Measurement-based kilo-voltage beam characterization and dose quantification for radiotherapy image guidance

This study develops a measurement-based approach to characterize the on-board imaging (OBI) system and subsequently perform Monte-Carlo (MC) simulations of imaging dose, independent of manufacturer´s specifications. This novel development differentiates the current study from existing MC dosimetry investigations relying on proprietary manufacture´s data specifications and rids the difficulty in obtaining and employing such data. The proposed approach consists of an equivalent spectrum module incorporative of intrinsic filtration and an equivalent filter module characterizing the added bowtie filter. Validation studies were performed for a variety of beam energies with both full- and half-bowtie filters. The equivalent spectrum module was validated by evaluating bowtie independence in spectrum estimation. A comparison of quarter value layers (QVLs) derived from full-bowtie spectrum estimations with QVLs determined from half-bowtie measurements and vice versa yielded an estimation-measurement consistency with a discrepancy of less than 2% for all energies evaluated. The end-to-end system was validated by comparing dose distributions from MC simulations based on the measurement-based source/filter model with dose distributions from film measurements for both the full- and half- bowtie scenarios. For all energies evaluated, the average root-mean-square-error (RMSE) is 6.5% for the full-bowtie scenario and 7.05% for the half-bowtie scenario. These results indicate the effectiveness of the proposed method to model the OBI source and filtration components for MC simulations.