Patients with acute lymphoblastic leukaemia (ALL) and an intrachromosomal amplification of chromosome 21 (iAMP21) comprise a novel and distinct biological subgroup. We prospectively screened 1,630 (84%) patients treated on the UK MRC ALL97 protocol for iAMP21 and herein present demographic, clinical and survival data on the 28 (2%) children found to harbour this abnormality. They had a common or pre-B ALL immunophenotype, were significantly older (median 9 versus 5 years) and had a lower white cell count (median 3.9 versus 12.4) compared to children without this abnormality. Notably, iAMP21 patients had a significantly inferior event free and overall survival at 5 years compared with other patients: 29% (95% CI 13%-48%) versus 78% (76%-80%) and 71% (51%-84%) versus 87% (85%-88%), respectively. As a result of this three-fold increase in relapse risk, newly diagnosed iAMP21 patients recruited to the current UK MRC ALL2003 trial are being treated on the high risk arm and are considered for bone marrow transplant in first remission
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