475 / 1096
S460
ESTRO 36
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(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
(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).
Conclusion
We observed that both the iMPM and the iMM dose are
predictive for trismus with a better dose-response fit for