ESTRO 36 Abstract Book

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S456 ESTRO 36 2017
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(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/m2 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
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. Cicchetti1, B. Avuzzi2, T. Rancati1, F. Palorini1, C.
Stucchi3, G. Fellin4, P. Gabriele5, V. Vavassori 6, C. Degli
Esposti7, C. Cozzarini8, C. Fiorino9, R. Valdagni10
1Fondazione IRCCS Istituto Nazionale dei Tumori,
Prostate cancer program, Milan, Italy
2Fondazione IRCCS Istituto Nazionale dei Tumori,
Radiation Oncology 1, Milan, Italy
3Fondazione IRCCS Istituto Nazionale dei Tumori, Medical
Physics, Milan, Italy
4Ospedale Santa Chiara, Radiotherapy, Trento, Italy
5Istituto di Candiolo- Fondazione del Piemonte per
l'Oncologia IRCCS, Radiotherapy, Torino, Italy
6Cliniche Humanitas-Gavazzeni, Radiotherapy, Bergamo,
Italy
7Ospedale Bellaria, Radiotherapy, Bologna, Italy
8San Raffaele Scientific Institute, Radiotherapy, Milan,
Italy
9San Raffaele Scientific Institute, Medical Physics, Milan,
Italy
10Università 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