MO‐F‐110‐02: A New Model‐Based Dose Calculation Algorithm for Kilovoltage X‐ Rays

Abstract

Purpose: There is an increasing need to develop fast and accurate dose calculation algorithms for kilovoltage (kV) x‐rays. This study describes a new model‐based method and presents initial results of its calculation accuracy. Methods: The new approach calculates the radiation dose to patients by calculating the dose to water‐like media first. This is done by assuming all voxels in a patient CT data set are water, but with densities scaled according to a CT‐number‐to‐density calibration. The dose distribution to water‐like media with physical density considered is calculated by summation of pencil beam percent depth‐dose curves, considering the incident x‐ray fluence profiles and empirical x‐ray scatter factors. The beam modeling parameters were obtained by the Monte Carlo method. The final dose distribution calculated for the actual physical media is obtained using medium‐dependent correction (MDC) factors. The MDC factor at a voxel is expressed as a function of an intermediate quantity, the effective bone depth, which is calculated from patient CT data. The accuracy of this new approach was tested for calculating the dose resulting from a kV cone‐beam CT scan by comparing to Monte Carlo calculations. Results: The calculated dose using the new approach was shown to agree with Monte Carlo within 3% for the mean dose to bone, and within 1% for the mean dose to soft‐tissue. The dose to bone was on the order of 2–3 times the dose to soft‐tissue. The computation time of the new approach is significantly less than Monte Carlo. Conclusions: The new approach was shown to be very promising and has potential to extend current model‐based dose calculation algorithms to the kV energy range.