Conformal and limited-modulation VMAT plans were produced, together with five hybrid IMRT/VMAT plans, in which 0%, 25%, 50%, 75% or 100% of the segments were sequenced for IMRT, while the remainder were sequenced for VMAT. Integrated portal images were predicted for the plans. The plans were then delivered as a single hybrid beam using an Elekta Synergy accelerator with Agility head to a water-equivalent phantom and treatment time, isocentric dose and portal images were measured.
Increasing the IMRT percentage improves dose uniformity to the planning target volume (p < 0.01 for 50% IMRT or more), substantially reduces the volume of rectum irradiated to 65 Gy (p = 0.02 for 25% IMRT) and increases the monitor units (p < 0.001). Delivery time also increases substantially. All plans show accurate delivery of dose and reliable prediction of portal images.
Hybrid IMRT/VMAT can be efficiently planned and delivered as a single beam sequence. Beyond 25% IMRT, the delivery time becomes unacceptably long, with increased risk of intrafraction motion, but 25% IMRT is an attractive compromise. Integrated portal images can be used to perform in vivo dosimetry for this technique.