[1] Univ Chicago, Comer Childrens Hosp, Sect Hematol Oncol & Stem Cell Transplantat, Chicago, IL 60637 USA
[2] Univ Chicago, Dept Publ Hlth Sci, Chicago, IL 60637 USA
[3] Univ Chicago, Chapin Hall, Chicago, IL 60637 USA
[4] St Jude Childrens Res Hosp, Dept Epidemiol & Canc Control, 332 N Lauderdale St, Memphis, TN 38105 USA