Potential benefit of MRI-guided IMRT for flank irradiation in pediatric patients with Wilms’ tumor

Purpose/Objective: Flank irradiation for Wilms' tumor (WT) is currently performed at our institute using a cone-beam computed tomography-guided volumetric modulated arc (VMAT _CBCT ) workflow. By adding real-time magnetic resonance imaging (MRI) guidance to the treatment, safety margins could be reduced. The study purpose was to quantify the potential reduction of the planning target volume (PTV) margin and its dosimetric impact when using an MRI-guided intensity modulated radiation therapy (IMRT _MRI ) workflow compared to the VMAT _CBCT workflow. Material/Methods: 4D-CT, MRI and CBCT scans acquired during preparation and treatment of 15 patients, were used to estimate both geometric, motion and patient set-up systematic (∑) and random (σ) errors for VMAT _CBCT and IMRT _MRI workflows. The mean PTV (PTV _mean ) expansion was calculated using the van Herk formula. Treatment plans were generated using five margin scenarios (PTV _mean ± 0, 1 and 2 mm). Furthermore, the IMRT _MRI plans were optimized with a 1.5T transverse magnetic field turned-on to realistically model an MRI-guided treatment. Plans were evaluated using dose-volume statistics (p<.01, Wilcoxon). Results: Analysis of ∑ and σ errors resulted in a PTV _mean of 5 mm for the VMAT _CBCT and 3 mm for the IMRT _MRI workflows in each orthogonal direction. Target coverage was unaffected by the margin decrease with a mean V _95% =100% for all margin scenarios. For the PTV _mean , an average reduction of the mean dose to the organs at risk (OARs) was achieved with IMRT _MRI compared to VMAT _CBCT : 3.4 ± 2.4% (p<.01) for the kidney, 3.4 ± 2.1% (p<.01) for the liver, 2.8 ± 3.0% (p<.01) for the spleen and 4.9 ± 3.8% (p<.01) for the pancreas, respectively. Conclusions: Imaging data in children with WT demonstrated that the PTV margin could be reduced isotropically down to 2 mm when using the IMRT _MRI compared to the VMAT _CBCT workflow. The former results in a dose reduction to the OARs while maintaining target coverage.