ESTRO 2023 - Abstract Book

S23

Saturday 13 May

ESTRO 2023

Results In total 434 HNC patients were included, of which 26.5% (n = 115) reported moderate-severe taste loss at 6 months post treatment. Visual inspection showed good conformity between the auto-segmented structures and the Stieb et al. guidelines. The mean dose to the full oral cavity was 31.0 Gy [IQR: 13.7 – 50.5] and for the tongue structure 27.3 Gy [IQR: 7.5 – 48.0]. The full oral cavity mean dose (OR=1.037; p=<0.0001) and combined parotid gland mean dose (OR=1.041; p=<0.0001) showed a larger dose response relation compared to the tongue mucosa (OR=1.031; p=<0.0001). The multivariable NTCP model showed similar results, as a model with the tongue mucosa mean dose (AUC: 0.724 95%CI [0.674 0.777]; R2=0.165) did not perform better than the reference model with oral cavity mean dose (AUC: 0.729; 95%CI [0.678 - 0.7802]; R2=0.178). Table 1: Comparison of the NTCP model performance with oral cavity mean dose and the auto-segmented taste bud bearing tongue mucosa mean dose.

Conclusion The taste bud bearing tongue mucosa structure did not outperform the oral cavity structure in the logistic regression NTCP model predicting late taste toxicity at 6 months. This might related to the limited mean dose differences between the both structures. This further underlines the complexity of predicting taste loss in HNC patients. MO-0056 NTCP Models for Late Tissue Fibrosis Following Breast RT are Validated in a Large Prospective Cohort A. Cicchetti 1 , E. Gioscio 1 , M.C. De Santis 2 , P. Seibold 3 , D. Azria 4 , D. De Ruysscher 5 , A.M. Dunning 6 , R. Elliot 7 , A. Seoane 8 , M. Lambrecht 9 , E. Sperk 10 , B. Rosenstein 11 , C. Talbot 12 , A. Vega 13 , L. Veldeman 14 , A. Webb 12 , T. Rattay 15 , C. West 16 , T. Rancati 17 1 Fondazione IRCCS Istituto Nazionale dei Tumori, Prostate Cancer Program, Milan, Italy; 2 Fondazione IRCCS Istituto dei Tumori di Milano, Radiation Oncology, Milan, Italy; 3 German Cancer Research Center (DKFZ), Division of Cancer Epidemiology, Heildelberg, Germany; 4 Montpellier Cancer Institute, Radiation Oncology, Montpellier, France; 5 Maastricht University Medical Center, Radiation Oncology (Maastro), Maastricht, The Netherlands; 6 University of Cambridge, Strangeways Research Labs, Cambridge, United Kingdom; 7 University of Manchester, Manchester Accademie Health Science Centre, Manchester, United Kingdom; 8 Vall d'Hebron Hospital Universitari, Vall d'Hebron Barcelona Hospital Campus, Medical Physics Department, Barcelona, Spain; 9 University Hospitals Leuven, Radiation Oncology, Leuven, Belgium; 10 Universitätsmedizin Mannheim, Medical Faculty, Mannheim, Germany; 11 Icahn School of Medicine at Mount Sinai, Radiation Oncology, New York, USA; 12 University of Leicester, Genetics and Genome Biology, Leicester, United Kingdom; 13 Fundación Pública Galega, Medicina Xenómica, Santiago de Compostela, Spain; 14 Ghent University, Department of Human Structure and Repair, Ghent, Belgium; 15 University of Leicester, Cancer Research Centre, Leicester, United Kingdom; 16 University of Manchester, Translational Radiobiology Group, Manchester, United Kingdom; 17 Fondazione IRCCS Istituto Nazionale dei Tumori di Milano, Prostate Cancer Program, Milan, Italy Purpose or Objective To validate NTCP models for radiation-induced fibrosis in breast cancer patients treated with whole breast RT. Materials and Methods We selected two Lyman models for moderate/severe (CTCAE v4.0 grade ≥ 2) late tissue fibrosis (Fib2+) published in the literature (Alexander et al. PMB 2007, Avanzo et al. EJMP 2012). Model details are reported in Table 1. Both models are based on the Biological Effective Uniform Dose (BEUD) of the PTV DVH to include the effects of fractionation and dose inhomogeneity. The α / β ratio for dose corrections was 3 Gy. Alexander’s model considers the risk of Fib2+ at 5 years post RT. Of note, Avanzo's model includes the possibility of heterogeneous follow-up (FU) through the use of a latency function (Tucker et al. IJROBP 2008) based on data from the EORTC trial 22881-10882. We tested models in a multicentre EU/USA observational study cohort of breast cancer patients treated with RT after conservative surgery between 2014 and 2016. We selected a subgroup of patients with RT schedules similar to those for the model’s development: prescribed dose between 40 and 50 Gy, conventional or moderate hypofractionation, and no boost dose. Patients with post-operative fibrosis and atrophy (before RT) were excluded from the analysis. We also applied the latency function to the computation of the NTCP risk for Alexander’s model since 5 years of FU data were unavailable for all the patients. Validation performance was evaluated through the area under the receiver-operating curve (AUC) and the calibration plot's slope and offset (calibration-in-the-large). Results A total of 425 patients had RT schedules overlapping with those in the published development cohorts. The Median FU time was 24 (12-60) months. We processed 392 PTV DVHs for the analysis. Thirty-three pts (7.6%) with post-surgery side effects were excluded. Interestingly, only 55% of them continued to manifest symptoms in the late phase after RT suggesting the presence of surgical alterations that are resolved with time despite additional radiation damage. Fib2+ was scored in 19.2% of the study group, with 15.7% and 3.6% grade 2 and 3 events, respectively. The calibration plots for NTCP models are shown in Fig1: calibration slope and offset of 1.09 and 3% for Avanzo’s model and 1.05 and 1% for