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Figure 1. Illustration of the beam 1 direction in the
calibration phantom with different tissue-equivalent
inserts.
Figure 2: Dose to water profile for one beam direction in
the Gammex RMI 467 phantom. The dose is laterally
integrated and the R80 is measured.
Conclusion
A comparison study between the use of SECT and DECT
images for proton dose distribution is performed to
understand the differences and potential benefit of DECT
for proton therapy treatment planning, using different CT
scanners. The final aim is to decrease uncertainty in dose
delivery, possibly allowing narrower treatment margin
than currently used. In most scenarios, the different
modalities of DECT produced results closer to the
reference, when compared with the SECT based
simulations. Small differences were found for the
different DECT scanners.
OC-0342 Monte Carlo simulations of a low energy
proton beam and estimation of LET distributions
T.J. Dahle
1
, A.M. Rykkelid
2
, C.H. Stokkevåg
3
, A. Görgen
2
,
N.J. Edin
2
, E. Malinen
2,4
, K.S. Ytre-Hauge
1
1
University of Bergen, Department of Physics and
Technology, Bergen, Norway
2
University of Oslo, Department of Physics, Oslo, Norway
3
Haukeland University Hospital, Department of Oncology
and Medical Physics, Bergen, Norway
4
Oslo University Hospital, Department of Medical
Physics, Oslo, Norway
Purpose or Objective
The physical advantage of protons in radiotherapy is
mainly due to the ‘Bragg peak’ of the proton depth dose
distribution. However, there is still a controversy on the
biological effects of protons, in particular around the
Bragg peak. This relates both to the variability of
biological systems and endpoints studied, but also to the
actual linear energy transfer (LET) in the biological
systems. To provide accurate estimates of the relative
biological effects of protons, high precision cell
experiments are needed together with detailed knowledge
of the LET at a given measurement depth. The objective
of this study was to estimate the LET distribution along
the depth dose profiles from a low energy proton beam,
using Monte Carlo (MC) simulations adjusted to match
measured dose profiles.
Material and Methods
Dose measurements were performed at the experimental
proton beam line at the Oslo Cyclotron Laboratory (OCL)
employing 17 MeV protons. A Markus ionization chamber
and GafChromic films were used to measure the dose
distribution at 28, 88 and 110 cm from the beam exit
window. At each position, measurements were performed
along the depth dose profile (using increasing thickness of
paraffin- and Nylon6 sheets). A transmission chamber was
used for monitoring beam intensity. The geometry of the
experimental setup was reproduced in the FLUKA MC code.
The dose profiles were calculated using FLUKA, and MC
parameters relating to beam energy and beam line
components were optimized based on comparisons with
measured doses. LET-spectra and dose-averaged LET
(LET
d
) were also scored using FLUKA.
Results
The measured pristine Bragg peak from the OCL cyclotron
covered about 200 µm (Figure 1a). The MC simulations of
the beam line were validated by comparing simulated dose
profiles with measured data (Figure 1a). The simulated
LET
d
increased with depth, also beyond the Bragg peak
(Figure 1a and Table 1). Also, LET
d
at target entrance
increased with distance from the beam exit window due
to the presence of air (Table 1). The LET spectrum was
narrow at the target entrance, and considerably
broadened at BP depth (Figure 1b).
Conclusion