Page 51 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

3.4 | Discussion
Thus far, adult [9–14] and pediatric [15–17] studies separately reported on abdominal organ motion, which complicates a straightforward comparison of results due to different methodologies. This study is the first to quantify and compare interfractional organ position variation in a relatively large number of pediatric and adult cancer patients. Our results show that renal and diaphragmatic position variation was generally smaller in children than in adults; the median 3D vector lengths of the right and left kidney were significantly smaller in children as compared to adults.
Diaphragm position variation was assessed by manual registration of the diaphragm as one complete structure, which forced us to make concessions regarding the best fit. Due to different anatomy of the left and right upper abdomen, with the spleen in the upper far left part of the abdomen and the largest part of the liver in the upper right part of the abdomen, it is conceivable that the position variation of the left and right diaphragm domes could be different. Considering the left and right diaphragm dome position variation as a surrogate for, respectively, left and right sided abdominal organ position variation, might, therefore, lead to the need of distinct definitions of PTV and PRV margins for left and right sided tumors and OARs. In a future study, therefore, it would be worthwhile to investigate the diaphragm position variation considering the left and right diaphragm domes separately. Further, the findings thereof could be compared to the findings for the left and right kidney position variations.
Since interfractional organ position variation was investigated relative to the bony anatomy, it is not to be expected that treatment position variations caused the differences between children and adults. Daily anatomical variations due to differences in organ filling and the amount of air in stomach or bowel were present in the patients who showed extreme values of organ position variation (Figure 3.1). When we repeated our analysis excluding these outliers, the difference in mean position variation of the right kidney in LR direction in pediatrics vs. adults was found to be significant as well, whereas other results did not change.
In this study, we registered CBCTs with 3D- and averaged 4D-CTs. The acquisition times of pediatric and adult CBCTs were 35–60 and 120 s, respectively; both included a sufficient number of breathing cycles to ascertain an average of the full range of motion. This is comparable with an averaged 4D-CT, whereas a 3D-CT is acquired in a relatively shorter time frame than a CBCT. Therefore, a 3D-CT might not sufficiently represent the anatomy during a radiation treatment course [21]. However, differences in results are expected to be practically negligible; when we based our interfractional organ position variation calculations on the first CBCT instead of the refCT, registration outcomes were similar, as shown in our previous study on organ motion in children [17]. Some children had 3D-CTs with 5.0 mm slice thickness, which could have led to an overestimation of interfractional organ position variation in the CC-direction.
The differences in organ position variation resulted in generally smaller Σ and σ values for kidneys and diaphragm in children than in adults. The availability of a considerable amount of CBCTs (5–30 per patient) enhanced the reliability of our analyses, and allowed for good estimations of Σ and σ. When entering the Σ and σ values from Table 3.2 in the PTV or PRV margin recipes [6, 7], renal safety margins could be calculated, resulting in smaller PTV and PRV margins for children than for adults. However, such calculations are only valid for geometrical uncertainties due to interfractional organ position variation, without taking other components, such as intrafractional motion and delineation errors into account.
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