Optimizing MR Sequences to Visualize Adrenal, Pancreas, Rectum and Liver Tumors for MR-Guided Online Plan Adaptation On Elekta Unity MR-Linac
D Lee1, P Renz1, A Kirichenko1, S Oh1, M Hwang1*, D Pavord1, J Sohn1, (1) Allegheny Health Network, Pittsburgh, PA
Presentations
PO-GePV-M-254 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
ePoster Forums
Purpose: Tumor visualization is essential for fast and precise target contouring during MR-guided online plan adaptation. Thus this study proposed an MR sequence and modified Elekta pre-set MR sequences for improving tumor visualization on Elekta Unity MR-Linac.
Methods: Cancer patients of adrenal, pancreas, rectum and liver underwent a 3-5 fractionated online adaptive stereotactic body radiotherapy which includes MR scans for tumor localization. Prior to MR scans, three MR sequences were optimized; (1) T1-weighted (T1W) for adrenal and pancreas with/without node, (2) T2 for rectum and prostate, and T2 with navigator (T2+Nav) for liver. Elekta pre-set T2 and T2+Nav sequences were modified to modT2 and modT2+Nav for improving tumor visualization. In each fraction, one of T1W, modT2 and modT2+Nav sequences, depending on treatment site, was utilized to acquire a planning MR image set to localize target in the presence of tumor shape and position changes. All tumors were contoured on the planning MR image set for further treatment.
Results: All tumors on T1W, modT2 and modT2+Nav image sets were clearly localized for online plan adaptation. Tumor motion was averaged on T1W image sets, resulted in dark boundary tumor and slightly smaller size tumor so tumor contour included dark area. Superior contrast between tumor and nearby organs on modT2 and modT2+Nav was very useful for fast (i.e., an average of 5 mins [2 mins to 9 mins] depending on tumor size and location) and precise target localization.
Conclusion: This study successfully utilized a unique T1W sequence which reflects average tumor motion for Adrenal and Pancreas tumors, and modT2 and modeT2+Nav sequences for rectum and liver, respectively. Thus, this can be a preferable tumor visualizing technique for achieving fast online treatment plan adaptations with more accurate MR-guided liver SBRT. Our approach can also provide freedom of MR sequences.
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