Actual delivered dose calculation on intra-irradiation cone-beam computed tomography images: a phantom study

Cone-beam computed tomography (CBCT) images acquired during volumetric modulated arc therapy (VMAT; ii-CBCT) can be used to calculate actual delivered doses (ADDs). However, such ii-CBCT images are degraded by scattered megavoltage x-rays (MV-scatters). We aimed to evaluate the dose calculation accuracy of the MV-scatter uncorrected or corrected ii-CBCT images acquired during VMAT deliveries. For MV-scatter correction on concurrent kilovoltage projections (P _MVkV), projections consisting only of MV-scatters (P _MVS) were acquired under the same MV beam parameters and gantry angles and subtracted from P _MVkV (P _MVScorr). In addition, the projections by kilovoltage beams were acquired for reference (P _kV). The corresponding CBCT images were reconstructed using the Feldkamp-Davis-Kress algorithm (CBCT_MVkV, CBCT_MVScorr, and CBCT_kV as reference). A multi-energy phantom with rods of various relative electron densities (REDs) was used to generate a CBCT-number-to-RED conversion table. First, CBCT_kV was reconstructed. Then, the mean CBCT-numbers within each rod were extracted, and a reference table was generated. Concurrent kilovoltage imaging with various field sizes was also demonstrated, and CBCT_MVkV and CBCT_MVScorr were reconstructed. The extracted CBCT-numbers of each ii-CBCT image were converted into REDs using the reference table. Next, the absolute differences of RED between the ii-CBCT image and CBCT_kV were calculated. Ten VMAT plans using a 10 MV flattening-filter-free beam were used for concurrent imaging of an anthropomorphic torso phantom. Moreover, an iterative reconstruction algorithm (IRA) was used for CBCT_MVScorr. The plans were recalculated for the corresponding CBCT_MVkV, CBCT_MVScorr, CBCT_MVScorr+IRA, and CBCT_kV with the reference table. Finally, the doses were evaluated using 3D gamma analysis (1%/1 mm). The median difference ranges between CBCT_MVkV/CBCT_MVScorr and the reference values were -0.58 to -0.10/-0.03 to 0.00. The median gamma pass rates of the doses on CBCT_MVkV, CBCT_MVScorr, and CBCT_MVScorr+IRA to the rate on CBCT_kV were 70.4, 99.5, and 98.2%, respectively. CBCT_MVScorr were comparable with CBCT_kV for calculating the ADD from VMAT.