Page 72 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

4.4 | Discussion
In this study, we quantified interfractional position variation of multiple abdominal organs in 20 children during radiotherapy and evaluated if organ position variation is mutually related and location- dependent. We found weak correlations between the position variations of contralateral organs. In the CC direction, right and left diaphragm dome position variations correlated moderately with the position variations of the liver and spleen, respectively. However, correlations between the position variations of the diaphragm domes and those of both kidneys were negligible. Furthermore, the largest magnitude of organ position variations was observed in the CC direction, followed by the AP and LR directions. We found that differences between group systematic and random errors of abdominal organs were small and insignificant. This comprehensive analysis of organ position variations at different anatomical locations increases the insight in possible consequences for margin definitions, which has not been reported on for children so far.
Nazmy et al. studied interfractional position variation of the liver and kidneys in 9 children (mean age: 4.1 years, SD = 1.6 years) using reference CT and CBCT scans [10]. They also found that, in the CC direction, the liver showed more motion than the kidneys. However, their range of observed position variations of the left kidney was smaller than that of the right kidney. In contrary, when we analyzed patients in our cohort of similar age (n=6; range 2.2-5.3 years) we found slightly larger position variations of the left kidney compared to the right kidney. Although, this comparison involves small sample sizes, a possible explanation might be the different methodology in choosing the point of interest. Nazmy et al. used the upper pole of the kidneys whereby kidney deformations might have been interpreted as translations, resulting in an overestimation of motion. We used the COM as point of interest because it is less sensitive to organ deformations. Although data on organ deformation would provide useful additional information on organ motion characteristics, analyzing organ deformation was outside the scope of the current study.
Our results are comparable to findings of Guerreiro et al. who used a similar methodology as we did [15]. They quantified interfractional position variations of the spleen, liver, and the healthy kidney in patients (n=15, mean age: 4 years) with Wilms’ tumors. Their ranges of interfractional position variation, and the systematic and random errors were generally somewhat smaller than our results, which could be explained by the fact that their cohort consisted of younger patients (age range 1-8 years). However, when we analyzed patients in our cohort of similar age (n=10; range 2.2-7.8 years), the systematic and random errors for all organs and directions in our cohort remained somewhat larger (for Σ; mean difference 1.0 mm, SD=0.6 mm, for σ; mean difference 0.7 mm, SD= 0.7 mm).
Using a 3DCT as a reference point to estimate interfractional position variation is arguable. The 3DCT represents a ‘snapshot’ of repeatedly changing organ positions during the respiratory cycle [27]. A CBCT captures in 35-60 seconds several complete respiratory cycles and averages the motion over the observed breathing phases into one blurred 3D image. To investigate the possible effect of respiratory motion differences on the 3DCT and the CBCTs, we recalculated our measurements using the first CBCT scan as the reference scan instead of the 3DCT. Differences between the respective calculations based on the refCT and the first CBCT were negligible (<1mm). Also, although projection images could enable the quantification of intrafractional motion as well [28], the low dose CBCT protocols that we used for most children [29] unavoidably result in poorer quality of projection images. Therefore, we were not able to distinguish organs on the two-dimensional projection images of these CBCT scans in order to investigate intrafractional motion of the liver, spleen, and kidneys.
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