Page 146 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Other
Other options, which are technically more challenging techniques, including gating or tracking can also be employed to account for respiratory motion. While these techniques are successfully implemented in radiotherapy for some adult cancer types, implementing these techniques in a pediatric setting is more complicated. It is often a prerequisite for an ethical approval that the technique is proven beneficial in adults. Also, the benefits of the techniques should certainly outweigh the additional patient load, treatment time and costs, compared to current standards. Explanation and training of the techniques make that the full procedure requires more time. Prospective studies are needed to carefully evaluate the feasibility and potential clinical (and long- term) benefit of these techniques in children.
8.4 | Imaging modalities
Although 3DCT is the standard imaging technique for treatment planning purposes, there is a growing demand for better and more imaging. By incorporating complementary information from multimodality imaging (e.g., 4DCT, MRI, positron emission tomography (PET)), improved target delineation and more accurate treatment planning might be achieved. Especially, in the abdominal and thoracic area, where moving structures can cause motion artefacts and high soft tissue contrast is crucial, achievements in imaging techniques and thereby improving image quality are continuously investigated. However, concerning additional dose from CT imaging and the ALARA principle (keeping doses As Low As Reasonably Achievable), there is always an ongoing discussion and clinical implementations move slowly forward in pediatric radiotherapy.
4DCT
This thesis showed the need for a more individualized treatment approach in pediatric radiotherapy. A pre-treatment 4DCT is an effective tool to determine intrafractional motion from respiration. There are different strategies for using 4DCT in treatment planning [11]. Solely accounting for the internal motion leads to the ITV, which includes the CTV plus an internal margin, covering the entire respiratory-induced motion. However, to account for the remaining geometrical uncertainties an additional margin is added to the ITV (as earlier discussed), leading to large PTV margins and increased dose to healthy surrounding tissues. An alternative approach, the mid-ventilation based PTV planning, leads to smaller margins, simultaneously accounting for respiratory motion and the other geometrical uncertainties [11, 67, 68]. Previously, the use of 4DCT was only reported by Panandiker et al. [9, 69]. Recently, others also reported the use of 4DCT in children and described the use of ITV-to-PTV margins [1, 8, 69–71]. However, the optimal strategy how to use 4DCT in pediatric treatment planning is yet unknown and needs to be investigated. More importantly, although we suggested in chapter 6 that the 4DCT should be effective for treatment planning purposes in children, we found in chapter 7 that respiratory-induced motion as measured on 4DCT is not always representative for respiratory-induced motion during treatment. Using such a single measurement could possibly lead to insufficient target coverage. Therefore, our results suggest to monitor respiratory motion on a more regular basis, and adapt treatment plans when necessary, which will both be discussed further in this and the following paragraph.
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