Towards MRI-guided radiotherapy for pediatric patients with abdominal tumors

Wilms’ tumor (WT) and neuroblastoma (NBL) belong to the most frequent abdominal tumors in children. Despite radiotherapy treatments are reported to be beneficial for cancer survival, they are associated with an increased risk of normal tissue toxicity. At the radiotherapy department of University Medical Center Utrecht, these patients are currently treated with photon therapy using a volumetric modulated arc therapy (VMAT) technique. The main goal of this thesis was to investigate the benefit of using novel radiotherapy techniques, such as magnetic resonance imaging (MRI)-guided photon therapy and intensity-modulated proton therapy (IMPT), to treat this patient category. In light of the work shown throughout the thesis, the main conclusions are: (1) Average motion and patient set-up uncertainties during radiotherapy treatment were found to be limited; (2) IMPT delivered by posterior-oblique radiation fields proved to be more dosimetric favorable and more robust against inter-fraction anatomical variations than available cone beam computed tomography (CBCT)- and MRI-guided photon techniques; (3) Accurate MRI-only photon and proton dose calculations were feasible with the
proposed synthetic CT generation method. Consequently, the work presented in this thesis contributed to build the rationale of introducing MRI-guided proton radiotherapy treatments for pediatric patients with abdominal tumors (WT and NBL). With MRI-guided proton therapy, further dose reductions might be foreseen compared to the currently available radiotherapy techniques. This technique seems to
be particularly promising for the pediatric population, for whom there is a special concern for normal tissue sparing to minimize the risk of radiation-induced complications after treatment.