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About 80% of absolute shift errors in all MR-P
xy
s were not
significantly different from MR-ref in all directions. No
specific trend between PAT-f and shift error was observed,
implying fusion accuracy could not be predicted by PAT-f
value. The errors of T2W were significantly smaller than
T1W in almost all of the shifts (p<0.05). Junior observers
performed significantly better than seniors when fusing
T1W images (p<0.05). For treatment verification purpose,
T1W MR-P
42
(88s) and T2W MR-P
22
(173s) were chosen since
they were the fastest sequences with comparable image
quality to MR-ref. However when considering fusion
errors, MR-P
44
for T1W (49s) and T2W (59s) images, with
an even shorter scan time, should be opted instead as its
verification accuracy had no significant difference in most
shift directions from MR-ref (Fig.2) despite significantly
degraded image quality.
Conclusion
The degradation of image quality by increasing PAT-f does
not necessarily affect the fusion accuracy. MR-P
44
with
acceptable verification accuracy and shortest scan time is
therefore proposed for MRI-guided RT in C and HN regions.
OC-0163 Online workflow for the First-in-Man study
on bone metastases at the MRI-linear accelerator
L.T.C. Meijers
1
, S.J. Hoogcarspel
1
, A.N.T.J. Kotte
1
,
C.N.N. Nomden
1
, G.G. Sikkes
1
, I.H. Kiekebosch
1
, E.N.
Groot de
1
, G.H. Bol
1
, B. Asselen van
1
, I.M. Jurgenliemk-
Schulz
1
, L.G.W. Kerkmeijer
1
, B.W. Raaymakers
1
1
UMC Utrecht, Radiotherapy department, Utrecht, The
Netherlands
Purpose or Objective
The first treatment on the MR-Linac in the UMC Utrecht
includes patients with bone metastases, the first-in-man
study. A fast workflow is required since all processes will
be performed in an online setting. The aim of the present
study is to test the feasibility of an online workflow for
these patients at the MR-Linac within a 30 minute time
limit.
Material and Methods
A workflow with offline and online procedures was
developed for the MR-Linac. To test this workflow, the
Alderson phantom was used, a human body shaped
phantom with tissue-equivalent material. The phantom
yields sufficient CT and MR contrast. The offline workflow
included acquisition of a reference CT scan and manual
delineation of the spinal cord and one lumbar vertebra
(vertebra = CTV, PTV-CTV margin = 5 mm). The online
workflow at the MR-Linac, shown in figure 1, included
setup of the patient (= Alderson phantom) and acquisition
of a transversal 3D T1-weighted TSE MR image (optimized
MR sequence for bone metastases). A deformable
registration was performed (ADMIRE, v1.13.5, Elekta AB,
Sweden) of the obtained MR-images with the reference CT
for automatic contour propagation and CT deformation.
Furthermore a 3-field IMRT technique treatment plan was
generated automatically using a research version of
Monaco (v5.19.01. Elekta AB, Sweden) with a prescribed
dose of 8 Gy. Additional to the MRI, independent position
verification was performed using two orthogonal MV
beams and the plan was delivered to the phantom. The
offline and online procedures were tested three times,
each time for a different lumbar vertebra. The duration of
the individual procedures within the online workflow was
measured.
Results
Time measurements of the individual procedures were as
follows (table 1); MRI acquisition time of 5:02 minutes,
deformable image registration, contour propagation and
generation of the deformed CT within a range of 40-44
seconds, development of an automatic treatment plan
within a range of 6:20-6:30 minutes and position
verification and dose delivery within a range of 2:32-2:42
minutes. The total time of these online procedures ranges
from 14:34-15:01 minutes. These time measurements do
not include the additional time for patient setup, data
transfer and the time needed for a physician to evaluate
the propagated contours and treatment plan at the MR-
Linac. This can potentially increase the total workflow
time.
Conclusion
From a technical perspective, the online workflow
developed for the first-in-man study on the MR-Linac can
be performed well within 30 minutes to treat patients with
bone metastases. Current work is focused on automation
of the data transfer process.
OC-0164 Set-up reproducibility on an MR-Linac
A. Betgen
1
, T. Vijlbrief
1
, L. Wiersema
1
, V.W.J. Van Pelt
1
,
J.J. Sonke
1
, U.A. Van der Heide
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Department of Radiation Oncology, Amsterdam,
The Netherlands
Purpose or Objective
MRI integrated Linacs are becoming available for
improving the accuracy of radiation therapy. The MR-Linac
(1ATL, Elekta AB, Sweden) is an integration of a 7MV linear
accelerator and a modified 1.5T Ingenia MRI (Philips
Healthcare, NL). Like on a conventional MRI, the table
only moves in longitudinal direction while the patient is in
the bore. No laser system is available. An indexation at