML (5-20% blasts in BM) with/without expression of CD66 on blasts;
   5. refractory ALL (5-20% blasts in BM) with/without expression of CD66 on blasts;
   6. refractory AML (> 20% blasts in BM) with expression of CD66 on blasts;
   7. refractory ALL (> 20% blasts in BM) with expression of CD66 on blasts;

2. be ≥ 1 year old and ≤ 18 years old;

3. must not be eligible for therapy of higher curative potential. Where an alternative therapy has been shown to prolong survival in an analogous population, this should be offered to the patient prior to discussing this study;

4. have a Karnofsky Performance Status ≥ 50 or Lansky Performance Status ≥ 30;

5. provide signed, written informed consent from parent or guardian;

6. be able to comply with study procedures and follow-up examinations;

7. have normal cardiac function without specific treatment;

8. have adequate organ function (as indicated by Table 3, chapter 5), within 30 days prior to 111In infusion;

9. patients who have received any other chemotherapy within the previous 2 weeks and must have recovered from acute toxicity of all previous therapy prior to enrolment;

10. be negative for human-anti-murine antibodies (HAMA). Exclusion criteria

1) patients with CNS disease; 2) patients with BM cellularity < 10%; 3) patients with ≥ 20% blasts in BM and no expression of CD66 on blasts; 4) patients who are positive for human-anti-murine antibodies (HAMA); 5) patients with compromised organ function (as indicated by Table 3, chapter 5), within 30 days prior to 111In infusion; 6) patients with extensive chronic GvHD; 7) patients with an active, uncontrolled systemic infection considered opportunistic, life threatening, or clinically significant at the time of treatment; 8) patients who are pregnant or lactating; 9) patients with any other severe concurrent disease, which, in the judgment of the Investigator, would make the patient inappropriate for entry into this study.

Yttrium-90 labelled monoclonal antibody against CD66