Assessment of target volume doses in radiotherapy based on the standard and measured calibration curves

In radiation treatments, estimation of the dose distribution in the target volume is one of the main components of the treatment planning procedure. To estimate the dose distribution, the information of electron densities is necessary. The standard curves determined by computed tomography (CT) scanner that may be different from that of other oncology centers. In this study, the changes of dose calculation due to the different calibration curves (HU-ρel) were investigated.

Dose values were calculated based on the standard calibration curve that was predefined for the treatment planning system (TPS). The calibration curve was also extracted from the CT images of the phantom, and dose values were calculated based on this curve. The percentage errors of the calculated values were determined.

The statistical analyses of the mean differences were performed using the Wilcoxon rank-sum test for both of the calibration curves.

The results show no significant difference for both of the measured and standard calibration curves (HU-ρel) in 6, 15, and 18 MeV energies. In Wilcoxon ranked sum nonparametric test for independent samples with P<0.05, the equality of monitor units for both of the curves to transfer 200 cGy doses to reference points was resulted. The percentage errors of the calculated values were lower than 2% and 1.5% in 6 and 15 MeV, respectively.

From the results, it could be concluded that the standard calibration curve could be used in TPS dose calculation accurately.