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 ), projections consisting only of MV-scatters (P ) were acquired under the same MV beam parameters and gantry angles and subtracted from P (P ). In addition, the projections by kilovoltage beams were acquired for reference (P ). The corresponding CBCT images were reconstructed using the Feldkamp-Davis-Kress algorithm (CBCT, CBCT, and CBCT 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 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 and CBCT 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 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. The plans were recalculated for the corresponding CBCT, CBCT, CBCT, and CBCT with the reference table. Finally, the doses were evaluated using 3D gamma analysis (1%/1 mm). The median difference ranges between CBCT/CBCT 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, CBCT, and CBCT to the rate on CBCT were 70.4, 99.5, and 98.2%, respectively. CBCT were comparable with CBCT for calculating the ADD from VMAT.