[1] Univ Toronto, Toronto, ON, Canada
[2] Princess Margaret Hosp, Dept Radiat Oncol, Toronto, ON M4X 1K9, Canada
[3] Univ Western Ontario, London, ON, Canada
[4] London Reg Canc Ctr, Dept Radiat Oncol, London, ON N6A 4L6, Canada
[5] London Reg Canc Ctr, Dept Epidemiol & Biostat, London, ON N6A 4L6, Canada