Page 140 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Intrafractional motion
Intrafractional motion in the abdomen and thorax is mainly caused by respiration and also needs to be accounted for when population-based safety margins are derived for treatment planning. In chapter 5, we found large variation in amplitude of respiratory-induced diaphragm motion between children (range 4.1-17.4 mm). Others found that intrafractional motion of the tumor bed and OARs was ≤ 5 mm [1, 8–10]. Since we only investigated the intrafractional motion of the diaphragm, which does not necessarily correlate with tumor or organ motion [9], using the diaphragm as a surrogate for abdominal and thoracic organ motion will certainly induce inaccuracies and uncertainties that need to be taken into account for treatment planning purposes. A summary of the currently available data on intrafractional organ motion in pediatric cancer patients is presented in Table 8.2. Since variation between patients was large (chapter 5 and 6), which was also found by others [1, 8–10], we suggested that a more individualized approach with pre-treatment 4DCT was necessary. A pre- treatment 4DCT provides patient-specific 3D data on the tumor position and OARs during several phases of the breathing cycle, which can then be incorporated in an individualized treatment plan [11]. However, in chapter 7, we found that respiratory-induced diaphragm motion in children determined on 4DCT does not accurately predict the daily respiratory-induced diaphragm motion observed on CBCTs. In addition, respiratory-induced diaphragm motion on CBCTs acquired within one treatment session with an interval of minutes already showed significant differences, meaning that the actual respiratory-induced motion during dose delivery can again differ from measurements on the CBCT, questioning the usefulness of ART. In chapter 5, we also showed that respiratory- induced diaphragm motion even varies within consecutive breathing cycles in children. To optimally account for intrafractional motion, real-time imaging might be the solution. Real-time imaging and other options to account for intrafractional motion will be further discussed in this chapter in paragraph 8.3 and 8.5.
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