ESTRO 2022 - Abstract Book

S191

Abstract book

ESTRO 2022

Conclusion The correlation between the external surrogate, which is assumed to be directly related to the diaphragm motion, and the internal tumour motion illustrated a moderate-high correlation for tumours proximal to the diaphragm. The correlation is low among tumours in the upper lobe of the lung.

PD-0231 Impact of respiratory motion for breast cancer proton therapy in free breathing

L.B. Stick 1,2 , M.F. Jensen 1 , C.J.S. Kronborg 1 , E.L. Lorenzen 3 , H.R. Mortensen 1 , P.W. Nyström 1,4 , S.E. Petersen 5 , P. Randers 5 , L.M.H. Thai 5 , E.S. Yates 6 , B.V. Offersen 5,6,7 1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 2 Rigshospitalet, University of Copenhagen, Department of Oncology, Copenhagen, Denmark; 3 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark; 4 Genetics and Pathology, Uppsala University, Department of Immunology, Uppsala, Sweden; 5 Aarhus University Hospital, Danish Centre of Particle Therapy, Aarhus, Denmark; 6 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 7 Aarhus University Hospital, Department of Experimental Clincial Oncology, Aarhus, Denmark Purpose or Objective Today, proton therapy for breast cancer is mostly performed in free breathing as breathing motion is assumed to have minor impact on the radiological path length when using an en face field arrangements. This study examines the effect of respiratory motion in patients with breast cancer receiving loco-regional proton therapy based on 4DCT scans. Materials and Methods Twenty-five patients with breast cancer (20 left-sided, five right-sided), treated at Danish Centre for Particle Therapy in 2019 and 2020, were included. Patients were immobilised in supine position with arms above the head. A planning CT scan in free breathing and a 4DCT scan, both with 2 mm slice thickness, were acquired for all patients on a Siemens Somatom Definition Edge scanner. The 4DCT scan was sorted in ten phases according to amplitude. Breast or chest wall, internal mammary nodes (IMN), interpectoral nodes and level 1-4 lymph nodes (level 1 irradiation was required for 15 patients) were contoured as clinical target volumes (CTVs) on the planning CT following the ESTRO guidelines. Relative biological effectiveness (RBE) was fixed at 1.1. Patients were prescribed either 40 Gy RBE in 15 fractions (nine patients) or 50 Gy RBE in 25 fractions (16 patients). Clinical planning objectives: CTV breast/chest wall V95% (relative volume receiving 95% of the prescribed dose) and CTV lymph nodes V90% should be at least 98%. Spot-scanning proton therapy plans using two to three en face fields, single-field optimisation and range shifter were created in Eclipse v13.7 (Varian Medical Systems). CTVs were robustly optimised using 14 scenarios: 0 mm ±3.5% and ±5 mm ±3.5%. The plan was recalculated on all ten phases of the 4DCT scan with fixed monitor units. All ten phases in the 4DCT scan were deformable image registered to the planning CT in Velocity v4.0 (Varian Medical Systems), and the deformed doses from the phases were accumulated on the planning CT. A paired, two-tailed Wilcoxon signed-rank test was used to compare dose metrics from the nominal plan and the 4D evaluation.