[1] McMaster Univ, Dept Pediat, Hamilton, ON L8N 3Z5, Canada
[2] Univ Plymouth, Peninsula Sch Med & Dent, Clin Neurol Res Grp, Plymouth PL6 8BX, Devon, England
[3] Mem Sloan Kettering Canc Ctr, Dept Surg, New York, NY 10065 USA
[4] McMaster Univ, Dept Clin Epidemiol & Biostat, Hamilton, ON L8N 3Z5, Canada