Diaphragm and abdominal organ motion during radiotherapy: a comprehensive multicenter study in 189 children

Background: For accurate thoracic and abdominal radiotherapy, inter- and intrafractional geometrical uncertainties need to be considered to enable accurate margin sizes. We aim to quantify interfractional diaphragm and abdominal organ position variations, and intrafractional diaphragm motion in a large multicenter cohort of pediatric cancer patients (< 18 years). We investigated the correlation of interfractional position variations and intrafractional motion with age, and with general anesthesia (GA).

Methods: In 189 children (mean age 8.1; range 0.4-17.9 years) from six institutes, interfractional position variation of both hemidiaphragms, spleen, liver, left and right kidneys was quantified using a two-step registration. CBCTs were registered to the reference CT relative to the bony anatomy, followed by organ registration. We calculated the group mean, systematic and random errors (standard deviations Σ and σ, respectively) in cranial-caudal (CC), left-right and anterior-posterior directions. Intrafractional right hemidiaphragm motion was quantified using CBCTs on which the breathing amplitude, defined as the difference between end-inspiration and end-expiration peaks, was assessed (N = 79). We investigated correlations with age (Spearman's ρ), and differences in motion between patients treated with and without GA (N = 75; all < 5.5 years).

Results: Interfractional group means were largest in CC direction and varied widely between patients, with largest variations in the right hemidiaphragm (range -13.0-17.5 mm). Interfractional group mean of the left kidney showed a borderline significant correlation with age (p = 0.047; ρ = 0.17). Intrafractional right hemidiaphragm motion in patients ≥ 5.5 years (mean 10.3 mm) was significantly larger compared to patients < 5.5 years treated without GA (mean 8.3 mm) (p = 0.02), with smaller Σ and σ values. We found a significant correlation between breathing amplitude and age (p < 0.001; ρ = 0.43). Interfractional right hemidiaphragm position variations were significantly smaller in patients < 5.5 years treated with GA than without GA (p = 0.004), but intrafractional motion showed no significant difference.

Conclusion: In this large multicenter cohort of children undergoing thoracic and abdominal radiotherapy, we found that interfractional position variation does not depend on age, but the use of GA in patients < 5.5 years showed smaller systematic and random errors. Furthermore, our results showed that breathing amplitude increases with age. Moreover, variations between patients advocate the need for a patient-specific margin approach.