Evaluation of Accumulated Dose In MRI-Guided Linac Adaptive Treatment Using Deformable Image Registration In Pancreatic Cancer
Abstract
Purpose
MRI‑guided linear accelerators (MR‑Linacs) enable real‑time adaptive radiation therapy by allowing daily plan adaptation based on anatomical changes. In pancreatic cancer, substantial interfraction motion and proximity to gastrointestinal organs at risk (GI OARs) make accumulated dose evaluation crucial. This study assessed the delivered cumulative dose across a 5‑fraction stereotactic body radiation therapy (SBRT) regimen using deformable image registration (DIR) to estimate the true delivered dose distribution.
Methods
We analyzed plans of five pancreatic cancer patients treated with 5 fractions of 10 Gy using an MR‑Linac adaptive workflow. The gross tumor volume (GTV) was expanded by 3 mm to generate the PTV, and PTV_OPT was created by subtracting the PTV from a 3 mm expansion of relevant OARs. OAR constraints followed the SMART protocol, including a limit of 33 Gy to less then 0.5 cc of any GI OAR. A new MRI was acquired at each fraction, and plans were adapted daily. After treatment completion, each daily MRI was deformably registered to the planning MRI using Velocity 3.2.1. Fraction doses were resampled and accumulated for analysis of target coverage and OAR constraints.
Results
Adaptive planning was primarily driven by OAR sparing, which in some cases reduced target coverage. DIR‑based summation across 5 fractions resulted in GTV coverage of 59.96%, 99.99%, 74.72%, 96.05%, and 98.26% to 45.7Gy for the five patients. Corresponding PTV_OPT coverage of 47.5Gy was 74.87%, 87.67%, 60.51%, 90.69%, and 78.65%. Accumulated dose evaluation indicated that selected GI OAR constraints were exceeded in several patients, despite meeting constraints during daily planning.
Conclusion
Because MR‑Linac systems currently lack integrated dose accumulation, off‑line DIR‑based dose summation provides an important estimate of the delivered dose. Fraction‑by‑fraction dose accumulation may improve understanding of OAR dose exposure and guide adaptive decisions in subsequent fractions, potentially improving treatment safety and accuracy in pancreatic SBRT.