Page 36 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

2.4 | Discussion
We quantified renal and diaphragmatic interfractional motion in 39 children using daily or weekly CBCTs and we calculated systematic and random errors. Our results show that kidney motion was largest in the CC direction, suggesting that margins should be applied anisotropically rather than isotropically. Moreover, a large variation in mean interfractional deviations and a wide distribution of random errors was found between patients. Therefore, our findings suggest an individualized approach to define appropriate margins in abdominal pediatric RT.
The large variation in interfractional organ motion between patients also suggests that the selection of the reference scan for estimating organ motion might be non-trivial. The refCT yields a short acquisition time over repeatedly changing anatomy during respiration cycles, which might not be the most representative as reference situation [19]. A CBCT has a slower gantry rotation and averages the motion over the observed breathing phases into one blurred 3D image. The timeframe of 35-60 seconds ascertains that the CBCTs show an average of the full range of motion. Therefore, we considered an alternative by calculating the mean interfractional motion based on the first CBCT as a surrogate of the reference scan for all patients. It turned out that differences between the respective calculations based on the refCT and the first CBCT were less than 1 mm, which is practically negligible (data not shown).
For younger and/or more mobile children, institution-based protocols determined whether or not to use a vacuum matrass for immobilization. Using a vacuum matrass might affect the breathing pattern and influence the baseline organ position. We did not analyze the effect of immobilization, since the interfractional organ motion with respect to the bony anatomy, as quantified in our study, is not affected by the immobilization systems used.
In one of the few studies published on extra-cranial pediatric organ motion, Nazmy et al. used the CBCTs of 9 abdominal neuroblastoma patients (mean age 4.1 ±1.6 years) to analyze renal interfractional motion relative to bony anatomy [16]. When we analyzed the 10 neuroblastoma patients (mean age 4.6 ±2.2 years) in our cohort separately, we found a slightly smaller range in interfractional motion in the CC direction (right kidney, range -1 to 8 mm vs. -4 to 10 mm; left kidney, range -4 to 5 mm vs. -4 to 8 mm). As point of interest we used the kidney COM, because it is less sensitive to kidney deformation, whereas Nazmy et al. used the upper pole of the kidney. Due to this method, Nazmy et al. might interpreted kidney deformations as translations, yielding an overestimation of the kidney motion. This is in line with the slightly smaller deviations that we found. Interestingly, all patients included in the neuroblastoma study [16] were treated under general anesthesia (GA), whereas GA was not used in any of the neuroblastoma patients in our study. Instead, a large proportion of young children in our cohort were positioned in a vacuum matrass and treated during their regular nap time as a means to establish immobilization.
Panandiker et al. studied extra-cranial pediatric organ motion using 4DCTs, in which respiratory cycles were captured as series of 8 phases [17]. They focused on renal intrafractional motion in 20 patients (median age 8 years, range 2-18 years), of whom 10 were treated under GA. The authors used an arbitrary baseline in 3D space to define organ motion, rather than bony anatomy as we did in our study. In accordance with our methodology, they used the geometric COM as primary calculation reference point. They reported a relationship between intrafractional kidney motion and age. Unexpectedly, we did not find such relationship for interfractional kidney motion with age. In accordance with Panandiker et al., both renal interfractional motion correlated with diaphragmatic motion. However, Panandiker et al. studied intrafractional motion mainly resulting from respiration,
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