The margin of internal risk volume on atrial septal and ventricular septal based on electrocardiograph gating 4DCT

Background The aim of this study was to quantify the margin of internal risk volume (IRV) on the atrial septum (AS) and ventricular septum (VS) based on electrocardiograph gating (ECG-gating) 4DCT. Methods Twenty patients were enrolled and received an ECG-gating 4DCT scan performed in breath-hold, and CT images were reconstructed at 5% intervals of the cardiac cycle for a total of 20 phases (0-95%). The contouring of the AS and VS were delineated in each phase, and the displacements and margin of the AS and VS were calculated. We fused the total of the AS and VS (0-95% phase), which were recorded as AS20 and VS20. The margins were applied to the AS and VS in every phase and revised according to the cover rate of AS20 and VS20. Results (I) The margins of the AS and VS according to displacements in the left-right, cranio-caudal, and antero-posterior direction were 3 mm, 3 mm, and 3 mm; and 3 mm, 3 mm, and 2 mm, respectively. (II) The volume of AS20 was (11.80±3.72) cm3, which was 2.9 times larger than the maximum volume of the AS. The volume of VS20 was (60.45±12.92) cm3, which was 1.6 times larger than the maximum volume of the VS. (III) The emendatory margins of the AS and VS in the left-right, cranio-caudal, and antero-posterior direction were 7 mm, 10 mm, and 7 mm; and 5 mm, 3 mm, and 4 mm, respectively. The emendatory margins were added to the AS and VS, and the coverage rates were (95.88±3.29)% and (95.24±2.54)%, respectively. Conclusions The margin of IRV on the AS and VS could cover the movement of AS and VS induced by heartbeat in the left-right, cranio-caudal, and antero-posterior direction respectively during thoracic radiotherapy.