481 / 1096
S466
ESTRO 36
_______________________________________________________________________________________________
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
acquired and analyzed using the offline image review
workspace in Mosaiq (v1.60, Elekta, Stockholm, Sweden)
to determine intrafractional patient movements. From
each CBCT, 3 translational and 3 rotational coordinates
were obtained.
Results
The average time between the patient setup CBCT and the
post treatment CBCT was 9 minutes (range, 6-14). The
average absolute translational variations (± 1 SD) obtained
from the post-treatment CBCT was 0.7 ± 0.7, 0.7 ± 0.8 and
0.5 ± 0.6 mm in the lateral, longitudinal and vertical
directions, respectively. The average absolute rotational
angles were 0.8 ± 0.7, 0.7 ± 0.4 and 0.8 ± 0.6˚ along pitch,
roll and yaw, respectively. Histograms of translational and
rotational deviations for all patients are shown in figure
1.
Conclusion
Near-rigid body immobilization, CBCT image guidance and
six degrees of freedom correction yields minimal
intrafractional motion and safe stereotactic spine
radiosurgery delivery. It is not easy to determine the
effect of rotational deviations. However, for treatment
plans with the isocenter plased in the center of the target
volume, which is the case for these patients, small
rotations would not result in large deviations in dose to
the target volume or adjacent OARs. There are different
approaches that could result in less patient motion and
increased precision in dose delivery. The combination of a
polyethylene sheet with a vacuum cushion would
presumably result in a more rigid immobilization.
Intrafractional imaging during treatment is another
alternative that could increase precision in dose delivery.
PO-0858 Intra-fraction motion quantification of head-
and-neck tumors using dynamic MRI
T. Bruijnen
1
, R.H.N. Tijssen
1
, M.E.P. Philippens
1
, C.H.J.
Terhaard
1
, T. Schakel
1
, J.J.W. Lagendijk
1
, C.P.J.
Raaijmakers
1
, B. Stemkens
1
1
UMC Utrecht, Radiotherapy, Utrecht, The Netherlands
Purpose or Objective
Previous research primarily focused on the effect of
deglutition on the accumulated tumor do se. However,
resting-state movements, such as respiratory-induced
tumor motion, has been largely overlooked. Nonetheless,
this may play an important role in the size of the
treatment volume of head-and-neck cancer. Here, we
investigate head-and-neck resting-state tumor motion in a
radiotherapy treatment position in order to provide
guidance for adequate internal target volume (ITV)
determination.
Material and Methods
Acquisition: 46 patients with head-and-neck cancer (6
nasopharyngeal/ 25 oropharyngeal/ 15 laryngeal)
underwent pretreatment clinical MRI scanning in a
radiotherapy treatment setup, including a custom-fit
immobilization mask. Two 2D sagittal dynamic acquisitions
(RF- and gradient-spoiled gradient echo; TE/TR=1.5/3ms;
voxel size=1.42x1.42x10 mm
3
;158 ms temporal
resolution), separated 10 minutes apart, localized to
intersect the tumor were acquired on a 3.0T scanner. GTV
delineations, as performed by a radiation oncologist, were
obtained from the treatment plans.