ESTRO 36 Abstract Book

476 / 1096

S461 ESTRO 36
_______________________________________________________________________________________________
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
Esposti 7, C. Cozzarini8, C. Fiorino9, 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
(gamma=0.7Gy/day), thus taking count of a possible
consequential effect between acute and late damage. The
figure reports calibration plot for all cases.
Conclusion
The study confirms formerly published results on effect of
abdominal surgery and dose to large rectal volumes as
potential risk factors for late FI.
Calibrations highlight a possible role of HF beyond linear
quadratic correction. Inclusion of time recovery
correction improved calibrations, but a further separate
effect of HF was still detectable. This might be related to
the selected alpha/beta (5Gy), which is currently
accepted for late rectal toxicity.
A more suitable value could be found for the longitudinal
definition used in these trials (i.e., toxicity starting in
acute phase and persisting during follow-up), instead of
using the assumption settled in studies focusing on
incidence of late peak events (such as rectal bleeding).
PO-0851 Artificial neural networks for toxicity
prediction in RT: a method to validate their
“intelligence”
E. Massari1, T. Rancati2, T. Giandini 1, A. Cicchetti2, V.
Vavassori3, G. Fellin4, B. Avuzzi5, C. Cozzarini6, C.
Fiorino7, R. Valdagni2, M. Carrara1
1Fondazione IRCCS Istituto Nazionale dei Tumori,
Diagnostic Imaging and Radiotherapy- Medical Physics
Unit, Milan, Italy
2Fondazione IRCCS Istituto Nazionale dei Tumori,
Prostate Cancer Program, Milan, Italy
3Cliniche Humanitas-Gavazzeni, Radiotherapy, Bergamo,
Italy
4Ospedale Santa Chiara, Radiotherapy, Trento, Italy
5Fondazione IRCCS Istituto Nazionale dei Tumori,
Radiation Oncology 1, Milan, Italy
6San Raffaele Scientific Institute, Radiotherapy, Milan,
Italy
7San Raffaele Scientific Institute, Medical Physics, Milan,
Italy