ESTRO 2020 Abstract book

S452 ESTRO 2020

within a cohort of centrally located NSCLC patients treated with SBRT. Material and Methods A total of 220 patients from 2 centers who underwent SBRT with curative intent (≤12 fractions) for centrally located NSCLC between 2006 – 2016 were eligible. A central tumor was defined as a tumor <2 cm from the esophagus and/or main bronchus or first branches of the bronchial tree. Commonly used fractionation schedules were 8 fractions of 7.5 Gy (31%) and 5 fractions of 9-12 Gy (37%). The majority was diagnosed with stage II disease (52%). External validation was performed in a separate cohort of 92 patients who underwent SBRT for central NSCLC in a 3 rd institute in which 72% of patients underwent 8 fractions of 7.5 Gy. The following parameters were analyzed: age, gender, Charlson Comorbidity Index, WHO performance scale, previous (lung)malignancies, availability of pathology, FEV 1 , tumor localization, operability, endobronchial tumor location, tumor size, PTV volume, disease stage and PTV D max / D min / D mean D 2% / D 50% / D 98% . A Cox proportional hazards model was used to build a nomogram to predict 6-months, 1-year, 2-year and 3-year OS. The bootstrap method was applied to internally validate the nomogram. Discriminatory ability was measured by the concordance index (C-index) while predictive accuracy was assessed with calibration plots. Results The final nomogram ( Figure 1 ) was based on five parameters: age, PTV volume, D mean (<100 Gy BED 10 vs. ≥100 Gy BED 10 ), WHO performance scale (0 vs. 1-4) and tumor localization (upper-/ middle- lobe or mediastinum vs. lower lobe). Median OS of the initial group was 28 months (95% CI 23-35) with a 1-year OS of 76% and a 2-year OS of 55%. The median OS of the external group was 31 months (95% CI 20-42), with a 1-year OS of 80% and a 2- year OS of 63%. The C-index of the nomogram (corrected for optimism) was moderate at 0.61. In the external validation cohort, the C-index was 0.60. Figure 2 shows the calibration plots for the 1-year OS in which the solid line displays the initial group and the dotted line the external validation group. This plot shows an agreement between the model’s predicted and observed values. The closer the points are to the diagonal line, the better the prediction.

Results 85 consecutive patients were enrolled. 9 patients were excluded because of poor-quality DW-MRI or interruption of radiotherapy. 54 patients experienced clinical partial response (sensitive group) and 22 with stable disease (resistant group). 51 patients were assigned to the training set and the subsequent 25 were allocated to the validation set. Primary tumor site was associated with treatment response in our study with a p -value of 0.030. None of ADC values and delta-ADC values were significantly correlated with treatment response. Furthermore, radiomics signature built from the training set which included 30 delta-RFs during the time range of 2 weeks showed a strong relationship to treatment response in the validation set ( p =0.032). The AUC value of the signature was 0.823 and 0.812 respectively ( p -value=0.942 in Delong test) in two sets within this time range. The combined model of tumor primary site and radiomics signature performed better and could successfully predict patients' response to cCRT. Conclusion The ADC map-based delta-radiomics during the early course of treatment can successfully predicts response to cCRT in patients with ESCC. PH-0721 Predicting overall survival in central NSCLC treated with SBRT: nomogram development and validation M. Duijm 1 , E. Oomen - de Hoop 1 , N. Van der Voort van Zyp 2 , H. Tekatli 3 , P. Van de Vaart 2 , M. Hoogeman 1 , S. Senan 3 , J. Nuyttens 1 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands ; 2 Haaglanden MC, Department of Radiation Oncology, The Hague, The Netherlands ; 3 Amsterdam University Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The treatment of centrally located NSCLC with SBRT is associated with higher probabilities of toxicity compared to peripheral lesions. A specific nomogram for this patient group could prevent that patients with low survival expectations are exposed to high risks of high-grade toxicities. Therefore, we developed and externally validated a nomogram to predict overall survival (OS)