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ESTRO 36 2017
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All measurements were performed in an PTW MP3
watertank except for TLDs and Gafchromic EBT3 film
which were performed in solid water.
Results
Figure 1 displays the results of the energy dependence
investigation for each detector in the study. The response
of each detector was normalised to 1 at 6MV.
Figure 2 displays a comparison between MC calculated
versus detector measured out-of-field dose.
Conclusion
In general the results of the energy dependence
investigation predicted the response of the detectors to
out-of-field radiation except for the case of the Pinpoint,
TLD and microDiamond detectors. Energy dependence was
thought to be the leading source of variation in detector
response to out-of-field radiation due to the relative
increase in low-energy photons. However, dose-rate and
angular dependencies can exist in detector responses but
were not investigated as part of this study. Other factors
such a charge multiplication and cable effects can
contribute to a change in response as observed with the
Pinpoint detector. This study highlights the need for
careful selection of appropriate detectors when accurate
out-of-field dosimetry is required and offers a guide and
improved understanding of detector response to out-of-
field radiation. The waterproof Farmer chamber showed
best agreement with MC calculated out-of-field dose and
is recommended for out-of-field dose measurements.
PO-0787 A compact and complete model for Bra gg
peak degradation in lung tissue
R. Dal Bello
1
, C. Möhler
1,2
, S. Greilich
1,2
, O. Jäkel
1,2,3
1
German Cancer Research Center DKFZ, Division of
Medical Physics in Radiation Oncology, Heidelberg,
Germany
2
National Center for Radiation Research in Oncology
NCRO, Heidelberg Institute for Radiation Oncology HIRO,
Heidelberg, Germany
3
Heidelberg Ion Beam Therapy Center HIT, Clinical
Research Group Radiotherapy with Heavy Ions,
Heidelberg, Germany
Purpose or Objective
Due to the lack of a reliable model, current analytical
treatment planning for proton and heavier ions cannot
account for the degradation of the sharp distal fall-off of
the Bragg peak caused by microscopic density
heterogeneities, which cannot be resolved by clinical CT.
Here, we present a systematic study of Bragg peak
degradation in stationary lung parenchyma to provide a
comprehensive
analytical
parametrization
for
implementation in treatment planning systems (TPS) –
aiming at the reduction of dose uncertainties in
radiotherapy of the lung.
Material and Methods
We developed a compact model describing the lung
parenchyma microscopic geometry based on few
geometrical and physical variables allowing for flexible
Monte Carlo (MC) simulations of lung specific features
(alveolar dimension, lung density) and breathing state
parameters (air filling state, water equivalent thickness
traversed, WET). To benchmark the accuracy of the
simulated model, we performed a MC study to assess the
specific contributions of the cumulative physical sources
of degradation and a series of transmission experiments
on lung-like phantoms with clinical proton and carbon
beams at the Heidelberg ion-therapy center (HIT). We
adopted the benchmarked model to provide a
parametrization of the Bragg peak degradation on the
beam and on the previously mentioned lung parameters.
Throughout this work, we tested and used a Gaussian
convolution of the undegraded Bragg peak (U. Titt et al,
2015) to parametrize the degradation. Furthermore, the
model was used to investigate the effects on clinical
spread out Bragg peak (SOBP) and on the relative
biological effectiveness (RBE).
Results
Fluctuations in the WET were found the major degradation
factor, contributing more than 75% (40%) to the
cumulative distal falloff widening for a carbon (proton)
Bragg peak. The simulated lung parenchyma model (Figure
1) was capable to reproduce the experimental data with a
slight underestimation of the degradation parameters, yet
guaranteeing the correct reproduction of all the relevant
characteristics in the degraded dose distribution. The
Gaussian filtration unified the description for different
beam particles and provided a compact and complete
characterization with specific dependencies with respect
to each lung parameter. Moreover, the description was
found independent from the initial beam energy resulting
in deviations mainly about the SOBP distal falloff while the
plateau remains unaffected. Finally, the impact on the
biological dose was mainly driven by changes to the
physical dose due to the limited deviations in the RBE.
Conclusion
We provide a comprehensive characterization of Bragg
peak degradation that can readily be implemented in a
TPS. Such implementation is crucial for a more complete
description of lung treatments, adding to the effect of
macroscopic structures (e.g. bronchi, CT resolvable) the