ESTRO 2021 Abstract Book

S264

ESTRO 2021

shown to have larger dosimetric impacts on both target and OARs (fig 2). This indicates that 4D dose reconstruction based on a pre-treatment 4DCT alone might underestimate the dose distortion.

Conclusion A surrogate driven, patient specific motion model based on simultaneous 4DCT and OTS acquisitions has been successfully developed and assessed. This can estimate deformable motion from the real-time motion surrogate acquired on treatment day, enabling incorporation of online motion irregularity into the 4D dose reconstruction workflow. OC-0361 Treatment planning and 4D robust evaluation for proton therapy of lung tumors with large motion V. Taasti 1 , D. Hattu 1 , F. Vaassen 1 , R. Canters 1 , M. Velders 1 , J. Mannens 1 , J. van Loon 1 , I. Rinaldi 1 , M. Unipan 1 , W. van Elmpt 1 1 Department of Radiation Oncology (MAASTRO), GROW – School for Oncology, Maastricht University Medical