Program Information
MLC Gamma Index: A New Method for the Evaluation of the Synchronization of Gantry Angle and MLC Leaf Movements
E Bangtsson , T Matzen*, ScandiDos, Uppsala, Sweden
Presentations
TU-C1-GePD-TT-3 (Tuesday, August 1, 2017) 9:30 AM - 10:00 AM Room: Therapy ePoster Theater
Purpose: When the MLC leaf edge positions and the corresponding gantry angles can be determined, it is possible to measure the synchronization between the aforementioned parameters. During a VMAT treatment the leaves may move fast between control points (up to 4 mm/degree at 1 RPM). Hence, a small gantry angle deviation (caused by uncertainties in the measurement and delivery) of about one degree can lead to significant, but probably irrelevant, deviations in the leaf edge position detection. Therefore, we introduce the new concept MLC Gamma Index which takes both leaf edge deviation and gantry angle deviation into account.
Methods: The MLC Gamma Index is calculated analogously to the dose deviation gamma index. The leaf edge deviation is computed for a multitude of gantry angle deviations. This gives a trajectory in the two dimensional leaf deviation-gantry angle deviation phase space. Said trajectory is normalized to user defined allowed deviations (for example 1mm/1deg). The MLC Gamma Index for the leaf in question is given by the smallest distance from the trajectory to the origin of the normalized coordinate system.
Results: For VMAT plans on two linacs (Elekta Agility MLC, Varian TrueBeam Millenium 120 MLC) measured with the upstream detector Delta4 Discover, the pass rate for the MLC Gamma Index (1mm/1deg) is higher than 95%. The pass rate for the leaf deviation (1 mm) is in the range 55%-65%, and for 5 mm deviation it is above 95%.
Conclusion: We can see that the apparently large measured leaf edge deviations originate in the uncertainty of the measurement of the gantry angle and/or the delivery thereof. These deviations give rise to false negative results. With the introduction of the composite property MLC Gamma Index we propose an intuitive and useful measure to reveal the false negatives as such.
Funding Support, Disclosures, and Conflict of Interest: The authors are employees of ScandiDos.
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