ESTRO 36 Abstract Book

S460 ESTRO 36 _______________________________________________________________________________________________

(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.

(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).

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

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