Dosimetric Verification of Real-Time Anatomy-Based Multiplane Dual Target Gating on a 0.35T MRI Linac
Abstract
Purpose
This work aimed to verify the dosimetric accuracy of a 0.35T MRI Linac using all five available tracking algorithms (Standard, Rigid, Small Mobile, Large Deforming, Complex Mobile) when tracking multiple targets simultaneously.
Methods
Following a recent upgrade to a 0.35T MR-Linac, multiplane cine imaging and the ability to gate on multiple distinct objects simultaneously became available for real-time anatomy-based gating during clinical treatment. To verify the dosimetric accuracy of the system when tracking two independent targets across different cine planes, two MRI4D QUASAR (IBA, London, Ontario, Canada) motion phantom motors and inserts were used. Targets were created based on the two motion phantom inserts which could then be driven independently. Ion chamber (IC) measurements were recorded for both inserts using all 5 available tracking algorithms (Standard, Rigid, Small Mobile, Large Deforming, Complex Mobile) and were repeated using motion correction. IC results for all tests were compared against reference measurements without either insert moving. Radiochromic film measurements were likewise acquired using the Standard tracking algorithm with and without motion correction. Gamma analysis was performed between stationary and moving films using an in-house developed MATLAB script. “Beam on Latency” was set to 1 frame to ensure that both targets were within their respective gating boundary prior to the system producing beam.
Results
All IC results were within 1.88% of the reference measurements with no significant differences found between tracking algorithms or the use of motion correction. Film measurements were within 99.90% using a 2%2mm local gamma criterion with a 10% threshold.
Conclusion
We verified the dosimetric accuracy of multiplane dual target gating on a 0.35T MRI Linac for all tracking algorithms both with and without motion correction. All measurements were within expected tolerances.