Investigation of tube voltage dependence on CT number and its effect on dose calculation algorithms using thorax phantom in Monaco treatment planning system for external beam radiation therapy.

INTRODUCTION

The accuracy of dose calculation algorithms depends on the electron density and computed tomography (CT) number of medium scanned. Our study aimed to verify the impact of different CT scanning protocols on Hounsfield unit (HU) and effect on dose calculation algorithms.

MATERIALS AND METHODS

CIRS thorax phantom with different density material plugs was scanned at varying tube voltages from CT scanner and HU values were measured in treatment planning system (TPS). Calibration curves of electron density at different tube voltages were plotted and used for dose calculation with different calculation algorithms at varying high energy megavoltage photon energies.

RESULTS

Insignificant difference is obtained in electron density curves plotted at different tube voltages. The mean variation in HU values was found at different tube voltages for bone, lung, and water are 896.75 (standard deviation [SD] 122.88), -799.25 (SD 5.74), and -17.5 (SD 0.57), respectively. The estimated values for change in HU values were 0.089, 0.258, and 0.121 for bone, lung, and water, respectively. Pencil beam (PB) convolution and collapsed cone algorithms show no significant dose difference, i.e., <1% variation and Monte Carlo (MC) shows maximum dose difference up to 1.4%.

CONCLUSION

Third-generation algorithms such as MC shows dependence on varying tube voltages in dose calculation. Calibration curves plotted at different kVp in TPS advised to be chosen wisely to avoid any dosimetric errors in different medium.