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Conclusion
A unique feature of strong inline magnetic fields is that
they act to reduce lateral electron scatter in lower density
mediums such as lung tissue. This work has demonstrated
this experimentally for the first time. Clinically, such
scenarios will arise in inline MRI-linac systems for
treatment of small lung tumors.
OC-0261 Monte Carlo Correction Factors for MRgRT
Reference Dosimetry
S. Pojtinger
1
, O. Dohm
2
, D. Thorwarth
1
1
University Hospital Tübingen, Department of
RadiationOncology - Section for Biomedical Physics,
Tübingen, Ger many
2
University Hospital Tübingen, Department of Radiation
Oncology - Division for Medical Physics, Tübingen,
Germany
Purpose or Objective
Magnetic-Resonance guided Radiotherapy (MRgRT) is a
new technique providing real-time MR-Im aging during
Radiotherapy (RT). Caused by the strong magnetic field,
trajectories of charged particles change, w hich results in
a different deposited dose in the ionization chamber.
Hence, a new chamber-specific correction factor k
B
,
taking into account the magnetic effect, has to be
introduced. In this work k
B
is determined for seven
commercially available chambers by Monte-Carlo (MC)
simulations.
Material and Methods
Simulations were made using the MC package EGSnrc.
Chambers were placed in 10cm depth, inside of a
30x30x20cm water phantom and dose was scored for
magnetic field strengths ranging from 0 to 5T in steps of
0.5T.
The orientation of chambers was perpendicular to the
central axis of the beam for thimble type chambers and
axial for plane-parallel chambers (see fig. 1). The
magnetic field was oriented perpendicular to the central
axis of the beam.
Simulated chambers were PTW 30013, PTW 30015, PTW
34045 Advanced Markus, PTW 23343 Markus, PTW 34001
Roos (PTW Freiburg), NE2571 (Phoenix Dosimetry) and
NACP02 (Scanditronix).
Modeling the PTW 30013 was based on plans provided by
manufacturer as well as on a µCT scan. For all other
chambers, geometries and material information were used
that have been provided and used in publications by Wulff
et al [1].
Incoming particles were simulated in 4 different ways:
1.
Commercial Elekta 6MV FFF accelerator has
been modeled as part of the MC simulation
(Elekta 6MV FFF(Beam Sim))
2.
A photon spectrum was calculated from an
earlier simulation of a commercial Elekta 6MV
FFF accelerator. This spectrum was used as an
input for modeling the beam in form of photons
diced from this distribution (Elekta 6MV FFF)
3.
The beam was simulated by photons sampled
from a photon spectrum that was calculated
from an earlier simulation of a commercial
Elekta 6MV accelerator with flattening filter
(Elekta 6MV)
4.
Photons were generated from a spectrum for a
6MV accelerator, published by Mohan et al [2]
(Mohan 6MV)
Results
Figure 1 shows the calculated dose D normalized to the
dose without magnetic field D
nb
.
This is the inverse of the correction factor k
B
that is given
for all simulation methods (at 1.5T) in table 1. Overall,
large variations were observed depending on the type of
ionization chamber and the magnetic field strength.
Though effects are stronger for plane-parallel chambers
one can find points of stable dose, e.g. k
B
=0.9997(19) for
PTW 23343 Markus at 3.5T.
Conclusion
If plane-parallel chambers are used, there is a strong
change in measured dose, as variations can range up to
10%. In contrast, the correction for thimble type chambers
is below 2%.
The results demonstrate that reference dosimetry for
MRgRT is possible, using the presented chambers, but
measured dose must be corrected by the calculated
factors k
B
.
References:
[1] Phys Med Biol. 2008 Jun 7;53(11):2823-36
[2] Med Phys. 1985 Sep-Oct;12(5):592-7
OC-0262 Implementation of patient specific QA for
daily adaptive MR-guided radiation therapy
M.A. Palacios
1
, T. Apicella
2
, D. Hoffmans
1
, T. R osario
1
,
M. Admiraal
1
, I. Kawrakow
2
, J. Cuijpers
1
1
V U Medical Center, Radiation Oncology Department,
Amsterdam, The Netherlands
2
ViewRay- Inc., Research & Development, Mountain
View, USA
Purpose or Objective
To report on the successful implementation of a patient
specific QA procedure for online treatment plan
adaptation under MR guidance.
Material and Methods
The ViewRay MRIdian system was recently ins talled at our
institution. It allows for a fast online treatment plan
adaptation based on the daily MR image and real time
monitoring of the anatomy of the patient during treatment
delivery. To facilitate the clinical implementation of
online treatment plan adaptation we developed an online
dosimetric verification procedure that uses an
independent MC dose calculation engine and
automatically checks relevant planning parameters. The
independent MC includes the MLC, couch, and imaging
coils in the simulation and takes into account the magnetic
field. It runs on a computer equipped with a 4-core Intel®
Core™ i3-2100 CPU @ 3.10GHz and 8 GB of RAM. A 3%/3mm
Gamma comparison with the dose distribution from the
TPS is performed before accepting an adapted treatment