2505 - Inter-Fraction Dynamics and Dosimetric Assessment during MR-Guided Online Adaptive Radiotherapy for Glioblastoma

S. Ding¹, Y. Yin², H. Liu³, B. Liu¹, Y. Li³, B. Wang⁴, M. Chen¹, M. Liu¹, X. Huang¹, Y. Mu⁵, and Y. Chen⁶; ¹State Key Laboratory of Oncology in South China, Guangdong Provincial Clinical Research Center for Cancer, Department of Radiation Oncology, Sun Yat-sen University Cancer Center, Guangzhou, Guangdong, China, ²Department of Radiation Oncology,Affiliated Dongguan Hospital, Southern Medical University(Dongguan People’s Hospital), Guangzhou, China, ³State Key Laboratory of Oncology in South China; Collaborative Innovation Center for Cancer Medicine; Department of Radiation Oncology, Sun Yat-sen University Cancer Center., Guangzhou, China, ⁴Sun Yat-sen University Cancer Center, State Key Laboratory of Oncology in South China, Collaborative Innovation Center for Cancer Medicine, Guangzhou, China, ⁵Sun-Yat sen University Cancer Center, Guangzhou, China, ⁶State Key Laboratory of Oncology in South China, Sun Yat-sen University Cancer Center, Guangzhou 510060, P. R. China., Guangzhou, China

Purpose/Objective(s): This study aimed to assess the feasibility and potential benefits of MR-guided online adaptive radiotherapy (MRgOART) for patients with glioblastoma. Materials/

Methods: Twenty consecutive patients with glioblastoma were treated with MRgOART of 60 Gy in 30 fractions by the 1.5 T MR-Linac. The MRgOART fractions employed daily MR scans and the contours were utilized to create each adapted plan. The gross tumor volume (GTV) and clinical target volume (CTV) were delineated on MRI of pre-treatment simulation (Fx0) and all fractions (Fx1, Fx2, Fx3 ... Fx30) to evaluate the inter-fractional changes. These changes were quantified using absolute/relative volume (?V), Dice similarity coefficient (DSC) and Hausdorff distance (HD) metrics. The reference treatment plans were generated using step-and-shoot IMRT and utilized 7-9 beam groups on original CT. Before the treatment, a synthetic CT (sCT) quality assurance (QA) process was performed to assess the dose accuracy of bulk relative electron density (rED) assignment for online MRI based treatment plan in terms of gamma analysis, point dose comparison and dose volume histogram (DVH) parameters. Then, the online adaptative treatment plans were obtained by re-optimizing based on the contours on daily pre-treatment MRI by “adapt to shape” workflow. Non-ART plans for each patient were generated by recalculating the dose from the reference plans on daily online MRI by “adapt to position” workflow. GTV and CTV coverage and organ at risk (OAR) constraints were used to compare non-adaptive and adaptive plans.
Results: For both criteria, the 1%/1mm (98.58%±0.15%) and 2%/2mm (99.88%±0.18%) gamma passing rate results of plans on sCT were always clinically acceptable in sCT QA process. A total of 20 patients with 600 fractions were evaluated. The results showed that large inter-fractional changes for GTV limited the efficacy of radiation therapy (DSC: 0.78~0.9, HD: 5.9~19.3mm, ?V: -26.9%~9.71%). The inter-fractional CTV changes were smaller (DSC: 0.88~0.94, HD:4.9~9.6mm, ?V: -2.43%~1.29%). The changes of target were related to the gap between surgery and radiotherapy and the mode of operation (p < 0.001). GTV coverage of non-adaptive plans was below the prescribed coverage in 228/600 fractions (38%), with 90 (15%) failing by more than 10%. For CTV coverage of non-adaptive plans, the changes were less than 5%. Online adaptative plans improved GTV and CTV coverage significantly (p < 0.001) to 99%. The adaptive plans also had lower dose to whole brain than non-adaptive plans (p < 0.001).
Conclusion: Significant inter-fractional tumor changes could be found during radiotherapy in patients with glioblastoma treated by the 1.5 T MR-Linac. The changes of target were related to the gap between surgery and radiotherapy and the mode of operation. Daily MR-guided re-optimization of treatment plans corrected for day-to-day anatomical variations and resulted in adequate target coverage in all fractions.