Page 90 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Similar as was found in adults [32], we found larger intrafractional motion variability compared to interfractional motion variability. This indicates that the respiratory motion during each fraction is more irregular than the average respiratory motion over all fractions.
Currently, due to lack of quantitative paediatric-based data, margins in abdominal and thoracic paediatric RT are generally based on data from adults. Our previous study showed that interfractional organ motion was smaller in children than in adults [7, 17]. Additionally, our present results show that respiratory-induced diaphragm amplitude motion in children is also smaller than reported in adults (10.7 mm in children vs. 16.4 mm in adults) [32]. This suggests that current clinically used margins might be too large and that development of guidelines for accurate margin definition in children is urgently needed, as was also recently recommended by the international paediatric radiation oncology society (PROS) [9]. One of the difficulties in developing guidelines, however, is that the amplitude of respiratory-induced diaphragm motion is not known prior to treatment. Although we did not find consistent correlations between respiratory motion characteristics and patient-specific factors, an even larger cohort might be needed to investigate if correlations in different age, height, or weight groups in children could be found in order to estimate prior to treatment what margin size would be appropriate based on those patient-specific factors. However, since we found large interpatient variability, indicating that respiration is patient-specific, accounting for respiratory motion probably requires a more individualized approach, such as 4DCT-controlled RT. Moreover, we found small interfractional motion variability, meaning that the respiratory-induced diaphragm motion per patient was stable over the course of treatment. Therefore, at least a single measurement of respiratory- induced organ motions, as by use of a 4DCT, at the start of treatment might be sufficiently predictive for respiratory-induced organ motion over the course of treatment and can therefore be used during treatment planning to compensate for this uncertainty. Additionally, the need for more accurate imaging with 4DCT, leading to more precise treatment and thus less risk of developing adverse events, should be weighed against the slightly increased imaging dose to the patient. For highly mobile targets in the thorax and upper abdominal region, application of 4DCT is becoming standard in our clinic. Breath-hold might not be effective in eliminating organ motion completely [39]. Moreover, although children are well capable of breath-holding [40] implementing this technique for young children might be a challenge. Alternatively, regularization of breathing might be a more promising option, with some kind of feedback of the breathing signal [41].
The PROS calls for more research on the implementation of modern technologies in paediatric RT [9]. Due to lack of financial support this has been difficult and therefore efforts in adults RT have been pragmatically transferred into a paediatric setting [9]. Even more than in photon therapy, proton therapy demands for high-precision localization of the target volume due to the sharp dose fall-off [29]. More data and knowledge on the internal physiological motion in children is therefore highly needed for optimal abdominal and thoracic proton therapy. Other modern radiation technologies, such as MR-guided RT and respiration-gated RT, are promising in predicting and monitoring the respiratory-induced organ motion over time and during treatment [42–44].
Another solution for better estimation of respiratory motion for margin definitions could be to guide patients’ respiration with an active video system [45]. We found that patients treated under GA had a significantly lower intrafractional amplitude variability than patients within the same age range treated without GA. To reduce intrafractional variability in patients treated without GA, the use of an active video system might be considered to calm the patients’ respiration and reach a more stable respiratory amplitude and cycle time. Previous studies also indicated that familiarizing patients with the RT equipment, staff and process reduces the anxiety and distress [20], which could be an important
87