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 (P (MVkV)) 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): P (MVkV)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 P (MVkV) 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 P (MVkV)s were acquired, projections consisting of MV‐scatter only (P (MVS)) were acquired with closing kV blades and subtracted from P (MVkV) (P (MVScorr)). Projections by kV beams only were acquired (P (kV)). The corresponding CBCT images were reconstructed (CBCT(MVkV), CBCT(MVScorr), and CBCT(kV)). 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 CBCT(kV) (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 CBCT(MVScorr) were decreased by 107.5 HU (1‐arc) and 42.9 HU (2‐arc) compared to those for CBCT(MVkV). The median GPRs for CBCT(MVScorr) were 94.7% (1‐arc) and 93.4% (2‐arc), while those for CBCT(MVkV) 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 CBCT(MVScorr) regardless of the number of arcs.