Page 89 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

5.4 | Discussion
We quantified and analysed respiratory-induced diaphragm motion and its variability within and between fractions during complete RT courses in 45 children treated with IGRT. Over all patients, interfractional variability was smaller than intrafractional variability. We found large ranges of respiratory-induced diaphragm amplitude motion and cycle time, indicating substantial differences between patients. Moreover, no clinically significant correlations were found between respiratory- induced diaphragm motion characteristics and patient-specific factors (age, height, and weight). Our study is the first to include such a large number of children and the availability of the corresponding daily or weekly acquired CBCTs comprised an extensive dataset. This allowed for an all-encompassing analysis of respiratory-induced diaphragm motion in children throughout their treatment course, which has not been reported for children so far.
In our institute, paediatric CBCTs are acquired using lower imaging doses than used for adults [23]. We considered using an automatic method to detect the diaphragm dome, as used by Rit et al. [32], however, lower imaging doses result in less projection images and reduced image quality, which hampers the use of an automatic method to detect the diaphragm dome. Therefore, in our study the diaphragm dome was manually tracked in the end-inhale and end-exhale phases in the main direction of respiratory motion (CC direction). Most patients (37/45) received half-fan width CBCTs, resulting in few projection images at the angle where the diaphragm domes are not superimposed. Manually tracking enabled us to include the projection images in which left and right diaphragm domes were superimposed, thus including as many data as possible. Although the top of the diaphragm was clearly visible and easy to distinguish, it was a time-consuming method. Therefore, diaphragm tracking was done by one observer, and inter-observer analysis was not feasible. With a range of 4–32, the number of CBCTs differed substantially between patients. Analysis of our group measurements when corrected for the number of CBCTs showed no significant changes (amplitude <0.5 mm and cycle time <0.1 s). Furthermore, respiratory-induced diaphragm motion does not necessarily correlate with tumour motion [37, 38]. Therefore, using the diaphragm as a surrogate for abdominal and thoracic organ motion has some inaccuracies and induces uncertainties that need to be taken into account for treatment planning by defining safety margins for tumours or OARs. Additionally, since respiratory- induced motion is only one component of geometrical uncertainties, other uncertainties due to interfractional motion, delineation errors, and setup variations, also need to be accounted for.
Panandiker et al. studied intrafractional organ motion using 4DCT, in which the respiratory motion was binned in a series of 8 phases [6]. Maximal diaphragmatic motion between inspiration and expiration phases was quantified. Probably due to their analysis method, they found that age and diaphragmatic motion were highly correlated, in contrast to our results. They divided their patients in a younger (2-8 years, n=11) and an older age group (9-18 years, n=9), and found a mean respiratory- induced diaphragm CC motion of 5.1 mm and 9.6 mm in the younger and older age group, respectively. When we analysed our results stratified in similar age categories, we found these values to be 9.4 mm for our younger group (n=16) and 11.6 mm for our older age group (n=29). However, all younger patients in Panandiker’s study had been treated under GA, while in our study only 7 patients were treated under GA (mean respiratory-induced diaphragm CC motion 8.9 mm). Moreover, extracting the respiratory motion from a 4DCT divided in 8 bins is a coarser method and more sensitive to artefacts, compared to our method. Our measurements were done in daily or weekly CBCTs, which comprise more data and enabled an analysis of respiratory motion during different days, while a 4DCT is a single moment in time.
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