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MHD starting 6 months post RT (Figure 1). NTCP models for all 4 mortality time endpoints included the GTV covariate (in 100% of best performing models). Additional covariates were age and current smoker at 6 months (AUC=0.73); MHD at 12 months (AUC=0.71); MHD, WHO PS and current smoker at 18 months (AUC=0.71); WHO PS, age and current smoker at 24 months (AUC=0.72). MHD was selected in 40%, 100%, 87% and 47% of best performing models (this was only 27%, 47%, 33% and 33% for MLD) at 6, 12, 18 and 24 months, respectively. The 12 month mortality NTCP model had the highest MHD OR=1.042 (p=0.006) and was selected. The probability of 12 month mortality can be calculated with the formula NTCP=(1+e -S ) -1 with S=- 1.528+0.0408*MHD+0.00570*GTV (Figure 2). In dataset 2, the 12 and 18 month NTCP models had respective AUCs of 0.60 (0.65 when adding WHO PS) and 0.67. MHD OR was 1.050 (p=0.11) at 12 month. Conclusion MHD is a risk factor independent from GTV volume for post RT mortality endpoints later than 6 months and before 18 months. A NTCP model for 12 month mortality could allow patient selection for proton therapy.

Figure: Graphical representation of the multivariable logistic regression model Conclusion Our data suggest that a seroma and an axillary lymphadenectomy are the most important clinical risk factors for late aesthetic outcome and that the effect of the maximum dose is small (n = 1.00). The breast V55 dose-volume metric is suggested to limit unfavourable aesthetic outcome after breast-conserving therapy. PV-0317 A NTCP model for mortality after chemo-RT for lung cancer including mean heart dose and GTV G. Defraene 1 , S. Arredouani 1 , W. Van Elmpt 2 , M. Lambrecht 1 , D. De Ruysscher 2 1 KU Leuven - University of Leuven, Department of Oncology- Experimental Radiation Oncology, B-3000 Leuven, Belgium 2 Maastricht University Medical Center+, Department of Radiation Oncology Maastro clinic- GROW School for Developmental Biology and Oncology, Maastricht, The Netherlands Purpose or Objective It is known that higher radiation dose to the heart is associated with increased mortality in lung cancer patients. We constructed a normal tissue complication probability (NTCP) model for mortality. Material and Methods Two prospective cohorts containing 388 and 98 curatively treated stage I-III lung cancer patients from 2 003- 2016 (dataset 1) and 2011-2016 (external validation dataset 2) were studied. Doses were for NSCLC: 66Gy/2Gy (concurrent chemotherapy) or 66Gy/2.75Gy (sequential or no chemotherapy); for SCLC: 45Gy/1.5Gy. Clinic al (WHO PS, age, current smoking, T, N stage and GTV volume (combining primary tumor and involved lymph nodes)) and dosimetric (mean lung dose (MLD) and mean heart dose (MHD)) variables were analyzed. In dataset 1, factors with p<0.1 in univariate Cox regression were included in multivariable Cox model building and in logistic r egression NTCP model building for the endpoints 6, 12, 18 and 24 month mortality. A best subsets regression according to AIC (combining the likelihood and a penalty on the number of model parameters) was performed. The best model was reported and covariates of the top 15 performing models were analyzed. NTCP models were validated in dataset 2 after a refit of model coefficients. Results Median follow-up time was 30.8 and 43.3 months in dataset 1 and 2, respectively. Univariate Cox modeling selected MHD (p<0.001), GTV (p<0.001), WHO PS (p=0.006), MLD (p=0.007), age (p=0.03) and current smoker (p=0.07). The best multivariable Cox model included MHD (HR=1.026, p<0.001), GTV (HR=1.002, p<0.001), current smok er (HR=1.39, p=0.03) and WHO PS (HR=1.24, p=0.03). Adding a MHD*GTV interaction did not change these results. Survival curves showed an increased mortality associated with higher