Evaluation of the Dose Distribution Robustness for Lung Tumor Tracking in Robotic Radiotherapy Using Four-Dimensional Computed Tomography

Respiratory motion of tumors and surrounding organs at risk (OARs) is a critical factor affecting dose distribution and treatment accuracy in radiotherapy. The CyberKnife Synchrony respiratory tracking system (Accuray Inc., Sunnyvale, CA, USA) improves targeting accuracy by estimating tumor position based on the correlation between implanted fiducial markers and external respiratory motion. However, in clinical practice, dose evaluation is commonly performed using computed tomography (CT) images acquired at a single respiratory phase, and the impact of respiratory phase variations on dose distribution has not been fully investigated. This study is a feasibility analysis aimed at demonstrating the potential utility of four-dimensional CT (4DCT)-based simulations for evaluating dose variations associated with respiratory phase differences during Synchrony-based treatments. Two patients with lung tumors treated using CyberKnife stereotactic radiotherapy (SRT) were included in the study. Treatment plans (reference plans) were created using breath-hold CT images acquired at the end-expiratory phase. Using the same beam parameters as the reference plans, dose calculations were performed on multiple respiratory-phase images derived from free-breathing 4DCT acquired during treatment planning. Dose-volume histogram (DVH) metrics, including clinical target volume (CTV) D99 and planning target volume (PTV) D95, as well as dose parameters for OARs, were compared with the clinical goals defined by the radiation oncologist. In Patient 1, dose reductions were observed in both the PTV and CTV at a specific inspiratory phase, resulting in the CTV dose falling below the prescribed level. In contrast, Patient 2 showed only minor dose reductions in the PTV and CTV, while prescribed dose coverage was maintained. No clinically relevant dose deterioration was observed for the OARs in either patient. The proposed evaluation method using 4DCT demonstrated the potential to identify the influence of respiratory phase variations on dose distribution in Synchrony-based respiratory tracking radiotherapy at the treatment planning stage. This approach may serve as a supplementary tool to conventional single-phase CT-based dose evaluation for assessing plan robustness against respiratory motion.