SU-E-T-622: A Rapid Hybrid VMAT-IMRT Planning Method Using an Abbreviated Beam Angle Optimization Search

To generate hybrid VMAT-IMRT treatment plans by utilizing an IMRT beam-angle-optimality (BAO) search under a commercially available TPS without the aid of custom optimization software or high performance computing.

The high dose modulation provided by IMRT beams can be exploited to improve the quality of VMAT treatments. To achieve this, a VMAT treatment was created where the gantry pauses at predetermined angles to deliver IMRT segments. To determine IMRT BAO, an IMRT plan ('poly-IMRT') was made with many beams (>30) equally spaced around the patient. For practical reasons and to reduce the total time to approximately 1 hour, BAO was approximated by removing one beam out of the set and noting the new objective score. Determining this'score penalty' for each of the beams serves as a proxy for true BAO. The hybrid plan was created by combining the VMAT arc with a user-determined number of top-ranked beams from the poly-IMRT set. The BAO from this approach was compared with a more rigorous method ('VMAT+1'), in which a VMAT plan was optimized with 1 IMRT beam at various angles, allowing a direct determination of objective score versus gantry angle. The overall hybrid planning process was demonstrated by creating separate plans for a SBRT lung patient, with dose normalized to the limiting maximum aorta dose.

Large score penalties from poly-IMRT coincided with large score benefits from VMAT+1, indicating both methods identified the same optimal beams. The VMAT, IMRT, and hybrid plans delivered the prescription dose to 84.3%, 85.6% and 87.7% of the PTV and had homogeneity indices of 1.38, 1.41, and 1.32 respectively. Normal tissue doses were within 0.5%.

The presented method can create hybrid VMAT-IMRT plans which combine delivery efficiency with improved target coverage. The planning process takes about an hour using a standard TPS.