Real-time tumor tracking during VMAT radiotherapy treatments based on 2D/3D registration using CBCT projections

Rotational radiotherapy treatments such as volumetric modulated arc therapy (VMAT) can have superior overall quality while having shorter delivery time with respect to conventional intensity modulated radiotherapy (IMRT). As with conventional treatments, intrafractional tumor motion is a major source of uncertainty in dose application. To ensure full tumor coverage the planning target volume (PTV) margins are enlarged. Tumor tracking using intensity based 2D/3D registration can reduce uncertainty and would enable margin reduction. The challenge for full arc rotational treatments is the poor tumor visibility at certain irradiation angles. In this work we investigate the feasibility of tumor tracking in VMAT treatments with partial arcs where the tumor is well visible. For three patients, we determined for which subset of angles it was possible to track the tumor using raw projections from CBCT acquisition. We then generated VMAT plans with the obtained partial arcs. Full arc VMAT plans were used as benchmark. For all cases is was possible to track the tumor in two arcs of about 90 degrees, typically with imaging around anterior-posterior (AP) or posterior-anterior (PA) projections. The plans with the partial arcs were clinically comparable to the plans with full arc in terms of target coverage and OAR sparing. The results indicate the feasibility of partial arc VMAT treatments with tumor tracking. Based on this we will investigate generation of plans with reduced PTV margins. These treatments could be delivered by gating the LINAC beam if the tumor motion exceeds a certain margin.