Abstract
Radiotherapy treatment plans using dynamic couch rotation during volumetric modulated arc therapy (DCR-VMAT) reduce the dose to organs at risk (OARs) compared to coplanar VMAT, while maintaining the dose to the planning target volume (PTV). This paper seeks to validate this finding with measurements. DCR-VMAT treatment plans were produced for five patients with primary brain tumours and delivered using a commercial linear accelerator (linac). Dosimetric accuracy was assessed using point dose and radiochromic film measurements. Linac-recorded mechanical errors were assessed by extracting deviations from log files for multi-leaf collimator (MLC), couch, and gantry positions every 20 ms. Dose distributions, reconstructed from every fifth log file sample, were calculated and used to determine deviations from the treatment plans. Median (range) treatment delivery times were 125 s (123–133 s) for DCR-VMAT, compared to 78 s (64–130 s) for coplanar VMAT. Absolute point doses were 0.8% (0.6%–1.7%) higher than prediction. For coronal and sagittal films, respectively, 99.2% (96.7%–100%) and 98.1% (92.9%–99.0%) of pixels above a 20% low dose threshold reported gamma <1 for 3% and 3 mm criteria. Log file analysis showed similar gantry rotation root-mean-square error (RMSE) for VMAT and DCR-VMAT. Couch rotation RMSE for DCR-VMAT was 0.091° (0.086–0.102°). For delivered dose reconstructions, 100% of pixels above a 5% low dose threshold reported gamma <1 for 2% and 2 mm criteria in all cases. DCR-VMAT, for the primary brain tumour cases studied, can be delivered accurately using a commercial linac.
Highlights
2.1 Beam modelling A TrueBeam (Varian Medical Systems, Sunnyvale, CA) linac was modelled in the Pinnacle3 (v9.10, Philips Medical, Madison, WI) and in-house AutoBeam (Bedford 2009, 2013) treatment planning systems (TPS)
TrueBeam is the only currently available machine capable of delivering dynamic couch rotation treatment plans, in principle DCRVMAT can be delivered on any VMAT capable C-arm linac
Absolute point dose measurements and relative output factors of jaw-defined fields used for TrueBeam model validation were within ±1% of predictions
Summary
Radiotherapy treatment plans that use dynamic couch rotation during volumetric modulated arc therapy (DCR-VMAT) improve organ at risk (OAR) sparing over coplanar VMAT, while maintaining planning target volume (PTV) dose coverage (Podgorsak et al 1988, Krayenbuehl et al 2006, Shaitelman et al 2011, Yang et al 2011, Popescu et al 2013, Smyth et al 2013, Fahimian et al 2013, MacDonald and Thomas 2015, Wild et al 2015, Papp et al 2015, Liang et al 2015, Smyth et al 2016, Wilson et al 2017, Langhans et al 2018, Lyu et al 2018, Dong et al 2018, Fix et al 2018, Smyth et al 2019). Clinical adoption of DCR-VMAT would be limited if its delivery is significantly slower than coplanar VMAT and requires patient treatment appointments to be extended beyond their current duration
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