ESTRO 36 Abstract Book

S202 ESTRO 36 2017 _______________________________________________________________________________________________

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.