Page 149 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

ART
To anticipate on day-to-day anatomical and respiratory variations, ART might be the solution. Pre- fraction imaging enables to evaluate patient-specific variations and customize treatment plans according to the observed anatomical changes [4]. ART based on CBCT imaging can be performed offline or online. Most CBCT-based ART approaches are performed offline. A re-planning strategy requires a new CT to create a new treatment plan when the anatomical changes cause changes in the extent that the planning criteria are no longer met. For a plan selection strategy multiple treatment plans are created, where for each fraction, the plan that best fits the anatomy of that day is selected. An online ART workflow based on CBCT imaging is a more complex time-demanding process [98], requiring daily re-contouring and re-planning, while the patient is waiting. For a number of treatment sites in adults, ART has demonstrated to offer clinically relevant improvements [98–102]. The need for an adaptive approach in children with tumors in the head and neck region was not consistent [103–105]. Daily monitoring was suggested to assess which patients would benefit from an adaptive strategy. Guerreiro et al. investigated the dosimetric impact of daily anatomical changes in the abdominal region in children [106]. They found target under dosage and over dosage of the assessed kidneys, although differences were small. Re-planning should be considered when normal tissue tolerance of OARs is reached, however, due to poor soft tissue contracts of CBCT imaging assessment of other abdominal OARs was unfeasible.
The effectiveness of an adaptive strategy depends on the quality of daily imaging. Present quality in pediatric CBCT imaging and the absence of real-time imaging cause restrictions and limit the possibility of ART in abdominal and thoracic areas in children. Daily imaging comes with the drawback of increased treatment times and additional doses, but improved CBCT reconstruction methods could minimize these limitations. Future studies are needed to investigate the feasibility of ART based on pediatric CBCT imaging and whether the increased clinical workload, due to evaluating pre-fraction imaging, will outweigh the potential gain in accuracy and dosimetric benefits.
MR-Guidance
The introduction of MR-guided treatment machines allow for daily and online visualization of the tumor and OARs, real-time verification of plan delivery, and accurate treatment adaptations. Recently, the first clinical adult patients were treated [107]. MR-guided approaches introduce superior imaging for ART compared to CBCT imaging, without any additional imaging dose. In children, MR-guided IMRT planning versus CBCT-guided VMAT planning showed promising results; smaller PTV margins and a reduction of normal tissue exposure [24]. The MR linac has the potential to provide a high-precision, adaptive, image-guided approach, with less toxicity in the surrounding healthy tissues, and will play an important role in the future of pediatric radiotherapy.
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