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Time‐resolved dose distributions to moving targets during volumetric modulated arc therapy with and without dynamic MLC tracking
Author(s) -
Ravkilde Thomas,
Keall Paul J.,
Grau Cai,
Høyer Morten,
Poulsen Per R.
Publication year - 2013
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4826161
Subject(s) - multileaf collimator , imaging phantom , tracking (education) , dosimeter , nuclear medicine , dosimetry , collimator , linear particle accelerator , biomedical engineering , computer science , medicine , physics , beam (structure) , optics , psychology , pedagogy
Purpose: The highly conformal doses delivered by volumetric modulated arc therapy (VMAT) may be compromised by intrafraction target motion. Although dynamic multileaf collimator (DMLC) tracking can mitigate the dosimetric impact of motion on the accumulated dose, residual errors still exist. The purpose of this study was to investigate the temporal evolution of dose errors throughout VMAT treatments delivered with and without DMLC tracking.Methods: Tracking experiments were performed on a linear accelerator connected to prototype DMLC tracking software. A three‐axis motion stage reproduced representative clinical trajectories of four lung tumors and four prostates. For each trajectory, two VMAT treatment plans (low and high modulation) were delivered with and without DMLC tracking as well as to a static phantom for reference. Dose distributions were measured continuously at 72 Hz using a dosimeter with biplanar diode arrays. During tracking, the MLC leaves were continuously refitted to the 3D target position measured by an electromagnetic transponder at 30 Hz. The dosimetric errors caused in the 32 motion experiments were quantified by a time‐resolved 3%/3 mm γ ‐test. The erroneously exposed areas in treatment beamˈs eye view (BEV) caused by inadequate real‐time MLC adaptation were calculated and compared with the time‐resolved γ failure rates.Results: The transient γ failure rate was on average 16.8% without tracking and 5.3% with tracking. The γ failure rate correlated well with the erroneously exposed areas in BEV (mean of Pearson r = 0.83, p < 0.001). For the final accumulated doses, the mean γ failure rate was 17.9% without tracking and 1.0% with tracking. With tracking the transient dose errors tended to cancel out resulting in the low mean γ failure rate for the accumulated doses.Conclusions: Time‐resolved measurements allow pinpointing of transient errors in dose during VMAT delivery as well as monitoring of erroneous dose evolution in key target positions. The erroneously exposed area in BEV was shown to be a good indicator of errors in the dose distribution during treatment delivery.