ESTRO 36 Abstract Book

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S180 ESTRO 36
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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
(LETd) 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
LETd increased with depth, also beyond the Bragg peak
(Figure 1a and Table 1). Also, LETd 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
MC simulations accurately modelled the dose distribution
around the Bragg peak and can be used to estimate the
LET at any given position of the proton beam with
optimized parameters. The LET spectrum varied
considerably with depth and such LET estimates are highly
valuable for future studies of relative biological
effectiveness of protons.
OC-0343 Experimental setup to measure magnetic
field effects of proton dose distributions: simulation
study
S. Schellhammer1,2, B. Oborn3,4 , A. Lühr1,2,5, S. Gantz1,2,
P. Wohlfahrt1,2, M. Bussmann6, A. Hoffmann1,2,7
1Helmholtz-Zentrum Dresden-Rossendorf, Institute of
Radiooncology, Dresden, Germany
2OncoRay - National Center for Radiation Research and
Oncology, Medical Radiation Physics, Dresden, Germany
3Wollongong Hospital, Illawarra Cancer Care Centre,
Wollongong, Australia
4University of Wollongong, Centre for Medical Radiation
Physics, Wollongong, Australia
5German Cancer Consortium DKTK, Partner Site Dresden,
Dresden, Germany
6Helmholtz-Zentrum Dresden-Rossendorf, Institute of
Radiation Physics, Dresden, Germany
7Faculty of Medicine and University Hospital Carl Gustav
Carus at the Technische Universität Dresden,
Department of Radiation Oncology, Dresden, Germany
Purpose or Objective
As a first step towards proof-of-concept for MR-integrated
proton therapy, the dose deposited by a slowing down
proton pencil beam in tissue-equivalent material is
assessed within a realistic magnet assembly. Furthermore,
radiation-induced activation and demagnetization effects
of the magnet are studied.
Material and Methods
The dose distributions of proton pencil beams (energy
range 70-180 MeV) passing through a transverse magnetic
field of a permanent C-shaped NdFeB dipole magnet
(maximum magnetic flux density Bmax = 0.95 T) while being
stopped inside a tissue-equivalent slab phantom of PMMA
were simulated (Figure 1). The beam was collimated to a
diameter of 10 mm. A radiochromic EBT3 film dosimeter
was placed centrally between the two phantom slabs
parallel to the beam’s central axis. 3D magnetic field data
was calculated using finite-element modelling (COMSOL
Multiphysics) and experimentally validated using Hall-
probe based magnetometry. A Monte Carlo model was
designed using the simulation toolkit Geant4.10.2.p02 and
validated by reference measurements of depth-dose
distributions and beam profiles obtained with Giraffe and
Lynx detectors (IBA Dosimetry), respectively. The beam
trajectory and lateral deflection were extracted from the
film’s planar dose distribution. Demagnetization was
assessed by calculating the dose deposited in the magnet
elements, and by relating this to radiation hardness data
from literature. A worst-case estimate of the
radioactivation of the magnet was obtained by taking into
account the most common produced mother nuclides and
their corresponding daughter nuclides.
Figure 1 : Simulation geometry.
Results
The Monte Carlo model showed excellent agr eement with
the reference measurements (mean absolute range
difference below 0.2 mm). The predicted planar dose
distribution clearly showed the magnetic fi eld induced
beam deflection (Figure 2). The estimated in-plane