TU‐C‐BRA‐04: A New Approach to Model‐Based Dose Calculations for Kilovoltage Beams

Abstract

Purpose: The additional radiation dose delivered to radiotherapy patients from repeated image guidance procedures is a growing concern. This study presents a new approach to model‐based dose calculation that overcomes the deficiencies of currently available model‐based algorithms in the kilovoltage regime. Method and Materials: Monte Carlo techniques were used to calculate the dose to patients resulting from kV‐CBCT scans for multiple scan sites including the head‐and‐neck, chest, and pelvis. Both dose‐to‐medium and dose‐to‐water were calculated using realistic simulated kV‐CBCT beams to find a correlation between the effective thickness of bone (dEB) traversed by an x‐ray beam to reach a voxel and the correction factor needed to correct the model‐based dose calculation. A program was written to facilitate the calculation of dEB in which the incident beam profiles and isocenter relative to a scanned patient were taken into account. Results: A strong correlation between dose correction factor and dEB was observed. A unique correction factor calibration curve as a function of dEB was derived and used to predict patient dose. Compared to gold standard Monte Carlo calculated dose distributions, the resulting mean dose error for each patient was less than 3% for bone and 2% for soft‐tissue. This was in contrast to model‐based calculations, which resulted in mean errors of up to −103% for bone and 8% for soft‐tissue. Conclusion: The derived correction factor as a function of effective bone thickness is capable of accurately calculating the dose to bone and soft‐tissue by making use of patient CT data and the incident kV‐CBCT beam information. The accuracy of this new approach is patient and scan site independent because it takes into account the spectral changes of the x‐ray beam. With the addition of a commissioned kV‐CBCT beam this new approach can facilitate inclusion of imaging dose in radiotherapy treatment planning.