ESTRO 2020 Abstract book

S913 ESTRO 2020

left-sided breast cancer patients with an indication for locoregional irradiation. Material and Methods Clinical Target Volumes (CTVs) (left-sided breast, level 1 to 4 axillary, interpectoral and internal mammary lymph node (LN) regions) and organs at risk (heart, lungs and right breast) were delineated on 4D-CTs of ten female thoracic patients. CTVs of the level 1-4 axillary lymph nodes, interpectoral lymph nodes and internal mammary lymph nodes were combined in CTV elective . Two beams were used in a pencil beam scanning set-up. Beam angles were chosen to be the most favorable for each patients´ anatomy and were either a combination of 50 and 350 degrees or 0 and 45 degrees. After treatment planning to a prescribed dose of 40.05 Gy (RBE) in 15 fractions was made on the time-averaged CT, the dose was calculated with Monte Carlo on all ten phases of the breathing cycle and the robustness to setup and range errors (5mm and 3%, respectively) was evaluated for those ten phases. Correlations were evaluated between the phases of the breathing cycle and the D98% of the CTV and the D mean of the heart. Results Eight out of thirty of the correlation coefficients were statistically significant: 0.12 and -0.12 for D98% of the CTV breast of patient 2 and 7, respectively; 0.20, 0.29, 0.13 and 0.2 for D mean of the heart of four patients. At the most extreme values of the 28 robustness scenarios, the clinical goals are met for all but two patients (see Table). For the first patient, the maximum values of the D2% for the CTV elective exceeded the prescribed dose by 7.7% for all phases of the breathing cycle. For the second patient, D mean of the heart was 1.02 Gy and a strong dependence of D mean on breathing phases was observed, probably because of a large heart movement with the breathing cycle. For this patient, the amplitude of the breathing motion was 1.4 and 1.9 mm for mid-sternum and nipple, respectively. In Fig. 1, the dose distribution and the result of the robustness analysis with their correlation analyses for patient 4 , where every dot represents one scenario, are shown.

Conclusion The effect of breathing motion on the robustness of proton therapy treatment plans for this patient group is minor and not of clinical significance. The mean heart dose was 0.41 Gy(RBE) with a standard deviation of 0.31 Gy(RBE) of proton therapy plans. Therefore, a deep-inspiration breath hold is not required to improve robustness for these patients . PO-1589 Comparison of diaphragm motion amplitude during free versus regularized breathing measured with MRI Z. Van Kesteren 1 , M.J. Parkes 2 , M.F. Stevens 3 , P. Balasupramaniam 4 , J.G. Van den Aardweg 5 , G. Van Tienhoven 1 , A. Bel 1 , I.W.E.M. Van Dijk 1 1 Amsterdam UMC - location AMC, Department of Radiotherapy, Amsterdam, The Netherlands ; 2 University of Birmingham, School of Sport- Exercise & Rehabilitation Sciences, Birmingham, United Kingdom ; 3 Amsterdam UMC - location AMC, Department of Anesthesiology, Amsterdam, The Netherlands ; 5 Amsterdam UMC - location AMC, Department of Pulmonology, Amsterdam, The Netherlands Purpose or Objective 4D imaging is current practice in radiotherapy of the upper abdomen, as it depicts the target motion during breathing. Motion due to breathing can be accounted for during treatment by applying treatment margins. However, larger margins inevitably means irradiating more healthy tissue. Also, motion in free breathing (FB) is highly irregular and not reproducible over multiple treatment fractions. Applying non-invasive ventilation in unsedated subjects may enable regularized (rapid shallow) breathing with smaller tidal volumes, hence smaller respiratory motion. In this study, we investigated amplitude and reproducibility of the right hemidiaphragm motion between FB and regularized breathing (RB) induced by non-invasive ventilation. Material and Methods Six healthy volunteers underwent two magnetic resonance imaging (MRI). Prior to these sessions, the volunteers were introduced to non-invasive ventilation and were trained in letting the machine dictate their breathing pattern at 22 breaths per minute. 4DMRIs were obtained by acquiring a T2W turbo-spin echo scan, resolution (1.3x1.6x5 mm 3 ). Prior to each coronal slice acquisition, the position of the right hemidiaphragm was determined using a 1D navigator. The 2D slices were sorted into 10 amplitude bins after outlier rejection (discarding images corresponding to the

Table: Nominal dose parameters for all ten patients

Figure 1 Dose distribution (a) and robustness analysis results (b) for patient 4.