ESTRO 2021 Abstract Book

S1258

ESTRO 2021

Digital Poster: Adaptive radiotherapy and inter-fraction motion management

PO-1534 Are interfraction variations more crucial than respiratory motion in lung SBRT? C. Huesa-Berral 1 , C. Juan-Cruz 2 , S. Van Kranen 2 , M. Rossi 2 , J. Belderbos 2 , J.D. Azcona 3 , J. Burguete 4 , J. Sonke 2 1 The Netherlands Cancer Institute - Radiation Oncology, University Of Navarra - Physics and Applied Mathematics, Amsterdam, The Netherlands; 2 The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands; 3 University of Navarra Clinic, Service of Radiation Physics and Radiation Protection, Madrid, Spain; 4 University of Navarra, School of Science, Physics and Applied Mathematics, Pamplona, Spain Purpose or Objective Respiratory motion and interfractional anatomical variations are sources of geometric uncertainty in lung stereotactic body radiation therapy (SBRT). The purpose of this study was to evaluate the separate and combined effects of respiratory motion and interfractional anatomical variations during the course of SBRT treatment. Materials and Methods Ten patients with peripheral early stage lung cancer treated with SBRT in three fractions (3 × 18 Gy or 3 × 15 Gy) and tumor peak-to-peak amplitude ≥ 1 cm were retrospectively selected for this study. The dose distribution (3D Ref ) was optimized on a 3D mid-position computed tomography (CT MidP ) derived from the 4D planning CT. Daily 4D cone beam CT (4D-CBCT) scans were acquired just prior to treatment for patient setup verification. To quantify the dosimetric impact of respiratory motion (4D Ref ) in the planning CT, the dose was recalculated in each phase of the 4D-CT and deformed back to the CT MidP by using deformation vector fields (DVF) derived from deformable image registration (DIR) between the 4D-CT and the CT MidP . Inter-fractional anatomical variations (3D Acc ) were evaluated by doing dose recalculation on a virtual CT (vCT) generated in the daily mid position derived from the daily 4D-CBCT (CBCT MidP ), using a CT MidP to CBCT MidP DIR. The dose was recalculated for each fraction and accumulated in the CT MidP . To determine the combined effect of respiratory and inter- fraction motion (4D Acc ), a vCT was created in each phase of the daily 4D-CBCT via CT MidP to 4D-CBCT DIR to recalculate dose. Subsequently, this dose was mapped back to the CT MidP and accumulated over the phases and ultimately over fractions. Dose differences between 4D Ref , 3D Acc and 4D Acc with 3D Ref were calculated per voxel in high gradient zones, as well as dose volume histogram (DVH) parameters for gross target volume (GTV) and organs at risks (OARs). Results Dose differences between 3D Ref – 3D Acc and 3D Ref – 4D Acc were similar: the difference in 3D Acc - 4D Acc was calculated and found negligible (mean = 0.28 Gy, SD = 0.93 Gy, IQR = 0.39 Gy) ( Fig 1 ). This was confirmed by DVH parameters calculated from 3D Acc - 4D Acc for GTV and OARs, where IQR for both were below 0.6 Gy ( Fig 2 ). The impact of 3D Ref – 4D Ref was smaller than 3D Ref – 3D Acc and 3D Ref – 4D Acc ( Fig 1) and it was also confirmed in terms of GTV and OARs ( Fig 2 ).

Figure 1: Dose differences between workflows implemented to quantify the effect of respiratory motion, daily anatomical variations and the combined effect of both on the treatment planning dose.