Seminal vesicle intrafraction motion 75 Discussion When observing the intrafraction motion as provided in Figure 2, it can be seen that seminal vesicle intrafraction motion in the left-right and anterior translation direction is similar to the prostate, while the prostate shows larger spread in the posterior and caudal direction. In addition, the seminal vesicles show slightly larger spread than the prostate in the cranial direction, but not in the caudal direction. Both the intrafraction motion of the seminal vesicles and prostate increases over time. Similar findings were reported by Gill et al.11 using a 2D cine-MR based study. They reported that the seminal vesicles move significantly more than the prostate in the cranial-caudal direction but not in the anterior-posterior or left-right direction. The spread in seminal vesicle movement in the cranial direction as observed from Figure 2 might be a reflection of how seminal vesicles are affected by rectum filling variation or gas pockets. These effects tend to push the seminal vesicles in the anterior and cranial direction. Similar observations were reported by Sheng et al.10 Influences from gas pockets on the seminal vesicles were observed in 18% of the fractions (Table 1). This motion is often combined with a rotation about the left-right axis, and this effect can be observed in the graph of Figure 2. In this graph, the spread in seminal vesicle motion is significantly larger than observed for the prostate. The spread in intrafraction rotation for the seminal vesicles is also significantly larger than observed for the prostate about the anterior-posterior and caudal-cranial axis. The fact that the seminal vesicles are more easily influenced by changes in the rectum as they are anatomically positioned partly surrounding the rectum, can be recognised from the rotation graphs. Previous studies showed that the seminal vesicle position was primarily affected by rectal gas, while a correlation with bladder volume was not as strong.10,19 However, Mak et al.19 reported that a strong correlation of seminal vesicle anteriorposterior movement with the position of the most posterior point of the bladder was observed. Smitsmans et al.20, quantified interfraction displacement of seminal vesicles using CBCT and reported a squared Pearson correlation coefficient (R2) between the left and right seminal vesicle in the anterior-posterior direction of 0.62 (Pearson's R = 0.79). They reported that no correlation between the seminal vesicles for the left-right displacement was found, while caudal-cranial translations were not considered in the study. Liang et al.21 found a R2 value of 0.7 (Pearson's R = 0.84) between the anterior-posterior motion of the prostate and seminal vesicles. In addition, they reported that no correlation was found in the other directions. In our study, we found Pearson correlation coefficients in the range of 0.72-0.76 for the left and right seminal vesicle with respect to the prostate in anteriorposterior and caudal-cranial translation direction (Table S3). However, rotation correlation coefficients of the seminal vesicles with respect to the prostate were weak (Table S3). Moreover, the Pearson's 95% confidence intervals indicate that the correlation differs per patient and that some patients have a very weak or no correlation. These confidence intervals indicate that it is useful to investigate seminal vesicle intrafraction motion for each individual patient specifically. The presented results show that the seminal vesicles move partly independently from the prostate, and our findings are consistent with previous studies.11,20–25 4
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