ESTRO 36 Abstract Book

432 / 1096

S417 ESTRO 36
_______________________________________________________________________________________________
calculation times considerably, making the presented PBA
a suitable candidate for integration in a treatment
planning system. The current work demonstrates that
proton MRI is feasible from a dosimetric point of view.
PO-0786 Energy dependence investigation for
detectors used in out-of-filed dosimetry
L. Shields1, L. Leon-Vintro2, B. Mc Clean3
1 St Luke's Hospital, Medical Physics, Dublin, Ireland
2 University College Dublin, Schoool of Physics, Dublin,
Ireland
3 St. Luke's Radiation Oncology Network, Medical
Physsics, Dublin, Ireland
Purpose or Objective
Traditionally, energy dependence of a range of detectors
used in radiotherapy has been investigated mainly in the
Cobalt-60 and 6-15MV photon range. However, when
considering detectors for use in out-of-field dosimetry, it
is more important that the energy dependence is
investigated over a much lower range. This study
examined (i) the mean incident energy of radiation out-
of-field for a 6MV photon beam and (ii) the energy
dependence of a range of clinically available detectors to
the typical energies experienced out-of-field and (iii)
Monte Carlo (MC) calculated and detector measured out-
of-field dose profiles.
Material and Methods
An Elekta Synergy Linac operating at 6MV and a water
phantom at 90cm SSD was defined in BEAMnrc. Phase
spaces were scored at 6 different planes in the water
phantom - 0.2, 1.4 (dmax), 5, 10, 15 and 20cm. Each phase
space file was analysed using the EGSnrc program package
BEAMDP to extract energy spectra from each of the phase
space files to examine the change in energy spectra with
increasing distance from the field edge and depth in the
phantom.
The energy dependence of each of the detectors was
examined using 70, 100, 125 and 200 kV beams on a
Gulmay D3225 Orthovoltage Unit and a 6MV Elekta Synergy
beam. The kV energies lied within the range of energies
which were found to be dominant out-of-field in a 6MV
beam. A dose of 1 Gy was delivered to each detector as
determined by their respective calibration protocols, and
the signal was recorded for all energies.
In-plane and cross-plane profiles were measured by each
detector and compared to MC calculated.
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 Bello1, C. Möhler 1,2, S. Greilich 1,2, O. Jäkel1,2,3
1German Cancer Research Center DKFZ, Division of
Medical Physics in Radiation Oncology, Heidelberg,
Germany
2National Center for Radiation Research in Oncology
NCRO, Heidelberg Institute for Radiation Oncology HIRO,
Heidelberg, Germany
3Heidelberg 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