ESTRO 2021 Abstract Book

S419

ESTRO 2021

patients IV-contrast was used during the acquisition of CBCT A and C. Prior to treatment a test CBCT session was performed to determine the feasibility of the IV contrast delivery. During the test session a CBCT with (F) and without (E) IV contrast was acquired. The automatic matches for all CBCTs and for all patients were evaluated offline. Comparison of 2 different mask expansions and for the following CBCTs A,C,D,E and F was performed. Setup translations and rotations were extracted for all matches. The visibility of the cardiac outer contour, septum, atrium, ventricle and vasculature as surrogate to the PTV position was qualitatively evaluated. Results A total of 12 CBCTs were acquired during the treatment. Couch corrections were carried out after A, and C and in 2 patients also after B. The vector length derived by D for the 3 patients was 0.6mm, 0.9mm and 1.7mm. The use of IV contrast was well tolerated. In figure 1 is shown that the visibility of the LV summit region is better in the CBCT with contrast compared to the one without, providing an extra certainty to the RTTs for the match. The results of the offline comparison between the 2 masks in scans with and without IV contrast are given in figure 2. The outcome of the auto-match was most consistent when using the contrast-enhanced CBCT scan and as expected with the mask match with 2 cm expansion, which was also used during the treatment.