Predictive value of pediatric respiratory-induced diaphragm motion quantified using pre-treatment 4DCT and CBCTs

BACKGROUND: In adults, a single pre-treatment four-dimensional CT (4D-CT) acquisition is often used to account for respiratory-induced target motion during radiotherapy. However, studies have indicated that a 4D-CT is not always representative for respiratory motion. Our aim was to investigate whether respiratory-induced diaphragm motion in children on a single pre-treatment 4DCT can accurately predict respiratory-induced diaphragm motion as observed on cone beam CTs (CBCTs). METHODS: Twelve patients (mean age 14.5 yrs.; range 8.6–17.9 yrs) were retrospectively included based on visibility of the diaphragm on abdominal or thoracic imaging data acquired during free breathing. A 4DCT for planning purposes and daily/weekly CBCTs (total 125; range 4–29 per patient) acquired prior to dose delivery were available. The amplitude, corresponding to the difference in position of the diaphragm in cranial-caudal direction in end-inspiration and end-expiration phases, was extracted from the 4DCT (A(4DCT)). The amplitude in CBCTs (A(CBCT)) was defined as displacement between averaged in- and expiration diaphragm positions on corresponding projection images, and the distribution of A(CBCT) was compared to A(4DCT) (one-sample t-test, significance level p < 0.05). RESULTS: Over all patients, the mean A(4DCT) was 10.4 mm and the mean A(CBCT) 11.6 mm. For 9/12 patients, A(4DCT) differed significantly (p < 0.05) from A(CBCT). Differences > 3 mm were found in 69/125 CBCTs (55%), with A(4DCT) mostly underestimating A(CBCT). For 7/12 patients, diaphragm positions differed significantly from the baseline position. CONCLUSION: Respiratory-induced diaphragm motion determined on 4DCT does not accurately predict the daily respiratory-induced diaphragm motion observed on CBCTs, as the amplitude and baseline position differed statistically significantly in the majority of patients. Regular monitoring of respiratory motion during the treatment course using CBCTs could yield a higher accuracy when a daily adaptation to the actual breathing amplitude takes place. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s13014-018-1143-6) contains supplementary material, which is available to authorized users.