TH-AB-303-06: Real-Time 4D Dose Reconstruction for Tracked Dynamic MLC Deliveries in the Presence of Respiratory Motion.

PURPOSE: This work provides a proof-of-concept study for real-time 4D dose reconstruction on 4DCT lung data treated with MLC tracking and its impact on the size of target margins.

METHODS: We have implemented online dose reconstruction by connecting our research platform for dynamic dose delivery to an Agility MLC at an Elekta Synergy linac with our in-house treatment planning software (TPS). Actual MLC apertures and (simulated) target positions are reported to the TPS every 40ms and 33ms respectively. The dose is then accumulated in real-time by utilization of pre-calculated dose-influence matrices based on a collapsed-cone algorithm. To investigate the impact of a potentially reducible safety margin we have created and delivered treatment plans designed for a conventional internal target volume (ITV)+5mm and for a tracking scenario of a lung patient. Both were planned to meet the criteria of RTOG 1021 (3-Fx, 9-beam). For the tracking plan, a maximum target volume (MTV)+5mm was established by delineating the gross target volume (GTV) on every 4DCT phase. These were rigidly aligned to the chosen reference phase, resulting in a unified maximum GTV to which a 5mm isotropic margin was added. The dose accumulation on a reference phase of the 4DCT (2×2×2mm(3)) was accomplished by the use of 3D deformation vector fields.

RESULTS: D95 for GTV on the reference phase was 98% of the RTOG prescribed dose for both the ITV+5mm plan and the MTV+5mm plan with tracking. V20 for ipsilateral lung and mean chest wall dose decreased by 10% and 12% respectively for the tracking scenario. Dose calculation and accumulation were performed in <30ms per MLC aperture.

CONCLUSION: For the patient data and motion conditions in this study, the reconstructed dose distributions show that target dose can be maintained using dynamic MLC tracking whilst the reduced margins yield lower doses to healthy tissue. We acknowledge support from Elekta AB under a research agreement. Research at The Institute of Cancer Research is also supported by Cancer Research UK under Programme C33589/A19727. We acknowledge NHS funding to the NIHR Biomedical Research Centre at The Royal Marsden and The Institute of Cancer Research.