ESTRO 36 Abstract Book

S456 ESTRO 36 2017 _______________________________________________________________________________________________

(iMM) and the ipsilateral medial pterygoid muscle (iMPM). It is unclear whether these muscles should be regarded as a joined Organ at Risk or separately. The aim of our study was to calculate and compare separate dose-effect relationships between trismus and 1) the dose to the iMM and 2) iMPM dose, taking into account the baseline MMO. Material and Methods For 83 patients, participating in an exercise program to preserve oral function in the period 2008 - 2014, pre- and post-RT (6 weeks) MMO measurements were available. Treated tumors were mainly located in the oropharynx (40%) and hypopharynx (31%). All patients received concomitant radiotherapy (35x2Gy) via IMRT or VMAT technique with cisplatin 100mg/m 2 at day 1,22 and 43. Pathological MMO (trismus) was set at ≤35mm as a functional cut-off. Exclusion criteria were trismus at baseline and gross tumor infiltration of the iMM or iMPM on planning CT. The muscles were retrospectively delineated. A logistic regression with bootstrapping resampling technique (n=2000) was applied to calculate model parameters. Dose-volume parameters (mean-, absolute- and relative dose) were calculated in 5 Gy steps. Results MMO showed a large range ( Fig A ) with 14 trismus cases (17%) post-RT. Baseline MMO was a significant predictor for trismus (p=0.005) with an optimal cutoff at 45mm. Women more often had a baseline MMO ≤ 45 (65%) compared to men (37%, p=0.02) and therefore had a higher trismus risk (30% vs 12%, p=0.04). Mean doses of the iMPM and iMM correlated significantly ( Fig B, Pearson coefficient 0.83, p<0.001) with a mean iMPM dose of 53.3Gy versus 30.3Gy for iMM (p<0.001). In general, dose parameters of the iMPM showed superior fits (lowest -2 Log Likelihoods, lowest p values, better goodness-of-fit statistics) compared to iMM; differences were not statistically significant. The best fit for the iMPM was with mean dose (odds ratio 1.165, p<0.001); for iMM mean dose was most predictive as well (odds ratio 1.070, p=0.002). Fig C&D shows the dose-response for iMPM and iMM for the ≤45mm and >45mm subgroups. Best fit for dose volume parameters was for the percentage receiving ≥65Gy (iMPM, p=0.001) and the percentage receiving ≥40Gy (iMM, p=0.003).

the iMPM. We conclude that the strong correlation between iMPM and iMM is caused by close proximity of the two muscles. However, the different shapes of the dose- response curves of both muscles suggest that they should be regarded as separate OARs and at least the iMPM should be delineated to estimate trismus risks. Furthermore, baseline MMO is highly predictive and is important to take into account in trismus models. PO-0850 Predicting late fecal incontinence risk after RT for prostate cancer:external independent validation A. Cicchetti 1 , B. Avuzzi 2 , T. Rancati 1 , F. Palorini 1 , C. Stucchi 3 , G. Fellin 4 , P. Gabriele 5 , V. Vavassori 6 , C. Degli Esposti 7 , C. Cozzarini 8 , C. Fiorino 9 , R. Valdagni 10 1 Fondazione IRCCS Istituto Nazionale dei Tumori, Prostate cancer program, Milan, Italy 2 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiation Oncology 1, Milan, Italy 3 Fondazione IRCCS Istituto Nazionale dei Tumori, Medical Physics, Milan, Italy 4 Ospedale Santa Chiara, Radiotherapy, Trento, Italy 5 Istituto di Candiolo- Fondazione del Piemonte per l'Oncologia IRCCS, Radiotherapy, Torino, Italy 6 Cliniche Humanitas-Gavazzeni, Radiotherapy, Bergamo, Italy 7 Ospedale Bellaria, Radiotherapy, Bologna, Italy 8 San Raffaele Scientific Institute, Radiotherapy, Milan, Italy 9 San Raffaele Scientific Institute, Medical Physics, Milan, Italy 10 Università degli Studi di Milano, Oncology and Hemato- oncology, Milan, Italy Purpose or Objective To validating a predictive model for late fecal incontinence (FI) on a recent population of prostate cancer patients (pts) treated with radical radiotherapy. NTCP model was derived from literature. Material and Methods Population included 267 pts treated with Intensity Modulate Radiation Therapy (IMRT) in 2010-2014. Prescribed dose was between 68 and 80 Gy with conventional and hypo-fractionated (HF, from 2.2 to 2.8 Gy) treatment. Rectal toxicity was scored using the LENT/SOMA questionnaire. Follow-up (FU) was considered up to 2 years. The study endpoint was late FI. We chose to validate a model for prediction of chronic fecal incontinence, as evaluated through multiple measures during follow-up. Mean FI was defined as the average score during the FU period after RT. Mean incontinence >1 was the considered endpoint. Pts with at least three out of four FU points in the first 2 years were included (the 2- year point was mandatory). Literature based multivariate model included: mean rectal dose (Dmean), previous diseases of colon and previous abdominal surgery (SURG). Dose distributions were corrected EQD in 2 Gy fractions (alpha/beta=5Gy). Results 256 pts were available. Mean grade>1 FI was scored in 28 patients (10.9%). Univariate logistic analysis confirmed the risk factors reported in literature, with similar Odds Ratios (OR) for Dmean (1.04±0.03 vs 1.05±0.04) and SURG (1.90±1.70 vs 1.50±0.50). As consequence, NTCP models including Dmean and Dmean+SURG were evaluated through calibration plot. The models showed a clear trend (increasing observed toxicity rates with predicted risk), but the observed toxicity rates were underestimated. We guessed this scenario could be due to a hidden effect of HF (OR=2.20, 8.6% vs 17.6%), beyond standard correction using LQ model for late effects. The first approach was to directly evaluate the impact of HF, by including it as a variable into model (keeping coefficients for Dmean and SURG fixed at previously published values). It clearly improved calibrations. A further step was to include the time recovery effect into EQD2 correction

Conclusion We observed that both the iMPM and the iMM dose are predictive for trismus with a better dose-response fit for