Room: Exhibit Hall
Purpose: To capture real time online CBCT during multiple breath hold VMAT delivery of SBRT lung patients using MV beam scatter and HU calibration correction.
Methods: The study is systematically divided in to following sequence 1) Development of in-house methodology to capture real time online CBCT during VMAT.2) Capturing of the same using CATPHAN 500 phantom in the presence of single arc 1 cm MLC sweeping 6MV -FFF VMAT plan with Small (5cm2) , Medium (10cm2) and large (20cm2) field sizes. 3) Creating CBCT calibration templates sequences for the above field sizes and compared with CBCT with no MV beam using TG-142 image parameters .4) Applying the above calibrated sequences in 3 clinical sites. 5) Finally capturing Real-time Online CBCT acquisition during multiple breath hold VMAT based DIBH Lung SBRT treatments.
Results: Successfully developed in-house methodology to capture real time online CBCT during VMAT. Calibration was performed with Small (5cm2) , Medium (10cm2) and large (20cm2) field size single arc 1 cm MLC sweeping 6MV-FFF VMAT plans. There were no difference in geometric distortion, Contrast , Slice thickness and pixel spacing between CBCT with and without MV scatter. But Uniformity Index, Spatial resolution , Modular Transfer function , Hounsfield Unit (HU) varied more than 10% in presence of 6MV FFF VMAT beam scatter. HU measurement analysis of CATPHAN shows a maximum variation of 22% for Teflon and 8% for air when compared with CBCT without MV scatter. After applying proper Calibration sequence for respective field sizes the variation of the above parameters have reduced significantly and were found to be within TG-142 tolerance limits.
Conclusion: Results from the study showed it is possible to capture real time online CBCT during multiple breath hold VMAT delivery of SBRT lung after applying appropriate MV beam scatter and HU calibration corrections.