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

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.

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).

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.

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.