Abstract Book

ESTRO 37

S555

Material and Methods Eight models relevant to dose escalation in patients with stage III NSCLC treated with chemo-radiotherapy (chemo- RT) were identified: four ≥Grade 2 acute esophagitis (AE) models (Huang et al 2012, Huang et al 2017, Oh et al 2017, Wijsman et al 2015), and four ≥Grade 2 radiation pneumonitis (RP) models (Appelt et al 2014, Huang et al 2011, Oh et al 2017, QUANTEC (Marks et al) 2010). Model- specific parameters were applied to the corresponding variables in a stage III NSCLC cohort (N=193) treated with chemo-RT to 50-80Gy@1.8-2.0Gy/fraction in 2004-2014. The endpoints were ≥Grade 2 AE and ≥Grade 2 RP (rates: 48% and 16%). Model performance was assessed using area under the receiver-operating curve (AUC) and p- values, and model calibration between predicted and observed rates in quintiles using the Hosmer-Lemeshow test (p HL ). Results Predictability of the four AE models was overall higher than that of the four RP models (AUC: 0.66-0.71 vs. 0.51- 0.61; p<0.0001 vs. 0.057-0.55; Table; Figure). Except for two AE models, and one RP model (p HL : 0.10, 0.91, 0.99), calibration was reasonable (p HL : 0.32-0.71). The best- performing AE model (Huang et al 2012) included concurrent chemotherapy and mean esophagus dose (D mean ), while the best-performing RP model (Appelt et al 2014) included age>63years, current smoker, former smoker, inferior/middle primary tumors, obstructive lung disease, and sequential chemotherapy) in addition to lung D mean .

the stable exhale phase. DWA is a system -specific arc technique (Vero SBRT system) that combines synchronized gantry-ring rotation with dynamic -MLC optimization. It was integrated in the RayStation TPS (RaySearch Laboratories, Sweden), which is using a collapsed cone dose calculation algorithm. Optimal plans were generated using multiple DWAs chosen from a set of template-based trajectories (Figure 1). All plans were evaluated using the RTOG high- and intermediate- dose spillage criteria: conformity index (CI), ratio of 50% isodose volume to the PTV (R50%), maximum dose 2 cm away from the PTV (D2cm), percent of normal lung receiving ≥ 20Gy (V20) and all other OAR constraints were evaluated. Dose delivery verification was performed with the Delta4 dosimeter. Results A total of 31 SBRT plans were analyzed. Coin lesions were planned with 51Gy/3fr (n=3). Subpleural or more centrally located lesions were treated with 48Gy/4fr (n=21) or 60Gy/8fr (n=7) depending on PTV volume and OAR constraints. Average PTV volume treated was 47.8cc (range, 7.9 - 130.3cc). Mean tumor diameter was 24mm (range, 8 - 39mm). Mostly 3 DWA were applied (range, 2 - 4) to allow intra-fraction cone-beam CT verification and to limit the amount of MUs per wave. Mean beam-on delivery time was 5.8 min (range, 2.9 - 10.1min). The template-based DWA solution could not be used in 2 cases, thus standard coplanar VMAT was used instead. The reason was collision detection during the dry-run test the first case, and insufficient planning CT scan length to allow non-coplanar dose calculation in the second case. The CI was >1.2 and < 1.5 for 7 plans, the R50% of 15 plans showed a minor violation. The cut-off volume that derived higher CIs and more low dose spillage was determined at 25cc. Except for peripheral tumors where a portion of the rib was included in the PTV, OAR constraints could be met. The average gamma (3%/3mm) passing rate for the DWA plans was 99.6% (range, 97.7- 100%). Conclusion DWA is a fast and smooth treatment approach that improves patient comfort and reduces the workload for RTTs. The current template-based solution presented some limitations, therefore the use of individualized DWA trajectories should be made available. The excellent QA results demonstrate a dosimetrically robust delivery. Better conformity and less violations on the R50% were observed for lesions larger than 25cc. PO-0997 Towards personalized dose-escalation in non- small cell lung cancer: Validation of published models M. Thor 1 , A. Jackson 1 , A. Iyer 1 , E. Bendau 1 , A. Fontanella 1 , A. Apte 1 , E. Yorke 1 , A. Rimner 2 , J. Deasy 1 1 Memorial Sloan Kettering Cancer Center, Medical Physics, New York- NY, USA 2 Memorial Sloan Kettering Cancer Center, Radiation Oncology, New York- NY, USA Purpose or Objective RTOG 0617 has demonstrated the need for patient- specific dose-escalation, derived from normal tissue complication probability (NTCP) models, of stage III non- small cell lung cancer (NSCLC) treatments. The goal of this work was to investigate the performance of eight previously published NTCP models for NSCLC in an independent cohort, and to identify models for future patient-specific dose escalation.