Page 37 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

while interfractional motion, as quantified in our study, results also from other anatomical variations, such as inconstant bowel filling. These differences in inter- and intrafractional motion need to be further investigated.
Although we did not find a correlation between interfractional organ motion and height, we found a suggestive difference in renal and diaphragmatic motion between children and adults, when comparing our results with the literature. Wysocka et al. determined the median interfractional motion of the kidneys and diaphragm relative to bony anatomy in 22 adults [8]. However, instead of CBCTs, follow-up CT scans were used for registrations. Since a CT has a short acquisition time, organ position strongly depends on the breathing phase during that time, which could lead to more extreme deviations when compared to CBCT-based analysis. We compared kidney motion under free breathing as reported by Wysocka et al. with our data. To compare the median interfractional motion, we calculated the median values as well, and in accordance with their study, we found the largest displacements in the CC direction (Figure 2.1). The median interfractional displacements of the kidneys and diaphragm were seemingly smaller in the children in our cohort than in their study. This indicates that abdominal interfractional organ motion in children may be different from that in adults, although some caution in interpretation is warranted given the above-stated differences in methodology.
Strengths of our study are the large patient number and the substantial amount of CBCTs (mean 14, range 5-33 per patient). With a range of 5-33, the number of CBCTs differed substantially between patients. Analysis and recalculations in which we weighted the data with the number of CBCTs showed no significant changes. We are the first to focus on the quantification of interfractional renal motion using a large amount of imaging data, acquired during IGRT. This allowed for a good estimation of the group systematic (Σ) and random errors (σ). Subsequently, these values can be used in a margin recipe, such as 2.5 Σ + 0.7 σ [6], to estimate an appropriate CTV-PTV margin. PRV margins can be calculated using 1.3 Σ + 0.5 σ [7]. As a note of caution, our study included only one component of the margin, i.e., the interfractional organ motion, and did not consider other effects (such as delineation uncertainties or set-up variation). Moreover, intrafractional abdominal motion due to respiration during treatment should also be investigated. The retrospective (daily or weekly) CBCTs used in this study can also be used for the quantification of intrafractional abdominal motion [20]. In order to properly calculate the margins, all components of organ motion should be taken into consideration. A better understanding of abdominal organ motion in children during RT is also essential to take full advantage of the state-of-the-art treatment approaches for children, such as intensity modulated RT and intensity modulated proton therapy.
2.5 | Conclusions
Renal and diaphragmatic interfractional motion was largest in the CC direction. Interfractional motion did not correlate with patient-specific factors and variation between patients was large. This suggests that individualized margin approaches might be required. The outcomes of this study are a first step towards guidelines for pediatric abdominal margin definitions. To provide a full insight into renal and diaphragmatic motion, inter- and intrafractional motion should be further analyzed. Such comprehensive insight into pediatric abdominal organ motion is essential for high accuracy in hitting the target and minimizing dose to healthy tissue.
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