ESTRO 2021 Abstract Book

S543

ESTRO 2021

inter-fractional respiratory changes are quantified using repeated 4D-CT scans during the treatment course. The dosimetric effect in terms of dose to target and organs at risks is determined for all phases of both planning 4D-CT and repeated 4D-CTs for both photon-based radiotherapy and proton therapy. The consequence of interplay effects for proton therapy is quantified by calculation of phase-specific doses that are summed deformably in a reference phase of the 4D-CT scan. Results: Respiratory motion and cardiac motion varies for the different target locations in esophageal cancer as illustrated in Figure 1. Inter- and intra-fractional motion is dominated by respiratory motion compared to cardiac motion and it is largest in the cranio-caudal direction. However, large differences are seen depending on the target location with largest movement in the proximal stomach. Treatment delivery techniques, matching strategies for daily setup, and margins should reflect the effect of respiratory and cardiac motion and additionally the different uncertainties for the varying anatomical locations. Soft-tissue matching can reduce the margins needed for inter-fractional motion by 2–3 mm compared to bony-anatomy matching. The dosimetric effect of respiratory motion and anatomical changes differs between photon and proton therapy and is for proton therapy very dependent on beam arrangements and optimization strategies as shown in Figure 2. When posterior beam arrangement is used for proton therapy, this technique results in less deviation in target coverage compared to photon therapy. The interplay effect for proton therapy can be large for a single-fraction delivery, but is generally averaged out after a few weeks of treatment. Conclusions: Respiratory and cardiac motion may be determined from planning and weekly CT scans, daily CBCT scans, or kV fluoroscopy images. The intra-fractional motion is dominated by respiratory motion. Selection of posterior beam arrangements for proton therapy reduces intra-fractional motion and results in less deviations in target coverage compared to photon therapy.