ESTRO 2021 Abstract Book

S617

ESTRO 2021

Conclusion We here propose a method to include all relevant normal tissue dosimetric descriptors for the prediction of toxicity endpoints in one synthetic score (WDS) which takes weighted effect sizes into account and which can be included in classical NTCP models. When NTCP approach was combined to the RADIODTECT©, the discrimination power was improved, with the pATM assay working in a synergistic way with the dosimetric risk. Larger size cohorts, validation and assessment of the clinical usefulness are ongoing before implementing this approach in clinical practice. PD-0785 Personalized fractionation of ultracentral lung tumors using modeled outcomes from treated patients A. Iyer 1 , I. Chen 2 , M. Thor 1 , A. Wu 2 , A. Apte 1 , A. Rimner 2 , D. Gomez 2 , J. Deasy 1 , A. Jackson 1 1 Memorial Sloan Kettering Cancer Center, Department of Medical Physics, New York, USA; 2 Memorial Sloan Kettering Cancer Center, Department of Radiation Oncology, New York, USA Purpose or Objective Use of stereotactic body radiation therapy (SBRT) for central non small cell lung cancer (NSCLC) tumors has been shown to improve local control compared to conventional fractionation. The likelihood of toxicity increases for ‘ultracentral’ tumors abutting the central organs at risk, such as the proximal bronchial tree (PBT) or esophagus. A previously published tumor control probability (TCP) model for early stage NSCLC has been validated in large cohorts of different fractionation regimens. Our group and others have derived normal tissue complication probability (NTCP) models for central and ultracentral patients. The goal of this work is to use such TCP and NTCP models to improve the therapeutic ratio for ultracentral patients. Materials and Methods The analysis included 63 patients with ultracentral NSCLC tumors treated at our center between 2008 and 2017 using hypofractionation. Four baseline protocols were considered: 10 Gy X 5, 7.5 Gy X 8, 7 Gy X 10, and 4 Gy X 15 fractions. EQD2 based normal tissue guidelines and limits were created for each fractionation using our current 10 Gy X 5 clinical planning criteria, and additional NTCP model based criteria for grade 3+ radiation pneumonitis (RP3+; guideline 5%, limit 10%), grade 2+ esophagitis (E2+; 30% limit) and lobar