ESTRO 2022 - Abstract Book

S414

Abstract book

ESTRO 2022

Results The key DVH parameters from the delivered dose relative to the planned dose are shown in fig. 2. Each point in the scatterplot represents a fraction, characterized by a cranio-caudal (CC) peak-to-peak respiratory amplitude and the maximum CC drift as measured during treatment. In more than 80% of the fractions, the measured D 99% of the GTV coverages was more than 95% of the planned D 99% . In only two fractions the actual D 99% of the GTV was lower than 90% of the planned D 99% due to a lateral 2.5 mm drift GTV drift during dose delivery towards areas with sharp dose gradients. The dose hotspots in the OARs remained below 110% of the planned D 0.5cc , as they were mostly resolved in the delivered dose maps due to the blurring effect of motion.

Conclusion We have successfully implemented a workflow that retrospectively calculates the delivered dose for abdominal MRgSBRT treatments based on the intrafraction motion. Analyses revealed that for the vast majority of fractions/patients the intrafraction motion observed during treatment delivery only modestly impacted the dose to the target and organs at risk. This increased our confidence that MRgSBRT can be safely executed for patients with abdominal tumors, potentially allowing dose escalation strategies. Furthermore, this workflow helps us to identify patients that, based on the intrafraction motion, would benefit from a re-assessment of the dose planning objectives/constraints for the remaining fractions if tolerances tend to be become exceeded.