ESTRO 2023 - Abstract Book

S828

Monday 15 May 2023

ESTRO 2023

Fig.1. Mean dose differences for OAR and target (sCT_orig-sCT_DL). A small systematic shift towards a positive dose difference was observed. This was probably due to the larger sCT_DL volume, providing more radiation attenuation. Conclusion DL MRI reconstruction was suitable for sCT generation with only minor, clinically negligible, differences in HU and calculated dose. The differences were systematic and probably due to the larger size of the sCT_DL, and believed to be the effect of improved MRI image quality, thereby affecting the sCT generation. PD-0980 Exploring conventionally unexploited degrees of freedom in treatment plan optimization N. Torelli 1 , J. Unkelbach 1 1 University Hospital Zurich and University of Zurich, Department of Radiation Oncology, Zurich, Switzerland Purpose or Objective In current clinical practice, a single treatment plan is optimized to be repeatedly delivered over the course of multiple fractions. In this work, we investigate the benefit on the quality of radiotherapy treatments of using different beam orientations in different fractions and allowing different dose distributions to be delivered in distinct fractions. Materials and Methods A direct aperture optimization (DAO) algorithm is developed, which combines a column generation-based method used to iteratively add apertures from promising beam directions to a treatment plan and a gradient-based DAO method used to adjust the shapes and intensities of all added apertures. The DAO algorithm simultaneously optimizes multiple non-uniform dose distributions to be delivered in different fractions (based on their cumulative biologically effective dose (BED α / β )), together with their corresponding set of aperture shapes and MU weights. Each set of apertures specifies a series of control points along a fraction-specific non-coplanar dynamic trajectory, which consists of a 360° gantry arc with dynamic bi directional couch rotation and is automatically determined during the treatment plan optimization. A 3-fraction SBRT plan is generated for a patient with four large liver metastases using the proposed planning approach (plan A) and benchmarked against a 3-fraction SBRT plan that delivers the same dose distribution in every fraction using coplanar VMAT arcs (plan B). Results Figure 1 illustrates the dose distributions obtained for both plans, together with the used beam configurations. Plan A delivers highly non-uniform dose distributions in different fractions, where each fraction treats complementary parts of the target volume to a high dose and at the same time distributes the integral dose more uniformly over the surrounding healthy liver (thereby exploiting fractionation effects). Depending on which parts of the target volume are treated in which fraction, different beam orientations are used in distinct fractions. For a similar tumor BED10 coverage in both plans (corresponding to a physical dose of 3 x 12 Gy), the mean liver BED3 is reduced from 47.3 Gy for plan B to 32.7 Gy for plan A (-31%). Both explored additional degrees of freedom improve the treatment plan quality (Figure 2). The mean liver BED3 is reduced by 16% compared to plan B for a coplanar VMAT plan generated using different non-uniform dose distributions in different fractions. A plan that utilizes different non-coplanar dynamic trajectories in different fractions, but delivers a near-uniform dose within the target volume in every fraction, instead, reduces the mean liver BED3 by 24% compared to plan B.