ESTRO 2021 Abstract Book

S1274

ESTRO 2021

Conclusion Robust treatment planning did not ensure target coverage in all patients, highlighting the need for adaptive strategies in head and neck patients. DR had the potential to reduce the adaptation rate by 50%, this could impact the clinical adaptive workflow by saving time and human resources. However, not all adaptations were systematically solved, rCTs still needed to be visually inspected to detect patients requiring full off-line adaptation, thus, fully-automation might not be possible for HN. PO-1550 Cone-beam computed tomography for direct dose calculation in online adaptive radiotherapy F. Kafil 1 , G. Torabi Kristensen 2 , P. Sibolt 2 1 Herlev and Gentofte hospital, Radiotherapy Research Unit, Department of Oncology , Herlev, Denmark; 2 Herlev and Gentofte hospital, Radiotherapy Research Unit, Department of Oncology , Herlev, Denmark Purpose or Objective Recent developments in artificial intelligence (AI) and automation has enabled fast solutions for online adaptive radiotherapy (oART). These include AI-driven deformable image registration between original planning computed tomography (pCT) to the daily cone-beam CT (CBCT), creating a synthetic CT (sCT) for automated plan re-optimization on the anatomy of the day. However, sCTs are inherently prone to uncertainties, both geometrical and related to assignment of Hounsfield units (HU). To minimize these uncertainties, this study assesses the feasibility of using CBCT for direct dose calculations in AI-driven CBCT- based oART. Materials and Methods In order to ensure consistency between CT and CBCT, HU to relative electron density (RED) calibration curves were acquired for the CT-specific Pelvis mode and CBCT Pelvis Large acquisition mode, using a dedicated electron density phantom. The CBCT images were iteratively reconstructed using the clinical (iCBCT_clin) as well as an improved reconstruction model in development mode with motion artifact reduction (iCBCT_dev). Using the acquired HU-RED calibration curves, dose calculations were performed on CT and CBCT scans of an anthropomorphic pelvic phantom. Furthermore, dose calculations were performed on