An MR-Based Method to Evaluate the Dosimetric Effect of Set Up Based On Stent Vs Soft Tissue Alignment in Pancreas SBRT
Z Han*, A Sudhyadhom, Y Hu, S Hsu, R Mak, M Huynh, R Van Dams, S Tanguturi, V Venkatachalam, J Leeman, Brigham & Women's Hospital and Dana Farber Cancer Institute, Harvard Medical School, Boston, MA
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PO-GePV-M-225 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: We aimed to develop an MR-based method to evaluate the dosimetric effect of set up based on stent vs soft tissue in pancreas SBRT. Due to the lack of soft tissue contrast, CBCT alignments in the treatment of pancreas SBRT are often dependent on fiducials, or stent in the absence of fiducials. However, there is no reliable way to study the dosimetric effect of using stent as a surrogate for alignment on regular linacs. The recently available MR linacs, which allow for soft tissue alignment, potentially can serve as a benchmark for such studies.
Methods: In this methodology study, we retrospectively examined two fractions for a stented pancreatic patient treated on our ViewRay MRIdian platform. We first generated predicted plans which took initial beam parameters and predicted dose distribution on treatment day anatomy using soft tissue for alignment. We then performed new image registrations to align stent to simulate the setup process on a regular linac. The derived additional shifts from soft tissue to stent were incorporated into the generation of new predicted plans. Feasibility of performing dosimetric comparisons of these plans was also examined.
Results: Both types of predicted plans had identical beam statistics and patient anatomy allowing for evaluation of dosimetric differences attributable solely to different alignment methods. Appearance of stent on MR was different than CT but aligning stent on MR was possible. Stent could be filled with air or liquid showing different MR contrast on different days. Dosimetric evaluation, including PTV coverage and OAR sparing, was feasible.
Conclusion: We demonstrated the feasibility of using MR based soft tissue alignment as a benchmark to study the dosimetric effect of set up using stent on regular linacs in pancreas SBRT. Applying this method to a sufficiently large cohort of patients is needed to draw statistically significant conclusions.
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