Abstract Book

S15

ESTRO 37

PV-0038 Esophagus toxicity after stereotactic radiotherapy of central lung tumor: NTCP modelling M. Duijm 1 , H. Tekatli 2 , E. Oomen-de Hoop 1 , W. Verbakel 2 , W. Schillemans 1 , B.J. Slotman 2 , S. Senan 2 , J.J. Nuyttens 1 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands 2 Cancer Center Amsterdam VU University Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The esophagus is an organ at risk in stereotactic radiotherapy for central lung tumors. We studied the correlation between esophagus toxicity and dosimetric/volumetric parameters in order to assess risks, and to derive a Normal Tissue Complication Probability (NTCP) model. Material and Methods Patients with a central lung tumor from 2 centers who received stereotactic or hypofractionated radiotherapy (≤ 12 fractions) were retrospectively analyzed. Doses were recalculated to an equivalent dose of 2 Gy (EQD 2 ) with an α/β ratio of 10 for acute toxicity (within 3 months). The esophagus was manually delineated and dose-volume histogram (DVH) parameters (D max , D 1cc , D 2cc , D 5cc ) were evaluated. The primary endpoint was esophagus toxicity as scored by CTCAE version 4.0. NTCP was calculated based on a logistic regression model and significant parameters (p-value < 0.05) were plotted into the logistic model curve. Overall survival was calculated using Kaplan-Meier analysis and groups were compared with the log-rank test. Results Two-hundred-and-thirty-one patients (with 252 tumors) were eligible with median follow-up of 16 months (range 0.3 – 84.3). No acute or late grade 3-5 esophagus toxicity was reported. Acute grade 1-2 toxicity was recorded in 38 patients (16.5%). All DVH parameters differed significantly between patients with toxicity versus those without toxicity; median D max of 62.2 Gy 10 versus 31.6 Gy 10 , median D 1cc of 40.8 Gy 10 versus 18.8 Gy 10 , median D 2cc of 35.6 Gy 10 versus 16.3 Gy 10 and median D 5cc of 24.1 Gy 10 versus 11.1 Gy 10 , respectively (p < 0.001 for all parameters, based on Mann-Whitney U test). Out of 27 patients who received a D max ≥ 54.7 Gy 10 (equivalent of 5 x 7.5 Gy), 37% suffered from grade 1-2 esophagus toxicity. A D max ≥ 60.0 (equivalent of 5 x 8 Gy) resulted in toxicity in 1 out of 14 patients. A D 1cc ≥ 44.7 Gy 10 (equivalent of 5 x 6.5 Gy) resulted in toxicity in 6 out of 12 patients. A D 1cc ≥ 49.6 Gy 10 (equivalent of 5 x 7 Gy) resulted in toxicity in 5 out of 8 patients. Logistic regression showed significant correlations between all analyzed DVH parameters and toxicity, therefore NTCP-curves were calculated for all parameters (Figure 1 a-b). A 50% probability of acute grade 1-2 esophagus toxicity was found at a D max of 67 Gy 10 , D 1cc of 42 Gy 10 , D 2cc of 38 Gy 10 and D 5cc of 30 Gy 10 . No significant differences in overall survival were found between patients with and without toxicity: 2-year survival rate was 44% versus 51%, respectively ( p = 0.428). Figure 1 a-b: NTCP models for the probability of grade 1- 2 acute toxicity in the esophagus

Conclusion As no grades 3-5 esophagus toxicity were observed in our cohort, we conclude that a D max of 60.0 Gy 10 (5 x 8 Gy) and a D 1cc of 49.6 Gy 10 (5 x 7 Gy) can be safely delivered using stereotactic radiotherapy. The NTCP-curves derived may serve as guidelines for central lung cancer undergoing stereotactic treatment. PV-0039 Outcome of elderly NSCLC patients treated with isotoxic RT dose-escalation using IMRT (NCT01166204) J. Van Loon 1 , D. De Ruysscher 1 , S. Peeters 1 , K. Verhoeven 1 , G. Bootsma 2 , A.C. Dingemans 3 , A. Van Baardwijk 1 , B. Reymen 1 , R. Wanders 1 1 MAASTRO Clinic, Radiation Oncology, Maastricht, The Netherlands 2 Zuyderland Medical Centre, Pulmonology, Heerlen, The Netherlands 3 Maastricht, Maastricht University Medical Centre, Maastricht, The Netherlands