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For accurate radiotherapy of thoracic and abdominal tumors, organ motion needs to be considered. Present research on organ motion in pediatric patients is limited to rather small cohorts. Therefore, we aimed to quantify interfractional diaphragm and abdominal organ motion, and intrafractional motion of the right hemidiaphragm in a large international cohort of pediatric cancer patients (<18 years). We also investigated possible correlations of motion with age and general anesthesia (GA).

In 188 children (median age 7.0; range 0.4–17.9 years) from six institutes, interfractional motion of both hemidiaphragms, spleen, liver and both kidneys was quantified using a two-step registration; CBCTs were registered to the reference CT (refCT) relative to the bony anatomy, followed by registration of the organ. We calculated the group mean, and the distributions of systematic and random errors (standard deviations Σ and σ, respectively) in CC, LR and AP directions. To quantify intrafractional motion (CC direction) in a subcohort of 79 children (median age 7.6; range 1.0–17.8 years), we used a semi-automated method selecting inhale and exhale positions of the right hemidiaphragm peak, and calculated the difference between the peaks (breathing amplitude). We investigated the correlation of inter- and intrafractional motion with age (Spearman’s ρ), and the difference in motion between patients ≥5.5 years, and patients <5.5 years treated with and without GA.

The group mean interfractional motion was largest in CC direction, but for all structures <1 mm, and not significantly different from the refCT (Bonferroni’s adjusted p>0.008). For all structures, we found no correlation between interfractional motion and age (ρ<0.1; p>0.05). Mean interfractional motion of the right hemidiaphragm in patients treated with GA was significantly smaller than in those treated without GA (all <5.5 years; p=0.004), resulting in smaller Σ and σ values (Table 1).
For intrafractional motion, the breathing amplitudes in children ≥5.5 years were significantly larger compared to patients <5.5 years treated without GA (p=0.02) (Table 2). In patients <5.5 years treated with GA, the breathing amplitudes (and random errors) were smaller compared to those treated without GA (Table 2), but there was no significant difference (p=0.4). We found a moderate but significant correlation between breathing amplitude and age (ρ=0.43; p<0.001).

In a large cohort of pediatric cancer patients receiving radiotherapy, we found that interfractional motion does not depend on age, but the use of GA in younger patients results in smaller systematic and random errors. Furthermore, our results show that the breathing amplitude increases with age. Differences in systematic and random errors between younger and older children are small, but variations between patients advocate the need for a patient-specific margin approach.