ESTRO 36 Abstract Book

S203 ESTRO 36 _______________________________________________________________________________________________

homogeneity.

calculate the peak-to-peak amplitude of the respiration- induced marker motion and the marker motion trajectory. The mean and standard deviation (SD) of the peak-to-peak amplitudes over the treatment course were compared between the left-right (LR), cranial-caudal (CC), and anterior-posterior (AP) directions; and between the proximal, middle, distal esophagus, and proximal stomach. Further, the SDs of the peak-to-peak amplitudes and marker positions at the inhalation and exhalation were calculated to assess the interfractional variability of amplitude and trajectory shape. The correlation between the mean peak-to-peak amplitude and these SDs was also assessed. Results Overall, the mean and SD of the peak-to-peak amplitudes were significantly larger in the CC than in the LR/AP directions (median of mean[SD] in LR/CC/AP (mm): 2.0[0.6]/6.4[0.9]/2.4[0.7]; p <0.05, Friedman with Wilcoxon signed-rank test). It was also found to be significantly larger for the distal esophagus (2.6[0.6]/7.3[1.2]/3.1[0.7]) and proximal stomach (2.2[0.9]/6.8[1.1]/4.2[1.1]) than for the proximal (1.4[0.4]/2.7[0.7]/1.3[0.4]) and middle (1.6[0.5]/3.2[0.6]/1.6[0.5]) esophagus in all three directions (Fig. 1; p <0.05, Kruskal-Wallis with Dunn’s test). Moreover, the SDs of peak-to-peak amplitudes and marker positions at the inhalation and exhalation were ≤2.1mm (median: ≤0.9mm) in all three directions, suggesting a small interfractional variability of the motion amplitude and a stable trajectory shape (Fig. 2). Further, a weak correlation (coefficient R: 0.54–0.71, p <0.001) was found between the mean peak-to-peak amplitude and the interfractional variability of amplitude and trajectory shape (Fig. 2), implying that in addition to the peak-to- peak amplitude, other factors such as stomach fillings could also influence the interfractional variability of amplitude and trajectory shape. Conclusion The amplitude and variability of the respiration-induced esophageal tumor motion were found to be dependent on direction and region. The limited interfractional variability suggests that using a single planning 4D-CT may be sufficient to take into account the respiration-induced esophageal tumor motion.

Conclusion The TORUS algorithm is able to automatically generate trajectories having improved plan quality and delivery time over standard IMRT and VMAT treatments. TORUS offers an exciting and promising avenue forward toward increasing our dynamic capabilities in radiation delivery. OC-0377 Limited interfractional variabi lity of respiration-induced tumor motion in esophageal cancer RT P. Jin 1 , M.C.C.M. Hulshof 1 , N. Van Wieringen 1 , A. Bel 1 , T. Alderliesten 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Respiration-induced tumor motion is one of the major sources of intrafractional uncertainties in esophageal cancer RT. However, the variability thereof during the treatment course is unclear. In this study, we investigated the interfractional variability of respiration-induced esophageal tumor motion using fiducial markers and 4D- CBCT. Material and Methods We included 24 patients with in total 65 markers implanted in/around the primary esophageal tumor. Per patient, a 3D planning CT (pCT) and 7–28 (median: 8) 3D- CBCTs were acquired. Using the fluoroscopy projection images of the 3D-CBCTs, 10-breathing-phase 4D-CBCTs were retrospectively reconstructed. First, for each 4D- CBCT, the 10 phases were rigidly registered to the pCT based on the vertebra. Next, each marker in each phase was registered to its corresponding marker in the pCT to