Image quality evaluation of intra‐irradiation cone‐beam computed tomography acquired during one‐ and two‐arc prostate volumetric‐modulated arc therapy delivery: A phantom study

PURPOSE To evaluate (a) the effects of megavoltage (MV)-scatter on concurrent kilovoltage (kV) projections (PMVkV ) acquired during rotational delivery, and (b) the image quality of intra-irradiation cone-beam computed tomography (ii-CBCT) images acquired during prostate volumetric-modulated arc therapy (VMAT) delivery. METHODS Experiment (1): PMVkV s were acquired with various MV beam parameters using a cylindrical phantom: field size (FS), MV energy (6 or 15 MV), dose rate (DR), and gantry speed. The average pixel values were calculated in a region on each PMVkV which were extracted at eight equally spaced gantry angles. Experiment (2): 11 one-arc and seven two-arc 15 MV prostate VMAT plans were used along with a pelvis phantom. One plan was selected from each of arc plans and its MV energy was changed to 6 MV. After PMVkV s were acquired, projections consisting of MV-scatter only (PMVS ) were acquired with closing kV blades and subtracted from PMVkV (PMVScorr ). Projections by kV beams only were acquired (PkV ). The corresponding CBCT images were reconstructed (CBCTMVkV , CBCTMVScorr , and CBCTkV ). The root-mean-square errors (RMSEs) were calculated in prostate region and 3D gamma analysis was conducted, in which the CBCT-number was used instead of doses between ii-CBCT images and CBCTkV (30 HU/1 mm). RESULTS Experiment (1): The MV-scatters were dependent on the FSs, MV energies, and DRs. Experiment (2): The median RMSEs for CBCTMVScorr were decreased by 107.5 HU (1-arc) and 42.9 HU (2-arc) compared to those for CBCTMVkV . The median GPRs for CBCTMVScorr were 94.7% (1-arc) and 93.4% (2-arc), while those for CBCTMVkV were 61.1% and 79.9%, respectively. GPRs for 6 MV plans were smaller than those for 15 MV plans. CONCLUSIONS The number of MV-scatters increased with larger FSs and DRs, and smaller MV energy. The MV-scatters were corrected on the CBCTMVScorr regardless of the number of arcs.