Page 148 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

(4D-) MRI
With MR imaging, additional ionizing radiation doses can be avoided. Also, MR imaging provides superior soft tissue contrast and a higher resolution in the longitudinal plane compared to (CB)CT imaging. For delineation purposes, this additional imaging information might improve the identification of the tumor and its boundaries and reduce inter-observer variation [80, 81]. Also, 4D- MRI provides a non-ionizing option to measure and characterize respiratory-induced abdominal organ motion and has been successfully used in children [10, 82]. However, Panandiker et al. showed that the serial acquisition of simulation CT, 4DCT, and MRI required approximately 65 minutes for each child [69]. Additionally, incorporating supplementary MR imaging next to CT imaging, provides two imaging modalities for decision-making, which is at the same time more time-consuming and difficult, affecting treatment efficiency and clinical workload. Therefore, efforts have been made to eliminate CT imaging to create an MR-only workflow, generating synthetic CT-imaging from MR imaging [83], and has also been shown feasible for children for both photon and proton treatment planning [84–86]. Additionally, with the introduction of MR imaging integrated into the treatment machines [87, 88], online visualization of abdominal and thoracic motion during irradiation becomes possible. These MR-workflows might become increasingly important in future pediatric radiotherapy.
8.5 | Treatment delivery options
The development of modern treatment technologies, such as MR-guided radiotherapy and particle therapy, is at the same time putting high demands on experienced cooperating multimodality teams to create and deliver accurate and precise treatment plans.
IGRT
Novel delivery techniques, like VMAT and IMRT, are slowly integrated in pediatric radiotherapy and could improve target conformality and reduce normal tissue dose [89–93]. For Wilms’ tumor patients, the VMAT technique is associated with less radiation dose to the remaining kidney and better PTV coverage than conformal 3D radiotherapy [89, 90]. For abdominal neuroblastoma patients, the replacement of conformal radiotherapy by IMRT is controversial [91–94], since IMRT also slightly increases integral doses compared to less sophisticated techniques [93, 94] (Figure 8.1). One must balance if this increased dose is considered as an acceptable trade-off with regard to improved target coverage.
Moreover, these delivery techniques demand the addition of pre-fraction imaging for precise patient positioning. Additional imaging dose is often not included in total dose calculations. Each CBCT increases the total dose by a finite percentage and causes scattered radiation to surrounding organs [95, 96]. Hess et al. proposed two pediatric IGRT decision trees, in which radiation oncologists are assisted to choose the most appropriate image modality in children and when CT is selected as the image modality, to assist in choosing optimal acquisition parameters [97]. For areas, with less motion expected, the individualized IGRT decision-making trees might help to define which patients would not necessarily benefit from daily IGRT, and will eliminate unnecessary dose to those patients. However, in the abdominal and thoracic areas, substantial interfractional position variation and respiratory motion is present and pre-fraction imaging is essential for higher accuracy.
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