ESTRO 2024 - Abstract Book

S5537

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

In total, 47 patients with a total of 63 lymph nodes were included in this study with 5 fractions each, resulting in total of 340 fractions that were analyzed. Of these nodes, 10 were located in the mesorectum, 11 were paraaortic nodes, 17 were parailiac nodes, and 25 were obturator nodes. The measured intrafraction motion of these nodes in the lateral, longitudinal, anterior-posterior direction, as well as the 3D displacement is shown as a box-and-whisker plot in figure 1. A statistically significant difference in the intrafraction motion was observed for the mesorectal nodes compared to the other locations for the anterior-posterior direction, as well as the 3D displacement. No statistically significant differences were found between the other locations and in other directions.

Figure 1 - Box-and-whisker plots (depicting the mean, 1 SD, 2 SD and outliers) of the intra-fraction displacement of lymph nodes for the different locations in the lateral, longitudinal and ant-post direction, as well as the 3D displacement.

For an isotropic PTV margin of 1, 2 and 3 mm, the percentage of fractions at which a lymph node moves outside this margin is shown in figure 2 for the different node localizations. For all but the mesorectal nodes, a 2 mm margin will ensure that the nodes are inside the PTV for more than 90% of the fractions during the entire treatment procedure. For the mesorectal nodes, a 3 mm margin is needed to achieve this. A 4 mm margin would capture intrafraction motion for all fractions.