Page 52 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Pham et al. conducted a systematic review on the degree of intrafractional renal motion associated with various breathing conditions in children and adults. In concordance with our results, they summarized that under free breathing the mean right and left kidney motion in children was considerably smaller than in adults [10].
The use of general anesthesia to immobilize young children during radiotherapy varies. In our cohort five children needed general anesthesia. Organ position variations in this small group did not differ from outcomes in children of similar ages treated without anesthesia. Pai Panandiker et al. studied intrafractional renal motion in 20 pediatric patients and found that renal motion in patients undergoing anesthesia was smaller than in patients who did not need anesthesia. They also concluded that renal motion correlated with age and height [16]. In our previous study, it appeared that pediatric age and height were interdependent factors (correlation coefficient R2 > 0.9); therefore, only the correlation between organ position variation and height was investigated whereby no significant correlations were found for interfractional motion [17]. In this study, we explicitly chose to include height, and not age, in the correlation analyses since adolescents can be taller than small adults. We found only negligible correlations between organ position variation and height; the low ρ values indicate that only a small proportion of interfractional organ position variation can be attributed to patients’ height. This means that other, underlying mechanisms cause differences in organ position variation in children and adults. There is a substantial difference in age between the pediatric and adult group (i.e., pediatrics: maximum age, 17.8 years; adults: minimum age, 34.1 years). Different physique including stature, body fat and elasticity of tissues and organs of the (pediatric and adult) patients might affect organ position variation; for example, the extent of tissue and organ elasticity could have influenced organ deformation. These and other – yet unknown – factors might have hampered establishing a possible correlation between position variation and patient height in our cohort. In addition, although our study includes a relatively large number of patients, even larger cohorts might be needed to uncover with statistical significance correlations between organ position variation and patient characteristics.
Interfractional organ position variation is an important component of geometrical uncertainties. Our results show that smaller PTV margins could be applied in children. However, it is important to further investigate other components, such as intrafraction motion and delineation errors to generate an all-encompassing definition of pediatric safety margins.
3.5 | Conclusions
In conclusion, we showed that interfractional renal and diaphragmatic position variation in children is smaller than in adults. Underlying, yet unknown mechanisms need to be investigated to explain these differences. Nevertheless, these results indicate that pediatric safety margins should be defined differently from adult margins. Other components of geometrical uncertainties have to be investigated to appropriately define pediatric PTV margins.
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