ESTRO 2022 - Abstract Book

S334

Abstract book

ESTRO 2022

pCT is on average -0.3% (±0.8%), which is considered to be the golden standard. Only the orgCBCT-pCT deviates significantly (p<0.001) over all dose statistics compared to the golden standard, resulting in 1.6%(±0.9%) average dose deviation. The mean heart dose is similar for all image sets and varies at max 7 cGy, which corresponds to a min-max change in risk of an ACE by 0.01%-0.06%.

Conclusion The conversion correction, deep learning correction and virtual correction can be applied for an accurate synthetic CBCT dose evaluation for photon radiotherapy breast cancer patients as there is no statistical difference relative to the golden standard. Future work is warranted for breast proton therapy.

PD-0396 Robust dose summation to evaluate diaphragm motion impact in proton therapy of esophageal cancer

R. Canters 1 , V. Taasti 2 , G. Vilches-Freixas 3 , F. Vaassen 2 , K. Van der Klugt 2 , M. Berbee 2

1 GROW School for Oncology, Maastricht University Medical Center+, Department of Radiation Oncology (MAASTRO), , Maastricht, The Netherlands; 2 GROW School for Oncology, Maastricht University Medical Center+, Department of Radiation Oncology (MAASTRO), Maastricht, The Netherlands; 3 GROW School for Oncology, Maastricht University Medical Center+, Department of Radiation Oncology (MAASTRO), Maastricht, The Netherlands Purpose or Objective Diaphragm motion can be a significant factor influencing target coverage in irradiation of esophageal tumors. Proton therapy is potentially more sensitive to anatomical variations than photon radiotherapy. In this study we evaluated the dosimetric quality of proton treatments, taking into account the patient anatomy changes during the treatment course and the correlation with diaphragm movement changes. Materials and Methods In this study, 35 patients treated with proton therapy for esophageal cancer using a 23x1.8Gy fractionation scheme were evaluated. All patients received a 4D CT planning CT (pCT), as well as weekly repeat 4D CTs (reCT). The clinical target volume (CTV) was delineated on each phase of the pCT, and the CTVs were combined to an internal target volume (ITV). Three posterior beams with a separation of 20 degrees were used, with in some patients an additional fourth beam from anterior direction. Dose calculations were performed on the average CT. The proton plans were robustly optimized with 5mm setup uncertainty and 3% range uncertainty, combined with a fixed margin of 3mm around the ITV. During treatment, all plans were re-evaluated on the reCTs using a robust evaluation with 2mm setup uncertainty and 3% range uncertainty, accounting for residual errors. If necessary, plans were adapted based on evaluation on the reCT. After deformable