ESTRO 2020 Abstract book

S824 ESTRO 2020

all patients. RayStation TPS (RaySearch Medical Laboratories AB) was used to create both clinical and new plans. The new plans contain a dose fall-off objective for both heart and lung, instead of manually tweaking the maximal equivalent uniform dose (EUD) for these OARs. The beam optimization function available in the 3D-CRT module was used to automatically determine the optimal beam angles, whereafter these beam angles were used in IMRT plan optimization. Average OARs doses were obtained and cumulative risks of lung cancer and cardiac mortality at age 80 years were calculated for both plans. Mortality due to contralateral breast dose was considered to be negligible. The total cumulative mortality risk was defined as the sum of these two risks and was calculated for three patient scenario’s; 1) no cardiac risk factors and no smoker (C-L-), 2) cardiac risk factors and no smoker (C+L-) and 3) cardiac risk factors and smoker (C+L+). A Wilcoxon signed rank test was performed to investigate significance between doses to OARs and mortality risks in the scenario’s. Results Statistically significant differences were found in doses to OARs between the clinical and new plans, where the mean dose ± SD was 2.55 ± 0.68 and 2.15 ± 0.55 to the lungs (p < 0.001) and 1.26 ± 0.49 and 1.08 ± 0.45 to the heart (p < 0.001), respectively (Figure 1). All plans had adequate coverage (PTV V95≥97% and V107≤2%). For all three patient scenario’s, the cumulative mortality risk was significant lower for the new plans, as shown in Figure 2. The largest difference in cumulative mortality risk was observed for young patients (40 – 45 years) who are smoking, where it decreased from 3.24% ± 0.71% to 2.73% ± 0.60% (p < 0.001).

Conclusion For the first time several complexity indices were computed for CK MLC plans. CK plan characteristics required to adapt the computation of two metrics. The computed indices permitted to compare modulation complexity of plans created by two optimizing systems. Further data are needed to examine correlations between complexity metrics and PSQA pass-rates. This study set a basis to compare CK plans modulation in a multicentre, multi-platform context. PO-1452 The effect of automatic beam angle optimization and new objectives on OAR dose and mortality. N. Bakx 1 , H. Bluemink 1 , E. Hagelaar 1 , J. Van der Leer 1 , M. Van der Sangen 1 , J. Theuws 1 , C. Hurkmans 1 1 Catharina Ziekenhuis, Radiation Oncology, Eindhoven, The Netherlands Purpose or Objective Breast cancer radiotherapy plan optimization often involves manual steps and is usually performed based on a not patient specific set of objectives. However, it is shown that the effect of irradiation to OARs differs among women with/without cardiac risk factors and non- smokers/smokers, leading to a variation in the risks of cardiac mortality or mortality due to radiotherapy induced lung cancer [1, 2]. Choosing the appropriate beam angle and using the right optimization objectives are important aspects in reducing dose to OARs. Currently, this is done manually in our hospital. This study investigates the effect of automatic beam angle optimization and a set of new objectives on OAR dose and mortality. Material and Methods Forty consecutive patients, diagnosed with left sided, node negative breast cancer were included. Breath hold CT scans and clinical treatment plans were available for

Conclusion Making use of automatic beam angle optimization and new objectives significantly decreases the dose to OARs in the