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

Abstract

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 setup systematic (P) and random (r) 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 P and r 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% ¼100% 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.

Evaluating the benefit of PBS vs. VMAT dose distributions in terms of dosimetric sparing and robustness against inter-fraction anatomical changes for pediatric abdominal tumors

“…However, it can still be argued if the chosen uncertainties are too large given the available online image guidance and verification techniques. In previous studies [20,37], we estimated the target safety margin to use in a CBCT-guided workflow for a group of 15 WT patients. When accounting for the treatment chain geometric accuracy, patient set-up and inter-fraction motion uncertainties, a target safety margin of 5 mm in all orthogonal directions was calculated [37].…”

“…In previous studies [20,37], we estimated the target safety margin to use in a CBCT-guided workflow for a group of 15 WT patients. When accounting for the treatment chain geometric accuracy, patient set-up and inter-fraction motion uncertainties, a target safety margin of 5 mm in all orthogonal directions was calculated [37]. For this reason, a 5 mm set-up uncertainty plus an ITV margin to account for the breathing motion were used in this study.…”

“…With MRI-guided systems [46], the use of decreased safety margins due to the better visualization of the target could be achieved allowing for real-time adaptive regimes without extra patient radiation burden [47]. In a previous study [37], we have quantified the dosimetric impact when using MRI-guided IMRT (IMRT MRI ) compared to the clinical VMAT workflow to treat WT patients (n=15). When using a PTV margin of 1 mm for the IMRT MRI (simulating a best-case scenario), the calculated D mean was 18% in the kidneys, 30% in the liver, 32% in the spleen and 68% in the pancreas.…”

Locoregional control using highly conformal flank target volumes and volumetric-modulated arc therapy in pediatric renal tumors: Results from the Dutch national cohort

“…In the past years, the SIOP-RTSG has developed a new flank target volume definition that adjusts for the postoperative changes and intra-fraction motion [17]. Combined with modern RT techniques, this new target volume allows highly conformal dose distributions that reduce the risk of late toxicity to the surrounding healthy tissue [18][19][20]. However, in these children, a potential underestimation of the area at risk may increase the number of locoregional recurrences compared to those treated with conventional flank RT.…”

The contribution of surgical clips for optimizing highly-conformal image-guided flank irradiation in pediatric renal tumors: A single center experience

“…For pediatric patients with a renal tumor treated with a dose distribution obtained by AP/PA photon beams, the contribution of surgical clips to demarcate the preoperative tumor borders has diminished since the implementation of 3D-image coregistration. When IGRT techniques will be used for flank irradiation, as recommended by the new SIOP-RTSG consensus, the postoperative shift of intraperitoneal organs has to be considered to allow sparing of normal tissue otherwise irradiated by a conventional AP/ PA dose distribution [15][16][17][18][19]. However, in patients with a tumor that shrouds most of the hemi-abdomen on preoperative imaging, it is difficult to reconstruct the retroperitoneal tumor bed postoperatively with imaging only.…”