241 / 1082
S228
ESTRO 36 2017
_______________________________________________________________________________________________
SABR treatments. The potential benefits may include real-
time, good quality image guidance without additional
radiation. We are optimistic that improvements in the
detector and collimation system, will lead to better image
quality, more accurate tumor localizations, and possible
3D reconstruction of patient’s anatomy within the
irradiated region.
OC-0438 The impact of a 1.5 T MR-Linac fringe field
on neighbouring linear accelerators.
T. Perik
1
, J. Kaas
1
, F. Wittkamper
1
1
The Netherlands Cancer Institute, Department of
Radiation Oncology, Amsterdam, The Netherlands
Purpose or Objective
In our institution a clinical prototype of the MR-
Linac(MRL)(Elekta AB, Stockholm, Sweden) was installed
in an existing treatment room. The MRL, which has a field
strength of 1.5 T, is neighboured by 3 clinical Elekta
accelerators at a distance (Isocenter MRL to gun linac) of
7.5 and 5.5 and 11 meters. The peripheral magnetic field
outside of the magnet core of the MRL, the fringe field,
may influence the beam steering of accelerators in
adjacent treatment rooms. This influence for a pre-
clinical prototype was described by Kok et al. in 2009. The
aim of this study is to investigate the influence of the
significantly reduced fringe field of the clinical prototype
on the beam steering of its neighbouring accelerators.
Material and Methods
A STARCHECK MAXI detector array (PTW Freiburg,
Germany) was mounted on all the neighbouring
accelerators with a frame that puts the array on isocentre
height. An inclinometer was attached to the gantry to
acquire a gantry rotation signal. For every available
energy, two 360 degree arcs (clockwise and counter
clockwise) were irradiated with a 40x40 field. A
measurement of beam profile was acquired in movie mode
at a frame rate 2.5 Hz. Beam symmetry (IEC) was
determined for every frame.
These measurements were done before and after ramping
up the MRL magnet, and a 3
rd
time after adjusting the
look-up tables (LUT) which correct the beam steering by
applying a gantry-angle dependent current to the steering
coils (2R and 2T). These LUTs were adjusted using the
accelerator internal monitor chamber.
Results
A change in beam symmetry as a function of gantry angle,
before and after ramping the magnet, of up to 4% (Linac
A) and 1% (Linac B) is observed, causing beam symmetry
on both linacs to be out of tolerance (IEC 102%). Linac C
did not show any significant change. Figure 2 shows the
LUT before ramping the magnet (pre) and after
adjustment (post) and the difference for Linac A for the
10 MV beam. After adjustment of the beam steering on
Linac A and B, the symmetry was within tolerance for all
gantry angles. Adjusting the LUTs took 1.5 hours per linac.
Conclusion
The influence of the MRL fringe field is less than described
by Kok for the pre-clinical prototype, but does still
influence the beam steering of the accelerators in
adjacent treatment rooms. The LUTs of 2 accelerators,
that were situated the closest to the MRL, but outside the
0.5 Gauss line, needed to be adjusted in order to get beam
parameters within tolerances. Adjusting the LUTs fully
corrected the influence of the magnetic fringe field of the
MRL. In case of an unexpected ramp down of the magnet
(i.e. quench) both neighbouring accelerators cannot be
used clinically before the LUTs are adjusted to the new
situation. Adjustment of the LUT can be done in a short
time by experienced personnel, without a dedicated
measurement
device.
References:
Kok et al., Phys. Med. Biol.
54
(2009) N409–N415
OC-0439 Treating patients with Dynamic Wave Arc: first
clinical experience
M. Burghelea
1
, D. Verellen
2
, J. Dhont
1
, C. Hung
3
, K.
Poels
4
, R. Van den Begin
1
, M. Boussaer
1
, K. Tournel
1
, C.
Jaudet
1
, T. Reynders
1
, T. Gevaert
1
, V. Simon
5
, M. De
Ridder
1
1
Universitair Ziekenhuis Brussel- Vrije Universiteit
Brussel, Department of Radiotherapy, Brussels, Belgium
2
GZA Ziekenhuizen- Sint Augustinus – Iridium
Kankernetwerk Antwerpen, Radiotherapy Department,