ESTRO 2020 Abstract Book

S225 ESTRO 2020

PD-0415 Toxicity and bronchial stenosis in central lung tumors treated with SBRT B.L.T. Rijksen 1 , M.M.G. Rossi 1 , B. Stam 1 , J.N.A. Van Diessen 1 , J.L. Knegjens 1 , J.S.A. Belderbos 1 1 The Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Stereotactic Body Radiation Therapy (SBRT) for central lung tumors, i.e. tumors ≤2cm from the proximal bronchial tree (PBT) or other mediastinal structures, is often delivered with a risk adapted strategy. The proximity of critical structures to the high dose area increases the toxicity risk e.g. dyspnea/cough due to bronchial stenosis. Jackson et al. reported 27% risk of bronchial stenosis grade ≥2 after SBRT for ultra-central long tumors. The Nordic HILUS-trial investigated toxicity in centrally located tumors and observed 28% ≥grade 3 toxicities in tumors ≤1cm from the PBT. In our institute, patients with a centrally located tumor are treated with SBRT using the EORTC Lung-Tech trial dose prescription and constraints. In this study we evaluated bronchial stenosis in these patients. Material and Methods Patients receiving SBRT 8x7.5Gy (BED 87.5Gy, α/β=10) between 2015-2019 were retrospectively analyzed. Central location was defined as ≤2cm from the PBT or mediastinal and/or pericardial pleura. If the PTV was adjacent to the PBT the dose constraint (D0.5cc) on the bronchial wall was <60Gy (126.0Gy, α/β=3), when the PTV was not adjacent D0.5cc <44Gy (74.8Gy, α/β=3). Dyspnea/cough and bronchial stenosis were evaluated using CTCAEv4. In a subgroup of patients with increased or new complaints of dyspnea/cough, FU CT-scans were analyzed for onset of bronchial stenosis by 3 thoracic radiation oncologists. Difference in dose to the bronchus in this subgroup between patients with or without stenosis was evaluated. Overall survival (OS) and median FU were also evaluated. Results In total 118 patients were analyzed with a mean age of 69.9 years and 87.2% had WHO ≤1. The median overall survival was 44.8 months with a median follow up of 18.3 months in the total cohort. Consisting of 65% cT1-2 NSCLC and 35% metastases. No acute toxicity ≥grade 3 was recorded. 28% of patients (n=33) developed grade ≤2 dyspnea/cough. In this subgroup the tumors of 15 patients were located ≤2cm of PBT and bronchial stenosis grade 2 was scored in 5 patients, all with tumor ≤1cm from the PBT. In 4 patients complete obstruction of at least one lobar and/or sublobar bronchus with atelectasis (figure 1) and in 1 patient partial stenosis of the bronchus was observed. Mean time to bronchial stenosis was 9.9 months from end of SBRT. The median D0.5cc for the bronchus was significantly different between the group with stenosis and without stenosis: 34.4Gy (50.2Gy, α/β=3) vs 18.3Gy (19.4Gy, α/β=3) respectively (p=0.005). No grade ≥3 stenosis, fistula or hemorrhage was scored. Conclusion Thirty-three (28%) of 118 patients treated with SBRT for a centrally located tumor experienced increased dyspnea and/or cough. In 15 (45%) of these 33 patients the tumor location was found ≤2cm of PBT. In this subgroup bronchial stenosis grade 2 was found in 5 patients, all with tumors ≤1cm from the PBT and all related to the bronchus dose. Based on this increased risk on bronchial stenosis we are reconsidering SBRT for tumors ≤1cm of PBT.

PD-0416 Daily adaptive strategy simulation in lung cancer pts treated with RTCHT with reduced CTV-PTV MARGIN M.L. Bonu 1 , P. Borgetti 1 , L. Spiazzi 1 , A. Taddeo 1 , G. Volpi 1 , A. Guaineri 1 , G. Costantino 1 , A. Donofrio 1 , A. Guerini 1 , D. Tomasini 1 , C. Toraci 1 , J. Imbrescia 1 , S.M. Magrini 2 1 Spedali Civili di Brescia, Radiation Oncology Departement, Brescia, Italy ; 2 Spedali Civili di Brescia and Brescia University, Radiation Oncology Departement, Brescia, Italy Purpose or Objective to evaluate interfraction target coverage and primitive tumour GTV shrinkage in a cohort of lung cancer patients treated with radical intent radiochemotherapy (RTCHT) and planned with 4DCT with reduced iCTV-PTV margins. Material and Methods Ten lung cancer patients treated from 2017 and 2019 with a Histological diagnosis of NSCLC or SCLC were identified. 4D CT acquisition were performed, iCTV-PTV margin of 6 mm in cranial-caudal direction and 4 mm in radial direction were applied. Dose prescription were 60 Gy in 30 fraction, PTV coverage prescription was D95%≥95%. IMRT versus V-MAT RT plan were generated. Daily online CBCT images were acquired. We retrospectively recontured GTV_T, CTV and Organs at risk (OAR) for all 30 CBCT for all patients (total 300 CBCT). A DVH representing each single fraction and a mean DVH representing all 30 fractions were generated using an in-house software (PRO- DVH). Mean plan_adaptive was compared to plan_reference. Z test was exploited to detect statistically significant differences between provisional and adaptive CVT coverage. A shrinkage GTV_T map was also generated. Results In eight of 10 patients, mean target coverage of plan_adaptive resulted equal or superior to plan_reference. In two cases CTV coverage resulted inferior to plan_reference (D95=90% versus D95=95% and D95%=87,5% versus D95=95%, respectively, all p<0,001). In one patient, the analysis of DVH_adaptive of each fraction showed a progressive reduction in target coverage in the last 10 fractions. No trends were identified for the second patient. Concerning GTV_T volumes, we observed a mean regression in the whole series of 48% (range 32%-65%), with a major trend of reduction between the VIII and the XX fractions (figure 1).