ESTRO 2020 Abstract Book

S823 ESTRO 2020

Material and Methods A programmable phantom with a sin 6 motion patterns (20- mm diameter “target”; period = 4 s; motion magnitude of 0.5, 1, 2 and 3 cm) were used for this simulation study. Air cavities of sizes 0, 0.5 and 1.0 cm were introduced near the target using three custom made inserts. For all inserts, 4DCT scans were acquired and VMAT treatment plans were generated using the ITV approach. The PTV was created by an isotropic 5 mm margin of the ITV. The clinical plans were optimized and dose distributions were calculated on the AIP. 4D dose calculations were utilized to evaluate the effect of an air cavity near the target cases by a poor pulmonary lung function. The calculations were performed on the static phantom scan with series of isocentre shifts corresponding to the target position of each 4DCT phase. The 4D dose calculations were accumulated and compared to the 3D dose calculations on the TPS. Dose metric of minimum dose, D 99% , maximum dose, D 1% , and mean dose, D mean were compared to obtain the dose discrepancy in the target of each air cavity insert. The same procedures were repeated to assess the dose deficiency due to set up errors of 0, 0.2, 0.3, 0.5, 0.6 and 1.0 cm in the D 99% . Results Figure 1 shows the dose discrepancy in the investigated dosimetric parameters between 3D and 4D dose calculations were found to be within -0.15% to 2.81 %, - 0.02% to 2.56% and -0.05% to 5.62% for the target of 0, 0.5 and 1.0 cm air cavity, respectively. Figure 2 illustrates the dose deficiency in the investigated D 99% between 4D dose calculations and the prescription dose was found to be less than 5% up to 5 mm setup error for targets of 0 and 0.5 cm air cavities, but exceeded 5% (clinically significant) with only 3 mm setup errors for target of 1.0 cm air cavity. Conclusion The evaluated dose discrepancy due to air cavity and deficiency due to setup errors during lung VMAT SBRT treatments resulted in non-negligible dose variations. Therefore, performing the 3D dose calculation based on the AIP, rather than the 4D dose calculations may not be adequate, particularly for a target near a large air cavity. Furthermore, target near an air cavity needs special care prior to treatment to ensure high accuracy in the target localization as they are more sensitive to setup errors. Future work includes patients study could offer a better understanding of the dosimetric effect of an air cavity near a moving tumor. PO-1523 Predictive factors of over Grade 2 radiation pneumonitis for advanced non-small cell lung cancer. K. Muraki 1 , E. Ogo 1 , H. Suefuji 1 , H. Eto 1 , C. Tsuji 1 , C. Hattori 1 , Y. Miyata 1 , T. Abe 1 1 Kurume University, Radiology, Kurume, Japan Purpose or Objective The standard of treatment for patients with locally advanced non-small cell lung cancer (LA-NSCLC) is the platinum based chemoradiotherapy (CRT) and followed by anti PD-L1 antibody durvulmab. However, we should withhold durvalumab for patients with Grade ≧ 2 radiation pneumonitis (RP). We aimed to identify dosimetric and radiobiological factors for predicting Grade ≧ 2 radiation pneumonitis after CRT for LA-NSCLC. Material and Methods We retrospectively analyzed data from 73 patients (RP Grade1 ： Grade2 = 43 : 30) with LA-NSCLC treated with CRT in our hospital between 2013 to 2018. The NTCP was calculated with the biological evaluation tool RayBiology in RayStation (Ver. 6.2). Univariate and multivariate analyses were performed to test the association between radiation pneumonitis and dosimetric and radiobiological Poster: Physics track: Radiobiological and predictive modelling, and radiomics

factors; the mean lung dose (MLD), the normal lung receiving 5 Gy (V5), 20 Gy (V20) ,absolute lung volume spared from a 5 Gy dose (VS5) and PTV/Lung volume ratio (%) were analyzed. The normal tissue complication probability (NTCP) for radiation pneumonitis was calculated with the Lyman–Kutcher–Burman (LKB) model and the Poisson-linear-quadratic (LQ) model. And clinical factors, including age, gender, tumor site, chronic obstructive pulmonary disease (COPD), forced expiratory volume in 1 second (FEV1) of pre-radiotherapy were analyzed. Radiation pneumonitis was diagnosed by radiologist on computed tomography. Results Thirty patients (41%) developed Grade ≧ 2 radiation pneumonitis. These patients were given 3 dimensional conformal radiotherapy (3D-CRT). They were prescribed 60Gy in 30 fractions (2Gy/fr/day) as the standard RT dose fractionation regimen in the definitive management of stage III LA-NSCLC disease. The MLD (RP Grade 1 : 12.5 vs Grade 2 : 14.6 Gy, p<0.001), V5 (30.4 vs 38.7%, p=0.001), V20 (22.7 vs 28.7%, p<0.001), PTV/Lung volume ratio(14 vs 20.3%, p=0.003), NTCP LQ model(2.7 vs 5.7%, p<0.001),and NTCP LKB model(6.3 vs 14.8%, p<0.001) were low at Grade 1 radiation pneumonitis. There were no significant differences in VS5 (RP 2325 vs 1978 cc, p=0.07). A receiver operating characteristics (ROC) curve analysis confirmed 10% and 31.7% as the best cut-off value of NTCP LKB model (odds ratios : 9.05; 95% confidence interval, 2.0 to 40.4; P=0.004) and V20 (OR : 9.05; 95% CI, 2.5 to 81.9; P=0.003) respectively for Grade ≧ 2 radiation pneumonitis (area under the curve ; AUC=0.817; AUC=0.835). Sensitivity of the NTCP and V20 were 76.7%, 80% respectively. Specificity of the NTCP and V20 were 65.1%, 62.8% respectively. Conclusion NTCP LKB model and V20 were the best predictive factors of symptomatic radiation pneumonitis after CRT for LA- NSCLC. Multivariate models that also include clinical variables were slightly more predictable. In order to discuss the role of predictive factors, additional validation should be performed by using cut-off value of NTCP and V20 prospectively. PO-1524 FET-PET radiomics predicting outcome after re-irradiation in recurrent glioblastoma M. Carles 1,2 , M.M. Starke 3 , M. Mix 2,4 , H. Urbach 5 , T. Schimek-Jasch 3 , F. Eckert 6,7 , M. Niyazi 8,9 , D. Baltas 1,2 , A.L. Grosu 2,3 , I. Popp 2,3 1 University Medical Center Freiburg, Department of Radiation Oncology- Division of Medical Physics, Freiburg im Breisgau, Germany ; 2 German Cancer Consortium DKTK- German Cancer Research Center DKFZ, Partner Site Freiburg, Freiburg, Germany ; 3 University Medical Center Freiburg, Department of Radiation Oncology, Freiburg im Breisgau, Germany ; 4 University Medical Center Freiburg, Department of Nuclear Medicine, Freiburg im Breisgau, Germany ; 5 University Medical Center Freiburg, Department of Neuroradiology, Freiburg im Breisgau, Germany ; 6 University Hospital Tübingen, Department of Radiation Oncology, Tübingen, Germany ; 7 German Cancer Consortium DKTK- German Cancer Research Center DKFZ, Partner Site Tübingen, Tübingen, Germany ; 8 University Hospital- LMU Munich, Department of Radiation Oncology, Munich, Germany ; 9 Cerman Cancer Consortium DKTK- German Cancer Research Center DKFZ, Partner Site Munich, Munich, Germany Purpose or Objective Radiotherapy for primary and recurrent glioblastoma (GBM) is conventionally planned on anatomical magnetic resonance imaging (MRI), where the target volume is defined as the area of tumor-related gadolinium enhancement on a T1-weighted sequence (Gd-T1-MRI). Recent studies have indicated that O-(2-[ 18 F]fluoroethyl)-