Magnitude and variability of respiratory-induced diaphragm motion in children during image-guided radiotherapy

Background and purpose: To analyse the variability of respiratory motion during image-guided radiotherapy in paediatric cancer patients and to investigate possible relationships thereof with patient-specific factors.

Material and methods: Respiratory-induced diaphragm motion was retrospectively analysed on 480 cone beam CTs acquired during the treatment course of 45 children (<18years). The cranial-caudal positions of the top of the right diaphragm in exhale and inhale phases were manually selected in the projection images. The difference in position between both phases defines the amplitude. The cycle time equalled inspiratory plus expiratory time. We analysed the variability of the intra- and interfractional respiratory motion and studied possible correlations between respiratory-induced diaphragm motion and age, height, and weight.

Results: Over all patients, mean amplitude and cycle time were 10.7mm (range 4.1-17.4mm) and 2.9s (range 2.1-3.9s). Intrafractional variability was larger than interfractional variability (2.4mmvs. 1.4mm and 0.5svs. 0.4s for amplitude and cycle time, respectively). Correlations between mean amplitude and patient-specific factors were significant but weak (p<0.05, ρ≤0.45).

Conclusions: Large ranges of amplitude and cycle time and weak correlations confirm that respiratory motion is patient-specific and requires an individualized approach to account for. Since interfractional variability was small, we suggest that a pre-treatment 4DCT in children could be sufficiently predictive to quantify the respiratory motion.