1. Department of Radiation Physics; The University of Texas MD Anderson Cancer Center; Houston TX 77030 USA
2. IROC Houston QA Center; The University of Texas MD Anderson Cancer Center; Houston TX 77030 USA
3. Department of Radiation Oncology; St. Jude Children's Research Hospital; Memphis TN 38105 USA
4. Department of Biostatistics; The University of Texas MD Anderson Cancer Center; Houston TX 77030 USA
5. Department of Statistics, Computing, Applications; University of Florence; 50121 Florence Italy