ESTRO 2020 Abstract Book

S774 ESTRO 2020

CK plan characteristics, i.e. several non-coplanar beams and mostly 1-2 segments per beams. For MCS, two different solutions (a-b) were examined. By definition MCS and LG values decrease with increasing modulation, whereas the relation is reversed for EM, PI, and PM. All Modulation indices were computed over a total of 80 clinically acceptable plans, created for Liver, Pancreas, Prostate and Spine SBRT. Among these plans, 24 Sequential and 24 VOLO plans, created for the same cases using the same clinical protocols, were compared in terms of modulation complexity. Pearson’s r was used to explore dependencies between modulation indices over the totality of VOLO plans (56). Relationships of each metric with PSQA gamma passing rates for 32 plans were also analysed. Correlation was regarded as weak for absolute r values in the range 0.2 - 0.39, moderate 0.4 - 0.59, strong 0.6 -0.79 and very strong 0.8-1. Results Average beam number per plan and segment number per beam were respectively 44.0 and 1.68. Mean values and standard deviations of modulation metrics over all treatment sites and for both optimizers are shown in table1. When compared to VOLO, Sequential plans exhibited a higher complexity showing lower MCS(a,b) and LG values and higher EM, PM and PI. Differences were significant for 4/5 metrics (Wilcoxon p<0.01). Among treatment sites, the lowest modulation degree was found for liver plans whereas the highest complexity was found for spinal plans. A very strong significant negative correlation (p<0.01) was observed between MCS(a,b) and PM, as well as between EM and LG (table2). (2%,1mm) gamma pass-rate (range 83%-99.1%) did not correlate significantly with any metrics and Pearson’s r absolute values were below 0.4.

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 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