Page 144 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

8.3 | Other options to account for motion
Besides the application of a safety margin to account for organ motion, other approaches are available to either manage or reduce organ motion. Compared to radiotherapy in adults, different approaches and strategies are used in pediatric radiotherapy. Often, the young age of the patient plays a role. Compliance, cooperation and understanding of the technique can be challenging in (young) children, and make it difficult to implement the technique in a pediatric setting.
Immobilization
The importance of patient immobilization has increased as treatment delivery techniques have become more complex and precise. Especially for children, lying still is challenging when being separated from their parents when left alone in the treatment room [47–50]. Therefore, to ensure a stable and reproducible patient position, children are often fixated in a customized vacuum matrass [23]. Interestingly, most of the published studies report the use of a customized vacuum matrass [1– 3, 8–10], also for adolescents, while in our department a vacuum matrass is rarely used. In our multicenter study in chapter 2, institution-based protocols differed in determining whether or not to use a vacuum matrass. However, interfractional position variation with respect to the bony anatomy, as quantified in chapter 2, is not affected by the immobilization system used and we therefore did not analyze patients treated with or without a vacuum matrass separately. It is therefore questionable if, when image-guided daily position verification is acquired, the use of a vacuum matrass would contribute to more accurate treatment delivery. The effect of being positioned in a vacuum matrass during treatment delivery on the intrafractional motion has not been studied, since the included patients in chapter 5, 6, and 7 were all treated at our department, and thus without a vacuum matrass.
General anesthesia
For some children, usually younger patients (< approximately 5 years [51, 52]), who cannot lie still during treatment, general anesthesia (GA) is used. In most studies, and similar to our anesthetic protocol, propofol was used as the drug of choice for sedation, meaning that no airway device was needed and patients maintained spontaneous breathing. There are no clear indications that GA has a consistent effect on the magnitude of organ motion [1, 8–10]. Similar to our results in chapter 3, Guerreiro et al. found that interfractional organ position variation in children treated under GA did not significantly differ from children who did not receive GA [1]. Regarding the effect of GA on respiratory-induced diaphragm motion, we found in chapter 5 that the variability of the amplitude during a fraction was significantly smaller in children treated under GA than in children of similar ages treated without GA. Kannan et al. also found very regular breathing in both amplitude and cycle time in children treated under GA, while those who did not receive GA were not as uniformly consistent [8]. In accordance to our findings in chapter 5, this did not translate to a difference in magnitude of the motion between the two groups. Guerreiro et al. also did not find a difference in intrafractional organ motion magnitude between both groups [1]. On the other hand, Uh et al. showed that for children with slow respiration rates (i.e., children receiving GA (< 8 years)), intrafractional organ motion tended to be larger [10]. Unlike Uh et al., Panandiker et al. found that intrafractional kidney motion was smaller for children treated under GA (< 8 years) than for children 141