Feasibility of Four-dimensional Adaptation of Volumetric Modulated Arc Therapy Based on Volumetric Modulated Arc Therapy-computed Tomography.

Purpose: Volumetric modulated arc therapy (VMAT) has been increasingly used for cancer patients due to the fast delivery and improved dose conformity. Adaptive radiotherapy (ART) can significantly decrease dose to normal tissues and allow for dose escalation. However, current imaging techniques cannot provide four-dimensional (4D) patient anatomy or dose information during VMAT, which is critical for ART that involves respiratory motion. A novel imaging tool named VMAT-computed tomography (VMAT-CT) has the potential to reveal intra-fractional patient information. The goal of this study was to evaluate the feasibility of 4D adaptive VMAT based on 4D VMAT-CT.

Materials and methods: A commercial QUASAR respiratory phantom and an in-house deformable lung phantom were used in this study, and lung VMAT plans, including 4D union plan and 4D ART plan, were generated for the phantoms. A real lung patient's plan was also used in this feasibility study. ART plans based on 4D VMAT-CT were created for the phantoms and the real patient when planning goals were not met. Dose escalation plan based on 4D VMAT-CT was also created for the real patient.

Results: Planning target volume (PTV) coverage for the QUASAR phantom was 85.5% after breathing pattern being changed, and went up to 95% after adaptive re-planning. PTV coverage for the deformable phantom was 93% after deformation and breathing pattern being changed, and went up to 95% after re-planning. Re-planning and dose escalation were feasible and can spare normal tissues for the real patient. 4D ART plan based on 4D VMAT-CT required smaller margins than 4D union plan while maintaining the same prescription dose coverage.

Conclusions: ART based on 4D VMAT-CT is feasible and would potentially facilitate re-planning and PTV dose escalation for VMAT patients who have the motion issue.