ESTRO 36 Abstract Book

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S84 ESTRO 36
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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. Betgen1, T. Vijlbrief1, L. Wiersema1, V.W.J. Van Pelt1,
J.J. Sonke 1, U.A. Van der Heide1
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
the tabletop for the patients’ positioning devices has to
provide a reproducible position. The treatment plan will
be adapted to the patients’ position, anatomical variation
and organ motion of the day. The aim of this study was to
characterize the accuracy of patient set-up with a table
indexing system in the absence of lasers and skin marks.
Material and Methods
This investigation was performed on a conventional MRI.
MR-scans were acquired at 3 different time points from 8
volunteers. The pelvis was chosen as anatomical region of
interest. The tabletop of this MRI is comparable with the
tabletop of the MR-Linac. A head support and a knee
support, indexed on the table, were used for stability and
reproducibility. The first MR-scan was defined as
reference scan and the following two MR-scans were
registered to this reference on bony anatomy. The setup
variability was analyzed in terms of group mean (M),
systematic (Σ) and random errors (σ) for both translations
and rotations. The results were compared to retrospective
set-up data of 79 patients treated for rectum carcinoma
(5x5 Gy), aligned with lasers and skin marks and measured
with CT-scan and Cone Beam-CT position verification.
Because the group of volunteers is relatively small, the
comparison to the rectal cancer patient group is on a
descriptive basis only.
Results
When comparing retrospective set-up data of rectal
cancer patients to the group of volunteers in this
investigation, for translations, the group mean for the
patient group seem to show a better set-up reproducibility
in the LR and CC direction as compared to the volunteers.
This resulted in group means closer to zero with
corresponding smaller Σ errors and σ errors. In the AP
direction, the mean and standard errors did not seem to
show apparent differences. For rotations the results for
both groups were comparable. The results are presented
in table 1.
Conclusion
For volunteers, without the use of laser alignment,
translations seem to be larger in LR and CC direction.
Rotations were comparable for both groups. However, for
daily practice, the impact of this increased uncertainty is
likely small relative to uncertainties of internal organ
motion that can be in the cm range. In daily on-line
corrections, the combination needs to be considered in
positioning pelvic cancer patients without skin marks, on
an MR-Linac.
OC-0165 TPUS vs CBCT: comparison of daily inter-
modality derived setup shifts for prostate
radiotherapy.
E.P.P. Pang1,2, K. Knight2, M. Baird 2, J.M.Q. Loh1, E.T.Y.
Chen 1, G.K. Low1, C.C.C. Yap1, A.H.S. Boo1, J.K.L. Tuan1,3
1National Cancer Centre Singapore, Division of Radiation
Oncology, Singapore, Singapore
2Monash University, Faculty of Medicine- Nursing and
Health Sciences Medical Imaging & Radiation Sciences,
Melbourne, Australia
3Duke-NUS, Graduate Medical School, Singapore,
Singapore