Cone beam CT verification for active breathing control (ABC)-gated radiotherapy for lung cancer

Radiotherapy dose escalation improves patient outcome in non-small cell lung cancer (NSCLC) [1–5]. To achieve this, motion compensation techniques are often used, e.g. active breathing control (ABC) which allows for relative tumour immobilisation during inspiration breath-hold [6–8]. Although both reduction in dose-volume parameters determining lung toxicity and good reproducibility have been demonstrated with ABC, inter-patient variability exists with significant tumour motion being reported. As a result, image guidance and tumour-directed localisation have been recommended [9,10].
Image guidance with kilovoltage cone beam computerised tomography (CBCT) has been shown to provide accurate bony and soft tissue setup information [11,12]. Deep-inspiration breath-hold with CBCT is also feasible [8] and has been integrated as a verification tool for correction protocols when treating patients with ABC [13]. Current CB imaging software allows automatic matching within a user defined region of interest (ROI) but the selected ROI volume is important and can affect image registration accuracy [14–16]. For patients with lung tumours undergoing free-breathing radiotherapy, studies have shown that ROIs centred on the carina and spine can produce accurate CBCT image registration with advanced tumours [17] whilst soft tissue tumour matching is more accurate with early-staged tumours [18]. With ABC-gated radiotherapy, the optimal automatic registration method for image-guided setup correction has not yet been established. The aim of this study was therefore to evaluate the effect of using four different ROIs (with the Synergy XVI R4.2 software, Elekta, Crawley, UK) with ABC-gated radiotherapy in order to determine the optimum volume for registration.