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ESTRO 36 2017
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PO-0861 Geometric validation of a 4D-MRI guided
correction strategy on the MR-Linac
T. Van de Lindt
1
, R. Koopman
1
, A. Van de Schoot
1
, I.
Torres-Xirau
1
, U. Van der Heide
1
, J.J. Sonke
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiation Oncology, Amsterdam, The
Netherlands
Purpose or Objective
Currently in radiotherapy, respiratory motion correction
strategies are performed by the use of 4D-(CB)CT.
However, moving targets in for example the upper
abdomen are not (clearly) visible on these images because
of low soft-tissue contrast. The introduction of an
integrated MRI and linear accelerator (MR-Linac) will allow
for daily MRI-guidance of the tumor. Therefore, the aim of
this study was to develop and validate a 4D-MRI guided
mid-position (midP) correction strategy on an MR-Linac.
Material and Methods
Experiments were performed on an MR-Linac (ATL1, Elekta
AB, Sweden), using the CIRS MRI-LINAC Dynamic Phantom
(CIRS Inc., USA). The moving cylinder was filled with
anisotropic MRI contrasts and a Perspex spherical target.
Motion was performed in CC direction using a Lujan 4
motion pattern with a 20mm amplitude and 4s period.
First, a T2-weighted MRI scan was acquired in midP. The
cylinder and target were segmented and the target was
expanded with a non-uniform margin (LR, AP:10mm;
CC:20mm). A density overwrite of 1 was assigned to the
structures and a treatment plan consisting of a single
anterior beam shaped around the PTV was created in
Monaco (Version 5.19.01 Research). Then, baseline shifts
in CC direction of 5, 10, 15 and 20mm were applied to the
phantom motion. For every shift, a retrospective self-
sorted 4D-MRI was acquired (axial single-shot TSE,
2x2x5mm
3
, TE/TR=60/400ms, 30dyn) and each phase was
registered to the midP reference image to calculate the
time average displacement. The plan was adapted
accordingly, performing a virtual couch shift (simple dose
shift) using aperture morphing in Monaco. All plans were
delivered while electronic portal imaging device (EPID)
cine images were acquired. The time average
displacement of the target was calculated from the EPID
images and geometric accuracy of the workflow was
quantified as the distance of the average position of the
target to the field edges in the EPID images.
Results
In Figure1, MRI and EPID images of the midP and a shifted
inhale and exhale position are shown. Table1 shows the
time average displacement of the target in the 4D-MRI and
the EPID images with respect to the reference as well as
the distance of the average target position to the field
edges.
The geometric accuracy of the 4D-MRI guided workflow
was 0.3±0.4mm in CC, which includes the 4D-MRI
registration accuracy.
Conclusion
4D-MRI guidance on an MR-Linac was shown to be feasible
and had sub-millimeter accuracy. Such a correction
strategy has great potential for moving targets that are
difficult to visualize on alternative image guidance
modalities.
Acknowledgements: This research was partly sponsored by
Elekta AB, Stockholm, Sweden. The authors would like to
thank CIRS Inc., Robert Spaninks (Elekta) and Jochem
Kaas, Natasja Janssen, Ben Floot and Marco van den Berg