ESTRO 2020 Abstract Book

S86 ESTRO 2020

heart relative to the planning-CT during the DIBH course was: X= -0.82 mm (standard deviation (SD) 6.5 mm); Y= +0.15 mm (SD 3.8 mm) and Z= -0.47 mm (SD 6.18 mm). The total (3D) mean motion was: δ tot = 0.96 mm (SD 4.8 mm). The displacements did statistically not significantly differ from the reference heart position on the simulation Ct- scan.

Conclusion Our study demonstrated superior 5-year DFS in patients stage I-IIA BC treated with BCT compared to mastectomy. Improved outcome was observed regardless of age, overall stage, comorbidity, intrinsic subtype, multicentricity and receipt of chemotherapy. Our data adds to previous research showing a benefit of BCT when compared to mastectomy in patients suitable for both treatments. PD-0177 Interfraction heart motion during Deep Inspiration Breath Hold (DIBH) radiotherapy measured by CBCT. S. Benkhaled 1 , D. Van Gestel 1 , A. Desmet 1 , D. Rodriguez 2 , Y. Jourani 2 , A. De Caluwé 1 1 Institut Jules Bordet-Université Libre de Bruxelles, Department of Radiation-Oncology, Bruxelles, Belgium ; 2 Institut Jules Bordet-Université Libre de Bruxelles, Department of Medical-Physics, Bruxelles, Belgium Purpose or Objective Deep Inspiration Breath Hold (DIBH) decreases the radiation dose to the heart, an important goal in left breast radiotherapy (RT). This study aimed to quantify the inter-fractional heart motion (3D) of DIBH during RT as measured by daily CBCT (Cone-Beam-Computed- Tomography), the hypothesis of the study being that patients might perform breath hold differently during their RT course. DIBH might improve during the course with the heart moving further away from the chest wall as the patient gains experience in performing DIBH, as well as it can deteriorate due to fatigue or lack of patient cooperation and motivation. This study is one of the first to look at 3D movement of the heart using daily CBCT acquired in DIBH at every treatment fraction. Material and Methods Fifteen consecutive DIBH treatments performed by tracking the respiratory cycles using a surface-guided- radiotherapy system were retrospectively analyzed. Target and organs at risk were delineated as per ESTRO guidelines. The prescribed dose was 40 Gy in 15 fractions. At the pre-RT consultation, we recommended our patients to practice apnea in order to increase the DIBH efficiency and endurance. After each delivered fraction, RT- technologists gave a feedback to the patient on their DIBH. Daily CBCT was acquired and retrospectively 3- dimensionally co-registered by the same radiation oncologist to the DIBH planning-CT. Heart position on the DIBH planning-CT was considered as the reference position. Two different offline co-registrations were performed: 1) on the thoracic wall (X bones ,Y bones ,Z bones ) and 2) on the heart (X heart ,Y heart ,Z heart ) to calculate heart displacement relative to the thoracic wall. The total 3D heart motion was calculated as [δ x = (√[X bones -X heart ] 2 + [Y bones -Y heart ] 2 + [Z bones -Z heart ] 2 )]. Results Two hundred twenty-five CBCTs from 15 patients were analyzed. High intra and inter-individual heterogeneity was observed. The mean additional displacement of the

Figure 1. Fusion of axial (A,C) and coronal (B,D) Ct-slices from the same level in DIBH planning CT (pink) and : the first (A,B) and the last (C,D) CBCT (green), showing inter- fractional heart motion in one patient.

Figure 2. Box plot showing mean heart displacement (mm) of the fifteen patients from the chest wall on each axis (X;Y;Z). Conclusion Few studies examined the 3D movement of the heart during a course of DIBH-RT using daily CBCT. During the course of DIBH, no significant inter-fractional heart displacement has been observed. However, we found a high intra- and inter-individual heterogeneity of the heart position. RT technologist should evaluate the heart position on the daily CBCT and give good feedback after