ESTRO 2023 - Abstract Book

S87

Saturday 13 May

ESTRO 2023

Conclusion Weight changes, that may occur between radiotherapy planning scan and actual treatment delivery, can significantly affect TS dose in patients undergoing spine SBRT. A linear equation can model these dosimetric variations and estimate TSmax in these circumstances guiding clinical decisions. OC-0125 Clinical evaluation of proton therapy of esophageal cancer using dose summation on weekly repeat CTs K. van der Klugt 1 , R. Canters 1 , V. Taasti 1 , J. Buijsen 1 , F. Vereijken 1 , T. Verstappen 1 , K. Limpens 1 , S. Hurkmans 1 , M. van den Bosch 1 , G. Vilches-Freixas 1 , M. Berbee 1 , F. Visser 1 1 GROW-School for Oncology, Maastricht University Medical Center, Department of Radiation Oncology (MAASTRO), Maastricht, The Netherlands Purpose or Objective In this study, we assessed the robustness of intensity modulated proton therapy (IMPT) in esophageal cancer for anatomical variations during the course of treatment. Moreover, we assessed the effect of the anatomical changes on actual dose coverage of the internal target volume (ITV) using robust dose summation on weekly repeat CTs (reCTs). Materials and Methods We included 60 esophageal cancer patients in this study, treated in our institute with neo-adjuvant (23x1.8Gy) (n=54) or definitive (28x1.8Gy) (n=6) chemo-radiotherapy. CTVs were defined on all phases of a 4D CT, and subsequently combined to an ITV. During treatment planning both a 3 mm isotropic margin around the ITV as well as 5mm setup/ 3% range uncertainty robustness were used (Figure 1). Patients were treated using 4, 3, or 2 beams IMPT (Figure 1). Each patient received weekly reCTs during treatment, on which the CTV and ITV were redelineated. Dose was recalculated on all reCTs and a robust dose summation was performed, using a 2mm/3% residual uncertainty. To assess the factors influencing ITV coverage, a multivariate linear regression analysis was performed using all parameters in table 1. Additionally, clinical treatment adaptations were evaluated. 0.10 Change in water equivalent depth of the CTV-heart interface in beam direction 0.002 Shift of the heart center of mass position 0.55 Heart volume change 0.13 Weight loss 0.02 ITV volume 0.10 ITV amplitdue 0.24 ITV amplitude change 0.17 Diaphragm amplitude 0.99 Diaphragm amplitude change 0.47 Diaphragm baseline shift <0.001 Air volume changes 0.74 Table 1: Multivariate regression analysis factors and results. Bold text indicates significant variables. Results On 89% of the reCTs, ITV voxelwise minimum V94% was larger than 98%, while in 92% of patients, the voxelwise minimum V94% of the summed dose was larger than 98%. A multivariate analysis showed that significant factors influencing ITV coverage are diaphragm baseline shift, weight loss, and the water equivalent depth of the ITV in the beam, direction (table 1, figure 2). Underdosage of the ITV on the reCT mainly occurred in the 1st and 4th week (16%, 7%, 6%, and 16% in week 1, 2,3 and 4 respectively). Seven treatment adaptations were performed because of underdosage, all of which occurred after the first reCT (figure 1). Factors in multivariate regression of ITV V94% p-value Number of beams 0.78 Weight of oblique beams