ESTRO 2024 - Abstract Book

S3661

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

along the arc, reducing maximum leaf travel. This is generalizable also to non-coplanar VMAT, in which VMAT arcs are applied with a non-zero table rotation angle. In this work, dynamic collimator rotation is optimized for non coplanar VMAT plans created using an in-house optimizer with direct aperture optimization. The influence of dynamic collimator rotation is systematically investigated for an academic and three clinically motivated cases.

Material/Methods:

Two full VMAT arc plans with static collimator angle or dynamic collimator rotation are created for a cylindrical phantom with a C-shaped planning target volume (PTV) encompassing an organ at risk (OAR). Four scenarios are created: the PTV and OAR structures are tilted by an angle of 0°, 15°, 30°, and 45° and treatment plans are created for each scenario. Static collimator angle plans are created for every combination of collimator angle settings from 2° to 172° in steps of 10° for the two arcs. For the dynamic collimator rotation, the collimator angle is aligned to the axis of minimum inertia of the PTV. On top of the dynamic collimator rotation, additional collimator angles are added. For the two arcs, the same combinations of angles as above are added as additional collimator angles. For a bilateral head and neck, a breast, and a prostate case, two-arc (non-coplanar) VMAT plans are created for each of the table rotation angles of -30°, -15°, 0°, 15° and 30° once with static collimator angles of 2° and 92° and once with dynamic collimator rotation for one arc and an offset of 90° on the dynamic collimator rotation for the second arc. The plans were optimized with the same case-specific objectives and were evaluated based on the objective function value at the end of the optimization.

Results:

For the static collimator angle plans in the 0° and 15° tilted scenarios, the lowest objective function value was observed for the combination of 2° and 92° collimator angle. However, in the 30° and 45° scenarios, there was not such a clear trend. On the contrary, dynamic collimator plans with the lowest objective function value always were with additional collimator angles of 2° and 92°. Comparing the lowest objective function values, the values were lower in the dynamic collimator plans compared to the static collimator plans by 0% (0° tilt), 26% (15° tilt), 32% (30° tilt), and 43% (45° tilt). In Figure 1, the objective function values of the plans are illustrated for the 30° tilted scenario.