Hydroxyurea versus busulphan for chronic myeloid leukaemia: an individual patient data meta-analysis of three randomized trials by Hehlmann, R, Heimpel, H, Hasford, J, Allan, N, Burnett, A, Shepherd, P, Oberg, G, Simonsson, B, Stromberg, U, Alison, R, Clarke, M, Evans, V, Godwin, J, MacKinnon, E, Richards, S, Trialist, C