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ESTRO 36
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In HDR brachytherapy, image guidance is crucial for
accurate and safe dose delivery. Accordingly, MR-guided
HDR brachytherapy is in development at our institution.
This study demonstrates the testing of a recently
developed MR-compatible afterloader, while operating
simultaneously with MR imaging, as well as an MR-based
method for real-time source position verification.
Material and Methods
Experimental set-up:
A prototype of an MR-compatible afterloader (Flexitron,
Elekta) was developed. This afterloader was made MR-
compatible by providing every part as well as the cover
with RF shielding. The source cable was replaced by a
plastic cable containing a piece of steel at its tip, serving
as a dummy source. The afterloader was placed next to
the MRI scanner and connected to a catheter positioned in
an Agar phantom (doped with MnCl2), see Fig. 1.
Afterloader management:
The afterloader was programmed to send the source (I) to
10 dwell positions, with a 10 mm step size, remaining 10 s
at each position, and (II) to 20 dwell positions, with a 5
mm step size, remaining 0.5 s at each position.
MRI acquisition:
While sending the source to its predefined dwell positions,
MR imaging was carried out on a 1.5 T MR scanner (Ingenia,
Philips) using a 2D gradient echo sequence (TR/TE 2.2/1.0
ms, slice thickness 10 mm, FOV 192x192 mm, acq. matrix
96x96, flip angle 30°, SENSE=2), scanning two orthogonal
slices interleaved with a temporal resolution of 0.114 s per
image.
HDR source localization:
The MR artifact induced by the magnetic susceptibility of
the metallic source was exploited. The artifacts (complex
data) were simulated based on the susceptibility induced
B0 field disturbance [1]. The localization was executed
offline in a post processing operation by phase-only cross
correlation [1,2], to find the translation between the
experimental image and the simulated artifact.
Results
The experiments demonstrated that the prototype MR-
compatible afterloader and the MRI scanner fully
functioned while operating simultaneously, without
influencing each other. The afterloader was able to send
the source to the predefined dwell positions when placed
next to the MRI scanner, without being attracted to or
being disturbed by the scanner. The HDR source positions
could be determined by the described localization method
(now accomplished offline), see Fig. 2. The average
distances between the determined 3D source positions for
cases (I) and (II) were 9.9±0.2 mm and 5.0±0.2 mm,
respectively. The short dynamic scan time (~0.15 s) and
the fast reconstruction/post processing (<0.15 s)
guarantee that source localization will be possible in real
time.
Conclusion
The MR-compatible afterloader developed in this study
and a commercial 1.5 T MRI scanner were demonstrated
to fully function while operating simultaneously, enabling
real-time HDR source position verification for MR-guided
HDR brachytherapy, using a phase-only cross correlation
localization method.
[1] Beld E. et al. 2015 Proc. Intl. Mag. Reson. Med. 24,
#4151.
[2] De Oliveira A. et al. 2008 MRM
59
1043-1050.
OC-0276 Toward adaptive MR-guided HDR prostate
brachytherapy – Simulation study based on anatomy
movements
M. Borot de Battisti
1
, B. Denis de Senneville
2
, G.
Hautvast
3
, D. Binnekamp
3
, M. Peters
1
, J. Van der Voort
van Zyp
1
, J.J.W. Lagendijk
1
, M. Maenhout
1
, M.A.
Moerland
1
1
University Medical Center Utrecht, Departement of
Radiotherapy, Utrecht, The Netherlands
2
UMR 5251 CNRS/University of Bordeaux, Mathematics,
Talence, France
3
Philips Group Innovation, Biomedical Systems,
Eindhoven, The Netherlands
Purpose or Objective
Dose delivery during a single needle, robotic MR-guided
HDR prostate brachytherapy may be impaired by: (1)
needle insertion errors caused by e.g. needle bending, (2)
unpredictable anatomy movements such as prostate
rotations (induced by the insertion or retraction of the
needle), prostate swelling or intra-procedural rectum or
bladder filling. In this study, a new adaptive dose planning
strategy is proposed to assess the second challenge. The