## 4 Discussions  
This study is an ancillary study of the RADIOSA trial (AIRC IG-22159) with the aim of investigating and characterizing the entity of the OAR motion due to shrinkage, displacement, and volume variation related to the filling or emptying status of the organs during SBRT treatment. Despite significant volume variations under the influence of the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**OAR** & \begin{tabular}{c} **Median s-CT Dmax** \\ **(Range) (Gy)** \\ \end{tabular} & \
\begin{tabular}{c} **Median CBCT Dmax** \\ **(Range) (Gy)** \\ \end{tabular} &
\begin{tabular}{c} **Median Dmax** \\ **Variation (\%)** \\ \end{tabular} \\ \hline Cauda & 5.54 (0.55–7.32) & 5.66 (0.47–7.32) & \(-\)1.87 \\ Bladder & 11.14 (0.62–19.55) & 5.89 (0.53–23.20) & \(-\)20.09 \\ Rectum & 14.57 (0.45–30.14) & 13.90 (2.96–30.51) & \(-\)4.11 \\ Ileum & 22.19 (5.97–29.62) & 20.31 (6.04–31.90) & \(-\)1.38 \\ Colon & 11.32 (8.8–30.21) & 13.85 (4.97–30.37) & \(-\)3.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: From left to right: the OAR considered, median CBCT, median s-CT, and median CBCT values of Dmax for all the patients. Figure 4: Percentage volume variation distributions for all the considered organs. organ motion, the impact on the dose is only modest in terms of the constraint compliance, with a good level of local control and toxicity profile. Although several studies on OAR motion in the hypo-fractionated treatment of primitive prostate tumors are available in the literature [22; 23; 24; 25; 26], to the best of our knowledge, none of them explored organ movement in patients with lymph node oligometastatic PCa. The first section of this work focused on the assessment of the variability in the OAR structures between the s-CTs and daily CBCTs in order to identify volume variations, displacement, deformation, and shrinkage, playing important roles in the definition of a good treatment delivery. The colon and ileum were found to be the most critical OARs, as we demonstrated that they needed a margin of 15 mm and 10 mm, respectively, to reach the 95% % threshold. This variability can be ascribed to the very mutable shape of these organs, which is related to their physiological activity. Indeed, it is very well known that the small bowel is subjected to several mechanical stresses, which strongly affect the shape and entity of the movements of the organ itself [27; 28; 29]. On this basis, the isotropic margin was adopted. A comparable entity displacement in the case of the ileum was observed by [PERSON] et al. [29] in patients receiving adjuvant pelvic RT for rectal cancer. Nonetheless, the main difference from our study was the modality of the displacement estimation. [PERSON] et al. [29] measured the absolute length by considering the fixed point of reference in the bony anatomy, estimating the distance between certain predefined points in the spine and other chosen points in the small bowel. Instead, the entity of displacement in our work was computed as the relative distance between the surface of the structure in the s-CT and in the CBCT. A similar recent study by [PERSON] et al. [30] observed a similar trend in terms of the volume variation and Dmax for the bowel and, analogously with our study, inter-observer variability was avoided by enrolling one physician to contour all the OARs of interest. Concerning the bladder and rectum, their anatomical differences can be mainly related to their filling status. Regarding the rectum displacement, our results are comparable with those of [PERSON] et al. [31] and [PERSON] et al. [32] (>10 mm). Similar values were also reported by [PERSON] et al. [33], who derived margins of 8-9 mm for the displacement of the rectum. A displacement greater than 10 mm was observed by [PERSON] et al. [34], who concluded that margins of up to 10-15 mm are sufficient for identifying rectum movement. For the rectal volume variation, [PERSON] et al.