ESTRO 36 Abstract Book

S455 ESTRO 36 2017 _______________________________________________________________________________________________

increase of lung damage up to a certain threshold (60 Gy), before adopting an asymptotic relationship. Patient response in the linear fitting was heterogeneous with a greater than 3 fold difference found in the IQR. These findings may aid in post-treatment response assessment and toxicity modeling in NSCLC patients undergoing escalated dosing regimens. PO-0848 Predictors of patient-reported incontinence after prostate cancer RT: results from a cohort study C. Cozzarini 1 , N. Bedini 2 , E. Garibaldi 3 , D. Balestrini 4 , P. Franco 5 , G. Girelli 6 , I. Improta 7 , F. Palorini 8 , V. Vavassori 9 , T. Rancati 8 , R. Valdagni 2,8 , C. Fiorino 7 1 San Raffaele Scientific Institute, Radiotherapy, Milano, Italy 2 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiation Oncology 1, Milano, Italy 3 Istituto di Candiolo- Fondazione del Piemonte per l'Oncologia IRCCS, Radiotherapy, Candiolo, Italy 4 Ospedale Bellaria, Radiotherapy, Bologna, Italy 5 Ospedale Regionale U.Parini-AUSL Valle d’Aosta, Radiotherapy, Aosta, Italy 6 Ospedale ASL9, Radiotherapy, Ivrea, Italy 7 San Raffaele Scientific Institute, Medical Physics, Milano, Italy 8 Fondazione IRCCS Istituto Nazionale dei Tumori, Prostate Cancer Program, Milano, Italy 9 Cliniche Gavazzeni-Humanitas, Radiotherapy, Bergamo, Italy Purpose or Objective To assess clinical and dose factors affecting the incidence of patient-reported urinary incontinence (INC) at three years after radical radiotherapy (RT) for prostate cancer of a large group of patients enrolled in a prospective, multi-centric trial in the period 2010-2014. Material and Methods Enrolled patients were treated in seven Institutions at different prescribed doses with conventional (74-80 Gy at 1.8-2 Gy/fr, CONV) or moderately hypo-fractionated RT (65-75.2 Gy at 2.2-2.7 Gy/fr, HYPO) in 5 fractions/week. Several clinical factors were collected for each patient: comorbidities, drugs, hormone therapies, previous surgeries, smoking, alcohol, age, and body mass index. In addition, the prescribed 2Gy equivalent dose (EQD2) was considered by applying an alpha-beta ratio of 0.8, 3 and 5Gy, according to values recently reported in the literature. INC was evaluated through the International Consultation on Incontinence Modular Questionnaire short form (ICIQ) filled in by the patients at start/end of RT and every 6 months until 5 years of follow up. In the current analysis, patients with ICIQ available at 30 and/or 36 months were considered (n=298;); the incidence of INC at 3 years was defined as the occurrence of an ICIQ value >12 at least once between 6 and 36 months. Univariable and backward multivariable logistic analyses were performed to build a predictive model. Results In total, 298 patients had the required minimum follow- up; patients with baseline ICIQ>12 (n=3) were excluded restricting the analysis to 295 patients (CONV: 149; HYPO: 146, 86% treated with IMRT). The median number per patients of completed questionnaires was 5 (range: 2-6): the incidence of ICIQ>12 was 5.1% (n=15) with a prevalence at 30/36 months equal to 4.1%. Main predictors at univariable analysis were age (p=0.01,OR=1.19), baseline ICIQ>0 (p=0.056, OR=2.9), previous TURP (p=0.04, OR=3.8) and EQD2 (p=0.003-0.02, OR=1.12-1.17 depending on alpha-beta). EQD2 calculated with alpha- beta=0.8Gy showed the best performances in terms of calibration plot and p-value and was included in the multi- variable analysis. Final results suggested a two-variable model including EQD2 (p=0.005,OR=1.13; 95%CI:1.04-1.24) and age (p=0.011,OR=1.19; 95%CI:1.04-1.37); the model showed good performances in terms of goodness of fit

(H&L test, p=0.55) and calibration plot (slope:1.02, R 2 =0.92). In Figure, the risk of 3-year INC vs EQD2 (alpha- beta=0.8Gy) is shown with the calibration plot of the final two-variable model. The validity of the model was confirmed in the HYPO subgroup. Conclusion The incidence of patient-reported 3-year INC after high- dose RT for prostate cancer dramatically depends on the prescribed dose (EQD2) and, secondarily, on the age of patients. A previously suggested low alpha-beta value (0.8Gy) for late INC resulted in a significantly better calibrated model, consistently with a high sensitivity of late I NC to fractionation.

PO-0849 Trismus after chemoradiation in head & neck cancer: relation with medial pterygoid and masseter dose O. Hamming-Vrieze 1 , S. Kraaijenga 2 , S. Verheijen 1 , M. Jonker 1 , L. Van der Molen 2 , J. Van de Kamer 1 , M. Van de Brekel 2 , W. Heemsbergen 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands 2 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Head and Neck Surgery, Amsterdam, The Netherlands Purpose or Objective Reduced maximal mouth opening (MMO) is a serious side effect that can occur after chemoradiation (CRT) in head & neck patients. Recent studies showed dose-effect relationships with both the ipsilateral masseter muscle