Poster - Thur Eve - 23: Statistical analysis and verification of the percentage depth dose calculation based on the tissue maximum ratio in external beam radiotherapy.

The aim of this study is to perform a statistical analysis to verify the calculated percentage depth dose (PDD) based on the tissue maximum ratio (TMR) with the PDD measurements taken in water. 6 and 15 MV photon beams produced by a Varian linear 2100 C/D accelerator were used. PDDs and TMRs were measured at various depths and field sizes (5 × 5, 10 × 10, 15 × 15, 20 × 20 and 30 × 30 cm ) using a PTW 31006 ionization chamber and a scanning water tank. By comparing the calculated and measured PDD results, it was seen that for larger field sizes the deviation between the calculated and measured PDD was smaller. Deviations between the calculated and measured results were found to be higher in the build-up regions of the 6 and 15 MV photon beams. For the statistical analyses, t-tests were performed using the measured and calculated PDDs for each field size but showed insignificant deviations for the 6 and 15 MV photon beams. The mean t-test values are 0.952 and 0.970 for the 6 and 15 MV photon beams, respectively. The difference between the calculated and measured PDD is within the acceptable range according to the ICRU reports (ICRU Report No. 24, 1976). We conclude that accurate calculation of the PDD using the measured TMR data is possible, which is useful as the PDD cannot be measured directly.