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) 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) workflow compared to the VMAT 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 and IMRT workflows. The mean PTV (PTV) expansion was calculated using the van Herk formula. Treatment plans were generated using five margin scenarios (PTV ± 0, 1 and 2 mm). Furthermore, the IMRT 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 of 5 mm for the VMAT and 3 mm for the IMRT workflows in each orthogonal direction. Target coverage was unaffected by the margin decrease with a mean V=100% for all margin scenarios. For the PTV, an average reduction of the mean dose to the organs at risk (OARs) was achieved with IMRT compared to VMAT: 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 compared to the VMAT workflow. The former results in a dose reduction to the OARs while maintaining target coverage.