Page 108 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

6.4 | Discussion
Respiratory motion characteristics of 90 patients, including 45 children and 45 adults, analyzed with identical methodology in 1004 CBCTs [16], were compared to reveal the effectiveness of 4DCT application for planning purposes in pediatric radiotherapy. This comprehensive dataset shows small but statistically significant differences in respiratory-induced diaphragm motion in children and adults. We found that respiratory motion in children during the treatment course is more regular, indicating that a 4DCT will be at least equally beneficial for planning purposes as it is in adults. Additionally, large ranges of mean amplitude and mean cycle time in both children and adults confirm that respiratory motion is patient-specific and requires an individualized approach (e.g., based on 4DCT) to account for. This was emphasized by weak correlations between all respiratory parameters and the patient-specific factors.
Unexpectedly, we found that interfractional variability of the amplitude was statistically significantly smaller in children than in adults, meaning that over all fractions the respiratory amplitude was more stable in children. This could be explained by the fact that, since it is known that patients experience radiotherapy as a stressful procedure [20], more attention in the clinic is paid towards comforting the child and reducing anxiety [21–23], while this is less introduced in the clinic for adults. This might have also led to a more constant cycle time during each fraction in children than in adults, as shown by the smaller intrafractional variability of cycle time in children.
These present results indicate that a single pre-treatment measurement of respiratory- induced motion with 4DCT could be a good representation for motion in children during radiotherapy. Moreover, large variation in amplitude and cycle time in both children and adults confirms that a more individualized approach with 4DCT can be effective for children as well. Recently, discussion is ongoing on radiation risks in children from medical imaging [24–28]. However, although 4DCT increases imaging dose 2-4 times compared to 3DCT [29, 30], it provides more detailed information on organ motion, leading to more precise treatment planning and potentially minimizing dose to healthy tissues. Recently, for this aim a pediatric-specific 4DCT scanning sequence and protocol was developed [31]. In our institute, we recently introduced 4DCT for children and applied the same 4DCT protocol as used for adults. However, when feasible, parameters were adjusted to achieve lower imaging doses. Similarly, in our institute, a low-dose protocol for CBCT imaging was developed and implemented for pediatric patients [32]. For all adults, CBCT imaging was acquired with 360 degrees rotation while pediatric CBCTs were acquired at lower imaging doses with 200-360 degrees rotations. This resulted in a variation in number of projection images (180 to 760) between children and adults. However, for each patient, a sufficient amount of projection images was available for tracking diaphragm motion, representing sufficient breathing cycles (approximately 10-30) for the calculation of our parameters.
Although the differences in respiratory motion characteristics between children and adults are smaller, our present results on intrafractional organ motion are in line with findings from our previous study, in which we demonstrated that interfractional abdominal organ motion in children differed from that in adults [33]. This underscores the need, also in children, for a more individualized approach using 4DCT to define safety margins. However, all-encompassing safety margins cannot be defined solely based on inter- and intrafractional motion. Setup variations and delineation errors [6], should also be taken into account, but to our knowledge have not been reported on for pediatric radiotherapy. Additionally, since respiratory-induced diaphragm motion is used as a surrogate for abdominal organ or tumor motion, uncertainties need to be taken into account when these results are used for treatment planning purposes. Therefore, assumptions regarding potential margin reduction and
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