Room: AAPM ePoster Library
CBCT images acquired during volumetric-modulated arc therapy (VMAT) delivery (intra-irradiation CBCT images) can be utilized for calculating dose-of-the-day. However, the image quality was degraded by MV scatter X-rays (MV-scatters). The purpose was to evaluate VMAT dose calculation accuracy on the intra-irradiation CBCT images with and without MV-scatter correction.
Ten 1-full-arc VMAT plans with 10 MV FFF beam were utilized. Each plan was initially calculated on patients’ planning CT images by AcurosXB (ver 15.6) algorithm on Eclipse. Then, the plans were exported as DICOM-RT plan, and then converted to XML file with adding concurrent kV imaging (125 kVp and 1.2 mAs) control points. An anthropomorphic torso phantom was imaged during each VMAT delivery via TrueBeam Developer Mode. To correct MV-scatter on the concurrent kilovoltage (kV) projections (P_MVkV), the projections consist of MV-scatter only (P_MVS) were acquired under the same MV beam parameters and gantry angles, and then subtracted from the P_MVkV (P_MVScorr). The projections by kV beam only were also acquired for the reference. Corresponding CBCT images were reconstructed by Feldkamp-Davis-Kress algorithm via iTools software (CBCT_MVkV, CBCT_MVScorr, and CBCT_kV). Iterative reconstruction algorithm (IRA) was also used for CBCT_MVScorr. Those images were imported to Eclipse and the doses of each plan were recalculated on the images. 3D gamma analysis (1%/1 mm/threshold 5%) were performed on the doses of intra-irradiation CBCT images to that of CBCT_kV as reference.
Recalculated doses on CBCT_MVkV were elevated by approximately 6% due to cupping artifact from MV-scatters. Median gamma pass rates of the doses on CBCT_MVkV, CBCT_MVScorr, and CBCT_MVScorr+IRA to that on CBCT_kV were 70.4% (range: 46.0-82.3%), 99.5% (range: 99.3-99.9%), and 98.2% (range: 96.0-98.8%), respectively.
MV-scatters on CBCT_MVkV was corrected by subtracting P_MVS from P_MVkV before CBCT reconstruction. As results, the dose calculation accuracy on CBCT_MVScorrs was comparable with CBCT_kV.
Funding Support, Disclosures, and Conflict of Interest: Varian Research Grant, JSPS KAKENHI (Grant no. 19K17265)