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S461
ESTRO 36 2017
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PO-0856 Systematic baseline shifts of lymph node
targets between setup and treatment of lung cancer
patients
M.L. Schmidt
1
, L. Hoffmann
1
, M.M. Knap
2
, T.R.
Rasmussen
3
, B.H. Folkersen
3
, D.S. Møller
1
, B. Helbo
2
, P.R.
Poulsen
2
1
Aarhus University Hospital, Medical Physics, Aarhus C,
Denmark
2
Aarhus University Hospital, Department of Oncology,
Aarhus C, Denmark
3
Aarhus University Hospital, Department of
Pulmonology, Aarhus C, Denmark
Purpose or Objective
Internal target motion results in geometrical uncertainties
in lung cancer radiotherapy. The lymph node (LN) targets
in the mediastinum are difficult to visualize in cone-beam
computed tomography (CBCT) scans for image-guided
radiotherapy, but implanted fiducial markers enable
visualization on CBCT projections and fluoroscopic kV
images. In this study, we determined the intrafraction
motion of mediastinal LN targets in both the setup CBCT
and fluoroscopic kV images acquired during treatment
delivery, and investigated the baseline shifts and
treatment accuracy of LNs for ten lung cancer patients.
Material and Methods
Ten lung cancer patients had 2-4 fiducial markers
implanted in LN targets by EBUS bronchoscope. A total of
26 markers were evaluated. The patient received IMRT
with daily setup CBCT for online soft tissue match on the
primary tumor. During treatment delivery, 5 Hz
fluoroscopic kV images were acquired orthogonal to the
MV treatment beam. Offline, the marker positions were
segmented in each CBCT projection and fluoroscopic kV
image. From the segmented marker positions, the 3D
marker trajectories were estimated from the
segmentations with sample rate of 11 Hz during CBCT
acquisition and 5 Hz during treatment delivery.
The 3D motion amplitude and mean position of each LN
marker as well as the intrafraction baseline shifts between
setup CBCT and treatment delivery were calculated.
Results
Figure 1 shows the internal motion of one marker at one
fraction. The motion is shown relative to the mean
position during the CBCT scan and corrected for the couch
shift between CBCT and treatment. For this marker, the
baseline shift was 4.8 mm cranially, 0.6 mm posteriorly,
and 0.7 mm towards right. Figure 2a shows the distribution
of intrafraction baseline shifts for all patients and LNs at
all fractions. Systematic LN baseline shifts occurred
between CBCT and treatment delivery in the cranial
direction (mean 2.4 mm (SD 1.9 mm)) and posterior
direction (0.8 mm (1.1 mm)). The frequency of cranial
baseline shifts exceeding 4 mm and 6 mm were 15 % and 4
%. The baseline shifts resulted in systematic mean
geometrical errors during treatment delivery of 2.8 mm
(cranial) and 1.4 mm (posterior)(Figure 2b) for the LNs.
These errors were substantially larger than the sub-
millimeter mean errors expected from the setup CBCT
based soft tissue tumor match when correcting for the
applied couch shifts.
In general, the largest LN motion amplitude was observed
in the cranio-caudal direction both during CBCT and
treatment delivery. The mean motion amplitudes during
CBCT and treatment delivery agreed within 0.2 mm in all
three directions.
Conclusion
Systematic cranial and posterior intrafraction baseline
shifts between setup CBCT and treatment delivery
occurred for mediastinal LN targets in lung cancer patients
and reduced the treatment accuracy. Intrafraction motion
amplitudes were stable throughout each treatment
fraction, as well as the treatment course.
PO-0857 Analysis of Intrafraction Motion in Image-
Guided Stereotactic Radiosurgery of Spinal metastases
J.G. Svestad
1
1
Oslo University Hospital The Norwegian Radium
Hospital, Department of Medical Physics, Oslo, Norway
Purpose or Objective
Stereotactic radiosurgery of spinal metastases include
tight margins and steep dose gradients to the surrounding
organs at risk (OAR). The proximity of the target to the
adjacent spinal cord and the aim of keeping the dose to
the spinal cord within tolerance require a high degree of
precision in dose delivery. This study aimed to evaluate
intrafractional motion using cone beam computed
tomography (CBCT) image guidance, for immobilized
spinal stereotactic radiosurgery patients, with correction
in all six degrees of freedom.
Material and Methods
Intrafractional motion during spine radiosurgery
treatment in 16 patients (26 fractions) was retrospectively
analyzed. All patients were immobilized in the BlueBAG
BodyFIX (Elekta, Stockholm, Sweeden) that uses a vacuum
pump to create a precise mold of the patient’s position.
Radiation treatment was performed using a Varian
Truebeam STx linear accelerator equipped with a
PerfectPitch 6 degrees of freedom couch (Varian Medical
Systems, Inc., Palo Alto, USA).
Following initial setup, a CBCT was acquired for patient
alignment and patient position was corrected in all six
degrees of freedom. Patients were resetup manually, and
the process was reinitiated if the ± 3˚ rotational couch
tolerance was exceeded. A post treatment CBCT was