Abstract
Background and purposeFor tumor tracking, a correlation model is used to estimate internal tumor position based on external surrogate motion. When patients experience an internal/external surrogate drift, an update of the correlation model is required to continue tumor tracking. In this study, the accuracy of the internal tumor position estimation for both the clinical available update at discrete points in time (rebuild) and an in-house developed non-clinical online update approach was investigated. MethodsA dynamic phantom with superimposed baseline drifts and 14 SBRT patients, treated with real-time tumor tracking (RTTT) on the Vero system, were retrospectively simulated for three update scenarios, respectively no update, clinical rebuild and 0.5Hz automated online update of the correlation model. By comparing the target positions based on 0.5Hz verification X-ray images with the estimated internal tumor positions regarding all three update scenarios, 95th percentile modeling errors (ME95), incidences of full geometrical coverage of the CTV by a 5mm extended PTV (P5mm) and population-based PTV margins were calculated. Further, the treatment time reduction was estimated when switching from the clinical rebuild approach to the online correlation model update. ResultsFor dynamic phantom motion with baseline drifts up to 0.4mm/min, a 0.5Hz intra-fraction update showed a similar accuracy in terms of ME95 and P5mm compared to clinical rebuild. For SBRT patients treated on Vero with RTTT, accuracy was improved by 0.5Hz online update compared to the clinical rebuild protocol, yielding smaller PTV margins (from 3.2mm to 2.7mm), reduced ME95,3D (from 4.1mm to 3.4mm) and an increased 5th percentile P5mm (from 90.7% to 96.1%) for the entire patient group. Further, 80% of treatment sessions were reduced in time with on average 5.5±4.1(1SD)min. ConclusionWith a fast (0.5Hz) automated online update of the correlation model, an efficient RTTT workflow with improved geometrical accuracy was obtained.
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