Speaker
Description
[Purpose] kV-CBCT imaging during MV beam irradiation technique has been developed on C-arm linac. The purpose of this study was to quantify the performance of the intra-irradiation imaging technique under a phase-gated condition.
[Method] TrueBeam Developer mode was used to perform the intra-irradiation imaging. Catphan504 phantom was used. To demonstrate respiratory movement, the phantom was placed on a moving platform. The phantom was moved along the gun-target direction with the sinusoidal wave of amplitude of 40 mm, and 15 BPM. The intra-irradiation imagings during MV beam delivery were performed. First, intra-irradiation projections were acquired under a gating window of 50% (30-70% phase) with various MV beam energies: 6 MV, 10 MV, 15 MV, 6 MVFFF, and 10 MVFFF. One half- (181 – 21 deg clockwise, small FOV) or one full-rotation (181-179 deg clockwise, large FOV) beams with dose rate and field size of 400 MU/min and 15×15 cm2 were used per MV beam energy. kV imaging parameters were 125 kVp and 600/1080 mAs (small/large FOV). Second, MV-scatter-only projections were acquired with kV collimators closed to correct the MV-scatter signal in the intra-irradiation projections. Third, the acquired intra-irradiation projections were corrected by subtracting the MV-scatter-only projections. Gated and static intra-irradiation CBCT images were reconstructed from corresponding MV-scatter-corrected intra-irradiation projections. The FDK algorithm was used for the image reconstruction. In addition, Gated and static standard CBCT images were reconstructed from standard kV projections with the FDK algorithm. Root-mean-square error (RMSEs) of CT-numbers of inserted rods in the phantom on each intra-irradiation CBCT image were calculated using the CT-numbers on the standard CBCT image as reference. Noise was also calculated as the standard deviation of CT-numbers on the uniform module in the phantom.
[Result] RMSEs of intra-irradiation CBCT images under static condition were approximately less than 30 HU; however, those under gated condition were relatively large (50-230 HU). Larger FOV showed larger RMSEs. RMSEs of the intra-irradiation CBCT images acquired during 6XFFF beam delivery were large compared to the images acquired during other MV beam energies. Noises of intra-irradiation CBCT images under static condition (13-17/21-23 HU for small/large FOV, respectively) were comparable to those of standard CBCT images (12.3/20.8 HU). Noises of intra-irradiation CBCT images under gated condition were 15-23/23-35 HU while those of standard CBCT images were 8.8/27.2 HU. For small FOV, noises of intra-irradiation CBCT images were larger.
[Conclusion] Performance of the intra-irradiation imaging technique during a phase-gated condition was quantified.
