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S858
ESTRO 36
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EP-1592 Higher biological dose to heart and lung in
IMPT of medulloblastoma patients due to increased LET
E. Rørvik
1
, L.F. Fjæra
1
, C.H. Stokkevåg
2
, S. Thörnqvist
1,2
,
K.S. Ytre-Hauge
1
1
University of Bergen, Department of Physics and
Technology, Bergen, Norway
2
Haukeland University Hospital, Department of Oncology
and Medical Physics, Bergen, Norway
Purpose or Objective
For medulloblastoma patients, proton therapy offers
increased dose conformity compared to conventional
radiation therapy. There is however growing concern
about the clinical consequences of the enhanced linear
energy transfer (LET) delivered to the normal tissue
surrounding the tumor volume. In these patients, the
biological dose received to the lung and heart may be
higher than reflected by treatment plans based on the
clinical practice of using a constant relative biological
effectiveness (RBE) of 1.1. The aim of this study was
therefore to investigate the biological dose to the heart
and lung following craniospinal Intensity-modulated
proton therapy (IMPT) using LET-based models for the RBE
of protons.
Material and Methods
An intensity modulated proton therapy (IMPT) treatment
plan was generated in a commercial treatment planning
system(Eclipse, Varian Medical Systems) applying two
cranial fields and two spinal fields. The dose prescription
was 23.4 Gy(RBE) given by 13 fractions. The plan was
optimized using a fixed RBE value of 1.1. The IMPT plans
were imported into the FLUKA Monte Carlo code where
dose and dose-average LET distributions (LET
D
) were
calculated. Subsequently, a published LET-based RBE
model for proton therapy was applied to these
distributions to quantify the biological doses for the heart,
lungs and the PTV.
Results
The Monte Carlo simulations revealed elevated
LET
D
values both in the PTV, heart and lung and
correspondingly higher biological doses as compared to
the doses obtained with an RBE of 1.1 (Figure 1). The dose
volume histograms revealed small elevation in mean dose
to the lungs (from 2.0 to 2.4 Gy) and the heart (from 0.7
to 1.0 Gy). However, the variable RBE model calculated a
significantly higher max dose in both organs, with a shift
from 21.8 Gy to 27.2 Gy for the lungs and 17.2 Gy to 21.2
Gy for the heart.
Conclusion
Including LET and biological dose models revealed an
increased dose to heart and lung compared to doses
calculated with RBE of 1.1, mainly due to the enlarged LET
values at the distal dose falloff of the beam. The
reproducibility of these results in a larger cohort of
patients will be investigated.
Electronic Poster: Physics track: (Radio)biological
modelling
EP-1593 Accuracy of TCP model for nasopharyngeal
cancer after more than five years average follow-up
M. Avanzo
1
, J. Stancanello
2
, G. Franchin
3
, S. Barbiero
4
, R.
Jena
5
, G. Sartor
1
, E. Capra
1
1
Centro di Riferimento Oncologico, Medical Physics,
Aviano, Italy
2
Oncoradiomics, Mastricht, The Netherlands
3
Centro di Riferimento Oncologico, Radiation Oncology
Department, Aviano, Italy
4
Casa di Cura S. Rossore, Radio-oncology, Pisa, Italy
5
University of Cambridge, Department of Oncology,
Cambridge, United Kingdom
Purpose or Objective
A model for radiation therapy (RT) based tumor control
probability (TCP) of nasopharyngeal carcinoma (NPC)
which includes the effects of hypoxia and
chemoradiotherapy was previously developed by fitting
TCP to clinical local control data from published
randomized studies and tested in a patient dataset with
limited follow-up time.
The purpose of this work was to validate the model by
comparison of estimated TCP and average overall local
control rate in a cohort of patients with long follow-up
time.
Material and Methods
96 patients treated with intensity modulated RT delivered
by LINAC or Helical Tomotherapy and neoadjuvant
chemotherapy for histologically proven nasopharyngeal
carcinoma were followed for an average time of 5.5 years.
TCP was calculated from individual GTV dose-volume
histograms, fractionation and overall treatment duration
data using a previously established model (Ref. 1) based
on the linear-quadratic model and Poisson statistics
modified to account for repopulation, chemotherapy,
heterogeneity of dose to the tumor, and hypoxia. The
model parameters were: α/β of 10 Gy, α of 0.396 Gy
−1
with
σ
α
of 0.07 Gy
−1
, density of clonogens 10
7
cc
-1
, fraction of
patients showing hypoxia 22%, oxygen enhancement ratio
(OER) of 1.417, chemotherapy enhancement factor 1.64,
T
pot
of 3 days and T
k
of 28 days.
The average TCP was compared to the local control rate
(LCR), defined as the absence of clinical and radiological
evidence of disease residuum in the GTV at the last follow-
up visit.
Results
The average calculated TCP in patients treated at our
institution was 86.9% with 95% confidence intervals (95%
CIs) of 74%–95.0%. At a mean follow-up time of 5.5 years,
15 patients developed local failure in the nasopharynx and
three had distant and local recurrences. Three patients
developed recurrence outside the treated volume. Local
control was therefore obtained in 81% (95% CI: 72-87.8%)
patients, and was in agreement with calculated TCP.
Conclusion
The TCP model shows was in agreement with LCR in
patients treated at our institution after a long follow-up
time.
References:
1. Avanzo et al. Med. Phys. 37 (4) 1533-1544, April 2010
EP-1594 Development of multivariable models for
acute toxicities in nasopharyngeal cancer radiotherapy
A. Cavallo
1
, T. Rancati
2
, A. Cicchetti
2
, N.A. Iacovelli
3
, F.
Palorini
2
, C. Fallai
3
, E. Orlandi
3
, E. Pignoli
1
1
Fondazione IRCCS Istituto Nazionale dei Tumori, Medical
Physics Unit, Milan, Italy
2
Fondazione IRCCS Istituto Nazionale dei Tumori,
Prostate Cancer Program, Milan, Italy
3
Fondazione IRCCS Istituto Nazionale dei Tumori,
Radiation Oncology 2, Milan, Italy
Purpose or Objective
To investigate the dose-response relationship for acute
toxicities in nasopharyngeal cancer (NPC) RT and to build
multivariable models aiming at the inclusion of other non-
dosimetric factors.
Material and Methods
A series of consecutive NPC patients (pts) treated
curatively with IMRT/VMAT + chemotherapy at 70 Gy (35-
33 fr, 2-2.12 Gy/fr) was considered. Clinical- tumor- and
treatment-related data were retrospectively collected:
age, gender, BMI, smoking history, histology, staging,
comorbidities, RT technique, overall treatment time.
Organs-at-risk (OAR) volumes and dose distribution for
each pt were also considered. Acute toxicities were