Page 151 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

during treatment in the dose optimization. Respiratory motion can cause the interplay effect, and is more difficult to account for. However, Boria et al. showed that in children with mobile tumors receiving proton therapy, the interplay effect was largely negligible for younger children (2-6 years) with minimal CC target motion (< 5 mm) [85]. For mobile tumors treated with particle therapy, a 4D- based approach is paramount to quantify respiratory-induced motion and determine which strategy should be used [124]. As suggested by Boria et al. with respiratory motion > 10 mm, techniques to minimize the motion such as breath holding or respiratory gating should be considered [85, 125, 126].
Ultimately, to optimally benefit from particle therapy in those cases subject to tumor and organ motion, real-time imaging and the capability to adjust for anatomical changes is paramount. Pediatric patients could also benefit from ion radiography [112, 127], where the ion source is also used as imaging technique and the patient is imaged in the actual treatment position. Bony structures were clearly visible, allowing for accurate patient set-up, while reducing the imaging dose compared to X- ray imaging [127]. It could also be used during treatment delivery for online tumor tracking and dose adjustments, potentially leading to an online adaptive ion approach, but due to technical challenges it is still in an experimental phase. This also applies for achievements in combining MRI and particle therapy, which might ultimately provide the optimal treatment modality including an online non- ionizing imaging technique [128–131].
More realistic solutions are MR-guided photon therapy and image-guided proton therapy. They will become easier available in the Netherlands in the near future and are expected to play an important role in future pediatric radiotherapy. Until then, based on the available imaging techniques per institute (e.g., in-room CBCT, 4DCT), the optimal imaging and treatment approach will have to be chosen for each individual child, and the outcomes presented in this thesis will be applicable in treatment planning for defining appropriate margins.
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