Page 138 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Interfractional position variation
Daily imaging and patient set-up verification can minimize or eliminate the interfractional set-up error [1, 2]. Since bony anatomy is used as a surrogate of the target, a residual error (i.e., the interfractional position variation) of the tumor or OARs (relative to the bony anatomy) remains after set-up verification. In order to account for it, it needs to be predicted by population-based measurements. This type of pediatric data was lacking and predictions were generally based on clinical experience and available adult data. In chapter 3 we demonstrated significant differences in interfractional position variation of abdominal organs between children and adults. Results from chapter 2, 3 and 4 could, in combination with other pediatric studies focusing on interfractional position variation [1, 3], give an indication for the interfractional component of safety margins for children in abdominal and thoracic radiotherapy (Table 8.1). Since Nazmy et al. solely reported ranges of interfractional position variation [3], we were unable to estimate the systematic and random errors and therefore excluded this study in Table 8.1. We showed large variation between pediatric cancer patients. Although we hypothesized that organ motion might be related to age, height, or weight, we did not find clear correlations between interfractional position variation and such patient-specific factors. Neither did Guerreiro et al. [1], who found slightly smaller values in overall younger patients compared to our cohort. Therefore, we suggested a more individualized margin approach. With the introduction of pre-fraction cone beam computed tomography (CBCT) imaging, the treatment plan can be individualized by re-optimizing the plan based on patient-specific variations that occur over the course of treatment [4]. The success of this adaptive radiotherapy (ART) approach relies on the imaging quality and modality and will be further discussed in paragraph 8.5.
On current CBCT images, the tumor is often not directly visible due to poor image quality, and bony anatomy is used as a surrogate for setup verification. In order to mitigate the interfractional position uncertainty, instead of using bony structures as a surrogate, target-based setup verification should be investigated. The use of fiducial markers in the target has been demonstrated to be feasible and accurate in adults with abdominal tumors [5, 6]. However, implementing markers is an invasive procedure, which is clearly unfavorable and not a standard procedure in pediatric radiotherapy. Nevertheless, the tumor is often surgically removed during pre-radiation treatment in children with abdominal tumors and surgical clips are placed at the tumor resection area. These clips, identifying the remaining tumor bed [1], or another organ in close proximity to the primary tumor [7], could be used as a surrogate for the target. In chapter 4, we showed that differences of systematic and random errors between abdominal organs were small, suggesting that for margin definitions, an organ closely located to the tumor could be used as a suitable surrogate. More importantly, image quality of pediatric CBCTs requires sufficient quality to enable rigid registration of surgical clips or abdominal organs. Future studies are needed to investigate the feasibility of using surgical clips or adjacent organs as a suitable surrogate instead of the bony structure and dosimetric consequences need to be compared.
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