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