ESTRO 36 Abstract Book

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S181 ESTRO 36
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deflection of the Bragg peak ranged from 0 cm for 70 MeV
to 1 cm for 180 MeV in comparison to no magnetic field.
No out-of-plane beam deflection was observed. Exposing
the film to 2 Gy at the Bragg peak was estimated to cause
a mean dose to the magnets of 20 µGy, which is expected
to produce negligible magnetic flux loss. The initial
activation was estimated to be below 25 kBq.
Figure 2: Simulated dose distribution of a deflected
proton beam (180 MeV, 10 7 primary particles) on a film
dosimeter.Conclusion
A first experimental setup capable of measuring the
trajectory of a proton pencil beam slowing down in a
tissue-equivalent material within a realistic magnetic field
has been designed and built. Monte Carlo simulations of
the design show that magnetic field induced lateral beam
deflections are measurable at the energies studied and
radiation-induced magnet damage is expected to be
manageable. These results have been validated by
irradiation experiments, as reported in a separate
abstract.
OC-0344 Experimental validation of TOPAS neutron
dose for normal tissue dosimetry in proton therapy
patients
G. Kuzmin 1, A. Thompson2, M. Mille1, C. Lee1
1 National Cancer Institute, Division of Cancer
Epidemiology and Genetics, Rockville, USA
2 National Institute of Standards and Technology,
Radiation Physics Division, Gaithersburg, USA
Purpose or Objective
In the last several years, the popularity and use of proton
therapy has been increasing due to its promise of a
dosimetric advantage over conventional photon therapy.
This is especially of great importance in pediatric patients
who have a higher risk of developing late effects. During
proton therapy 90% of scatter dose is from neutrons, which
can travel out of the treatment field and can be highly
biologically effective. In order to conduct epidemiological
investigations of the risk of long term adverse health
effect in proton therapy patients, it is imperative to
accurately assess radiation dose to normal tissue. Tool for
Particle Simulation (TOPAS) based on the GEANT4
Simulation Toolkit may be a computational option for
normal tissue dosimetry to support large scale
epidemiological investigations of proton therapy patients.
While previous works have benchmarked TOPAS for proton
dosimetry within treatment fields, there is a lack of
validation for neutron scatter and energy spectrum. In the
current study, we measured the energy spectrum of
scattered neutrons using a simple physical phantom
coupled with a series of Bubble Detectors irradiated by
Californium-252 neutron source.
Material and Methods
We conducted the neutron measurement under the
collaboration with National Institute of Standards and
Technology (NIST). We employed Bubble detectors (BTI,
Canada) to measure the neutron dose and energy
spectrum with good spatial resolution. The detectors
provide six energy thresholds from 10 keV to 10 MeV
allowing to validate dose and the neutron energy
spectrum. To simulate neutron scatter, a polyethylene
cylindrical phantom was milled and the bubble detectors
were placed inside. The phantom was then irradiated with
a Californium-252 neutron source to simulate the
secondary neutrons. We also simulated the experiment in
TOPAS to compute the neutron dose and energy spectrum
for comparison (Figure 1).
Results
The measured spectrum was unfolded and shows to be in
good agreement with the simulation. On average, the
percent difference in the spectrum was less than 31%
(Graph 1) and the percent difference of dose was under
23%. The agreement was best at the neutron energies 10
keV – 100 keV (19 %) and worst at 2.5-10 MeV (91 %). Better
statistics are needed for the higher energy spectrum
region. We plan to conduct the measurement three times
to minimize statistical errors and plan to extend the
validation to anthropomorphic physical phantoms.
Conclusion
We validated the dose and energy spectrum of scattered
neutrons computed from TOPAS Monte Carlo code by the
measurements using Bubble Detector. We plan to utilize
TOPAS dose calculation system coupled with patient-
specific proton therapy data for normal dose calculations
to support epidemiological studies of proton therapy
patients.
Proffered Papers: Treatment planning applications
OC-0345 Comparing cranio spinal irradiation planning
for photon and proton techniques at 15 European
centers
E. Seravalli1, M. Bosman2, G. Smyth3, C. Alapetite4, M.
Christiaens5, L. Gandola6, B. Hoeben7, G. Horan8, E.
Koutsouveli9, M. Kusters10, Y. Lassen 11, S. Losa4, H.
Magelssen 12, T. Marchant13, H. Mandeville3, F.
Oldenburger14, L. Padovani15 , C. Paraskevopoulou16, B.
Rombi17, J. Visser14, G. Whitfield13, M. Schwarz 17, A.
Vestergaard18, G.O. Janssens19
1UMC Utrecht, Department of Radiation Oncology,
Utrecht, The Netherlands
2University Medical Center Utrecht, Radiotherapy,
Utrecht, The Netherlands