ESTRO 36 Abstract Book

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S153 ESTRO 36 2017
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irregularities. The limitation of CBCT for needing bony
landmarks, surrogates, the need for large tissue density
differences or the retrospective binning to assess motion
data will be solved when using MRI. So MRI is at the very
least a much better CBCT in the sense that it provides
direct visualization of target and surrounding structures.
CBCT guided proton therapy is lagging behind on the much
needed image guidance offered by MRI and hybrid MRI
radiotherapy systems will improve position verification.
On-line MRI will also enable on-line re-planning strategies
that are not, or only for some sites, feasible with CBCT as
an input. This on-line re-planning fits seamlessly into the
large research interest of the radiotherapy community to
adapt the dose more to the actual anatomy and deliver
more conformal dose distributions, currently being
implemented via library of plans or off-line re-planning
strategies.
Moreover, integrated MRI allows imaging during radiation
delivery. This way, assumptions on anatomical stability or
motion as determined on pre-treatment data can be
verified. Also, the intra-fraction volumetric imaging
provides the input for dose reconstruction, so even if the
pre-treatment assumptions are failing and the anatomy is
moving/deforming unexpectedly, one can reconstruct
exactly what the dose delivered is. This can be used for
off-line re-optimization for remaining fractions.
Additionally, as this dose reconstruction can be done in
near real-time, one can also built adaptation triggers on it
such as gating and ultimately intra-fraction re-planning
strategies. The latter would be truly interventional
radiosurgery where the dose distribution is continuously
adapted to the mobile anatomy.
Another advantage of integrated MRI radiotherapy systems
is the capability to assess functional parameters such as
perfusion or water diffusion, from the patient in
treatment position. This can provide great insight in
treatment response and temporal behavior during the
course of radiotherapy.
In summary, there is a clear desire from the image guided
radiotherapy community to use more and better imaging
prior and during radiation delivery. MRI guided photon
therapy can fulfill this desire and will contribute to more
precise radiation delivery and to a more hypo-fractionated
approach. With that hybrid MRI radiotherapy systems will
become the first choice for radiotherapy and CBCT guided
proton therapy is mainly indicated in case the integral
dose is treatment limiting, e.g. for pediatrics.
SP-0299 Against the motion
A. Lomax
1 Paul Scherrer Institute PSI, CPT, Villigen PSI, Switzerland
Abstract not received
Proffered Papers: Intra-fraction motion management
OC-0300 Proof of tumor position during SBRT delivery
using (limited-arc) CBCT imaging
C. Hazelaar1, M. Dahele 1, B. Slotman1, W. Verbakel1
1 VU University Medical Center, Radiotherapy,
Amsterdam, The Netherlands
Purpose or Objective
SBRT requires accurate patient positioning and robust
positional verification during irradiation itself is desirable.
We investigated if CBCT scans reconstructed from
(collimated) fluoroscopic kV images acquired during
irradiation, including over a limited arc length, can
provide information on the average tumor position, for
spine and lung SBRT.
Material and Methods
In total, 38 fluoroscopy datasets (1 dataset/arc) of 16
patients treated with spine SBRT were used for full-arc
CBCT reconstruction. The kV images were continuously
acquired at 7, 11, or 15 frames/s with a field size ranging
from 10.5x9cm² to 26.6x20cm² (full field) during
flattening filter free VMAT delivery. For reconstruction, a
standard “spotlight” mode template was modified to suit
our data, i.e. full 360° trajectory, full fan, no filters, and
100 kV. The FDK filtered back projection algorithm was
used to reconstruct the CBCTs and the scans were
matched to the planning CT in Offline Review (Varian
Medical Systems, Palo Alto, CA). For validation purposes,
the resulting match values were compared to the average
spine offset values found using template matching +
triangulation of the individual kV images. For lung SBRT,
limited-arc CBCTs were reconstructed from fluoroscopic
images acquired during irradiation of a lung lesion
embedded in a 3D printed anthropomorphic thorax
phantom and of one patient treated in breath-hold. In
order to determine which arc length is required to obtain
sufficient image quality for reliable CBCT-CT matching,
multiple limited-arc CBCTs were reconstructed using arc
lengths from 180° down to 20° in steps of 20°.
Results
3D spine CBCT-CT registration revealed mean positional
offsets of -0.1±0.8 mm (range: -1.5–2.2) for the lateral, -
0.1±0.4 mm (range: -1.3–0.7) for the longitudinal, and -
0.1±0.5 mm (range: -1.1–1.3 mm) for the vertical
direction. Comparison of these match results to the
average spine offsets found using template matching +
triangulation showed mean differences of 0.1±0.1 mm for
all directions (range: 0.0–0.5 mm). For limited-arc CBCTs
of the lung phantom, the automatic CBCT-CT match
results were ≤1mm in all directions for arc lengths of 60-
180°, but in order to perform 3D visual verification, an arc
length of at least 80° was found to be desirable. 20° CBCT
reconstruction still allowed for positional verification in 2
dimensions. The figure illustrates a limited-arc CBCT over
80° for a phantom and 100° for a patient.
Conclusion
Using standard techniques, we have been able to obtain
CBCT reconstructions of planar kV images acquired during
VMAT irradiation. For treatments consisting of partial
arcs, e.g. lung breath-hold treatments, limited-arc CBCTs
can show the average tumor position during the actual
treatment delivery. It is anticipated that this capability
could be implemented clinically with few modifications to
current treatment platforms. This could substantially
improve positional verification during irradiation.
OC-0301 Target position uncertainty during visually
guided breathhold radiotherapy in locally advanced
NSCLC
J. Scherman Rydhög1, S. Riisgaard Mortensen1, M.
Josipovic1, R. Irming Jølck 2, T. Andresen3, P. Rugaard
Poulsen4, G. Fredberg Persson1, P. Munck af Rosenschöld1
1Rigshospitalet, Department of Oncology- Section of
Radiotherapy, Copenhagen, Denmark
2DTU Nanotech and Nanovi Radiotherapy A/S,
Department of Micro-and Nanotechnology- Center for
Nanomedicine and Theranostics, Lyngby, Denmark
3DTU Nanotech, Department of Micro-and