255 - Changes in High-Grade Glioma Target Volume and Prescription Dose Coverage between Postoperative MRI and MR-Simulation
Presenter(s)
S. Thrower1, W. Talpur2, M. Farhat2, K. A. Al Feghali3, B. Curl4, M. K. Rooney2, O. Haisraely2, W. Floyd5, K. K. Brock6, and C. Chung7; 1Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 2Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 3Department of Radiation Oncology, University of Texas MD Anderson Cancer Center, Houston, TX, 4The University of Texas, Austin, TX, 5Department of Radiation Oncology, MD Anderson Cancer Center, Houston, TX, 6Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 7Institute of Data Science in Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX
Purpose/Objective(s): Current practice guidelines for high grade glioma (HGG) radiotherapy (RT) require that target volumes be delineated on post-contrast T1 and T2 MRI. Ideally, the MRIs used for target delineation are taken at the time of CT simulation for treatment planning (MR-Sim). However, many clinics elect to use post-operative MRI sequences (Postop-MRI) for target volume delineation due to difficulties with scheduling and lack of reimbursement. The purpose of this study is to determine whether using Postop-MRI for target delineation, instead of MR-Sim, significantly impacts gross tumor volume (GTV) and clinical target volume (CTV) definitions, and whether these differences meaningfully affect prescription dose (V100%) coverage.
Materials/Methods: We studied a retrospective cohort of 50 patients who received MR-Sim-based RT for HGG at our institution and had Postop-MRI available. Target volumes were contoured on the Postop-MRIs (PO-GTV, PO-CTV) by radiation oncologists. Planning target volumes were derived from a 3 mm expansion on the GTV (PO-PTV60) and CTV (PO-PTV50). RT plans were created based on the Postop target volumes, using a simultaneous integrated boost approach, with 60 Gy prescribed to the PO-PTV60 and 50 Gy to the PO-PTV50, while minimizing dose to normal tissues. Optimized plans were normalized to cover 95% of the PO-PTV60 with 60 Gy, to minimize the effects of planning differences. The dose distribution from the PO-based plan was evaluated on the MRSim-based target volumes (MRSim-GTV, MRSim-CTV), which were contoured for clinical use.
Results: The median (min, max) dice score was 0.74 (0.37, 0.92) between the PO-GTV and MRSim-GTV, and 0.86 (0.47,0.98) for the PO-CTV and MRSim-CTV. A Wilcoxon signed-rank test showed a statistically significant change in the V100% of the PO and MRSim targets (p < 0.0001). Out of 50 cases, when evaluating the PO-based dose, only 22 had a V100%> 98% for the MRSim-GTV, and only 22 had a V100%>98% for the MRSim-CTV. On average, the V100% dropped 7.9% (0%, 52%) and 6.3% (-3.4%, 55%) for the GTV and CTV, respectively. No correlation was found between the length of time from surgery to simulation and the magnitude of target volume changes.
Conclusion: This study demonstrates the value added by MR-Sim to the practice of RT for HGG. Our results support the importance of requiring universal MR-Sim-based treatment planning in future clinical trials, to ensure that the outcomes of the study are not due to insufficient coverage of the tumor volume due to changes between Postop-MRI and treatment planning at sites that do not have MR-Sim. We hope these results lead to clinical guidelines that strongly encourage, if not require, MR-Sim for HGG, and justify the coverage of MR-Sim by insurance companies.