ESTRO 2020 Abstract Book

S1046 ESTRO 2020

Conclusion Based on our analysis, the most appropriate sequences for planning PBT for head and neck tumours are a combination of T2W TSE mDixon and T1 SPIR with intravenous contrast. Considering clarity of named anatomical structures has facilitated an optimisation process of individual sequences. Using interdisciplinary VGA as an audit tool, we have implemented a continuous quality improvement (CQI) process for PBT treatment planning in head and neck cancers using MR. We anticipate this process will soon be implemented for other treatment sites at our institution. This philosophy of CQI will ensure sequence selection is evidence-based, the inclusion of appropriately selected sequences, and the removal of sequences felt to be less useful. PO-1877 Survival and dosimetric parameters in stage III NSCLC patients undergoing radical chemo- radiotherapy I. Remmerts De Vries 1 , M. Ronden 1 , P. De Haan 1 , F. Spoelstra 1 , N. Haasbeek 1 , M. Dahele 1 , I. Bahce 2 , S. Senan 1 , W. Verbakel 1 1 Amsterdam UMC, Radiotherapy, Amsterdam, The Netherlands ; 2 Amsterdam UMC, Lung, Amsterdam, The Netherlands Purpose or Objective The standard of care treatment of stage III NSCLC is concurrent chemo-radiotherapy (cCRT). In less fit patients or patients that refuse cCRT it is sequential chemo- radiotherapy (sCRT) or radiotherapy (RT) alone. We explored the relationship between dosimetric parameters and survival after radical treatment. The main endpoint was overall survival (OS), calculated from the last day of RT. Material and Methods Patients were considered eligible if they were treated for stage III non-small-cell lung cancer (NSCLC) between 2015- 2017 and received radical intent cCRT, sCRT or RT alone with a total dose of ≥50 Gy delivered in ≥15 fractions. Patients were excluded if they underwent surgery, SABR or previously had radiotherapy on the thorax. The planning target volume (PTV) was the internal target volume (ITV) plus a 1 cm margin. Plans aimed to limit the volume of total lung receiving at least 20 Gy (V20) ≤35%, and limited the contralateral lung V5 as much as possible. In this period no hard constraints were used for limiting doses to the heart or esophagus. The following prognostic factors were collected: PTV, mean lung dose, total lung V20, V10, V5, contralateral lung V5, esophagus V40, V50, V60, V65, mean heart dose, heart V25, V40. OS was measured using Kaplan-Meier graphs. Predictors of OS were assessed by Cox proportional hazard analysis and expressed as hazard ratios and 95% confidence intervals. Results Between 2015-2017, a total of 129 stage III NSCLC patients underwent treatment to a dose of ≥50 Gy at our center. All patients were treated using a full or hybrid IMRT/VMAT technique, and 115 (89%) received a dose of ≥60Gy. The median PTV was 578 cm3, with a range of 48- 2053 cm3. The Mean lungdose (Gy), mean total lung-PTV V20, V10, V5 and contralateral lung V5 (in %) were 13 (12- 22), 22.3 (5.5-37.2), 30.9 (11.2-52.9), 41.9 (9.2-78.3) and 22.7 (0-69.9), respectively. The mean heart dose (Gy), mean heart V40 and V25 (%) were 14 (0.5-44), 14.4 (0- 60.4), 19.3 (0-65.6). Mean esophagus V65, V60 and V40 (%) were 5.6 (0-36), 21.6 (0-83) and 45.1 (0-94). Patients receiving sCRT or RT alone had a worse OS (HR: 2.8; 95%CI 1.6-4.9; p=0.001 and 3.1; 95%CI 1.6-6.1; p=0.001). The only significant predictor of overall survival was the size of the PTV in cm3 (HR PTV >700 cm3: 2.03; 95%CI 1.2-3.3; p=0.005). Doses to the heart and esophagus were not significant predictors for OS. Kaplan-Meier graphs of the

OS (Figure 1) and OS related to mean heart doses (Figure 2) are shown below.

Conclusion Patients undergoing cCRT for stage III NSCLC had survivals similar to that reported in the literature. Using our IMRT/VMAT technique with a focus on contralateral lung sparing the PTV size correlated significantly with survival, but not the doses to the lungs, esophagus and heart. PO-1878 Factors affecting the dose received by CIED during photon radiation therapy. W. Szyszka 1 , E. Konstanty 2 1 The Greater Poland Cancer Center, Departement of Radiotherapy II, Poznań, Poland ; 2 The Greater Poland Cancer Center, Department of Medical Physics, Poznań, Poland Purpose or Objective Accurate CIED (Cardiac Implanted Electronic Devices) dose assessment is necessary during radiotherapy treatment to prevent the risk of CIED malfunction occurrence. The study aimed to measure the CIED doses and perform statistical analysis in terms of the correlation between measured dose relative to the irradiated area, irradiation technique and total dose of the PTV area. Material and Methods Dose measurements on CIED in 33 patients of the Greater Poland Cancer Center were performed during the first fraction using semiconductor detectors placed on the patient's skin, directly above the implanted pulse generator. The study analyzed dose measurements in all