ESTRO 37 Abstract book

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ESTRO 37

resulted in AUC 0.75 (95% CI 0.64; 0.85), the Hosmer- Lemeshow test was not significant and the loglikelihood was -51.8. The optimal LKB model parameters were EUD 3.6 (50) = 63.3 Gy (95% CI 59.2;68.1), n = 1.00 (95% CI 0.62;1), m = 0.23 (95% CI 0.16;0.37), dmf (seroma) = 0.83 (95% CI 0.73;0.95) and dmf (ALND) = 0.84 (95% CI 0.75;0.94). Bootstrap validation resulted in AUC 0.74 (95% CI 0.61; 0.83). The loglikelihood was -54.3, this is a lower value than in the multivariable logistic regression model and indicates a better calibration of the multivariable logistic regression model.

Conclusion Modest differences between planned and delivered dose were found in a relatively large patient cohort. Delivered dose yielded steeper NTCP models with improved fits but differences were not significant. Moreover, the difference between the models were small. Consequently, the potential clinical relevance of NTCP models based on accumulated dose for oral mucositis, xerostomia and dysphagia in head and neck cancer radiotherapy is likely to be limited. PV-0316 Development of a prediction model for unfavourable aesthetic outcome after breast- conserving therapy I. Kindts 1 , G. Defraene 2 , A. Laenen 3 , S. Petillion 1 , E. Van Limbergen 1 , T. Depuydt 1 , C. Weltens 1 1 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium 2 KU Leuven – University of Leuven, Department of Oncology - Experimental Radiation Oncology, Leuven, Belgium 3 KU Leuven – University of Leuven, Leuven Biostatistics and Statistical Bioinformatics Centre L-Biostat, Leuven, Belgium Purpose or Objective The aim of the present study was to develop a normal tissue complication probability (NTCP) model for late unfavourable aesthetic outcome (AO) after breast- conserving radiation therapy. Material and Methods The BCCT.core software was used to evaluate the AO of patients treated at one institution with breast-conserving radiation therapy, based on standardized photographs. Follow up periods ranged from two to six years. Literature data have shown there is no measurable change in AO measured by BCCT.core two to six years after surgery. Individual radiotherapy plans were assessed and the radiotherapy doses were recalculated to biologically equivalent doses using an α/β-value of 3.6 Gy. Uni- and multivariable logistic regression analysis was performed to study the predictive value of clinicopathological and dosimetric variables for an unfavourable AO. The Lyman Kutcher Burman (LKB) model was fit to the data with dose modifying factors (dmf). Model performance was assessed with the Area Under the Curve (AUC) of the receiver operating characteristic curve and bootstrap sampling. Calibration was evaluated using a Hosmer-Lemeshow test for the multivariable logistic regression model and a comparison with the LKB model was done with the loglikelihood. Results Forty-four of the 121 analysed patients (36%) developed unfavourable AO. In the optimal multivariable logistic regression model a larger breast volume receiving ≥55 Gy (V55), a seroma and an axillary lymph node dissection (ALND) were independently associated with an unfavourable AO. In individual cases, the probability of unfavourable AO can be calculated using the following formula: NTCP = 1 / (1+e -S ), where S = -2.6759 + (0.0574 x V55) + 1.5546 (if seroma) + 1.2029 (if ALND) (Figure). Bootstrap validation

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 2003-2016 (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. Clinical (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