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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.
Image analysis: The two dynamic MR series were analyzed
separately to quantify typical one minute tumor
displacements along two orthogonal directions; superior
(S), inferior (I), anterior (A) and posterior (P). All time-
points affected by non-respiratory associated tongue
motion or deglutition were manually discarded from the
analysis. One time-point was selected as the reference
and all other points were non-rigidly registered to the
reference using a validated optical flow algorithm [1].
Motion fields were computed for all the pixels inside the
tumor and combined into a single distance metric by
assessing the maximum contour coordinates (FIG1-A+B).
Typical 10-minutes displacements were investigated by
computing the difference
Results
Mean maximum 1-minute tumor displacements amounted
to 2.08 (SD 2.34) mm in (S), 2.26 (SD 1.48) mm in (I) and
1.66 (SD 0.93) mm in (A); 1.65 (SD 1.23) mm in (P) (FIG1-
D). However, there was strong inter-subject variability
within the laryngeal and oropharyngeal subgroups, with
laryngeal tumors exhibiting periodic displacements up to
14 mm in (S) (FIG1-C). The typical 10 minute shifts were
smaller than 2 mm for all patients (not shown in figure),
with means values of 0.62 (SD 0.44) mm in (S) ; 0.64 (SD
0.58) mm in (I); 0.49 (SD 0.51) mm in (A); 0.41 (SD 0.36)
mm in (P).
Conclusion
Although tumor displacements were small, there were
three subjects that exhibited resting-state displacements
larger than 5 mm. This suggests individualized ITVs for the
laryngeal tumors and oropharyngeal tumors, instead of
applying 5 mm margins in both I and S directions for
laryngeal tumors. The 10-minutes intrafraction shift was
smaller than 2 mm across all the patients and directions
and did not show any outliers.
PO-0859 Impact of 4DCBCT reconstruction algorithm
and surrogate on motion representation
E. Steiner
1
, C.C. Shieh
1
, V. Caillet
2
, N. Hardcastle
2
, C.
Haddad
2
, T. Eade
2
, J. Booth
2
, P. Keall
1
1
University of Sydney, Radiation Physics Laboratory-
Sydney Medical School, Camperdown, Australia
2
Northern Sydney Cancer Centre- Royal North Shore
Hospital, Radiotherapy Department, St Leonards,
Australia
Purpose or Objective
Lung tumour motion exceeding the observed motion from
planning 4D computed tomography (4DCT) is of concern in
stereotactic ablative body radiation therapy (SABR).
4D cone-beam CT (4DCBCT) facilitates verification of
tumour trajectories before each treatment fraction and an
accurate patient setup. This work aims to assess the
impact of the selection of the reconstruction algorithm
and surrogate for binning on the motion representation in