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the 4DCBCTs using implanted Calypso beacons in the lung
as ground truth.
Material and Methods
4DCBCTs were reconstructed from projections for
treatment setup CBCT for 1-2 fractions of 6 patients using
the prior image constrained compressed sensing (PICCS)
method and the FDK (Feldkamp-Davis-Kress) method. For
both methods reconstructions were performed based on
the internal Calypso motion trajectories (three beacons
per patient) or an external respiratory signal (Philips
Bellows belt). The Calypso beacons were segmented for
all 10 bins of the 4DCBCTs and the beacon centroid motion
compared to the motion range from the images. Paired t-
tests were performed on the mean size of excess beacon
motion and the proportion of scanning time with motion
larger than represented in 4DCBCT in order to identify a
superior reconstruction method.
Results
All methods for 4DCBCT reconstruction failed to capture
sudden motion peaks during scanning and underestimated
the actual beacon centroid motion (see Fig. 1), which is a
result of phase-based binning and averaging the images in
a bin. For the SI direction in general, reconstructions using
the belt signal, led to a representation of a larger motion
range (PICCS: 4.88±3.30mm, FDK: 4.81±3.35mm) than the
Calypso-based reconstruction (PICCS: 4.71±3.22mm, FDK:
4.76±3.29mm). However, the difference was not
significant, as for none of the other directions. For
comparison also the Calypso motion during the treatment
exceeding the 4DCBCT motion range is shown in Figure 1.
Figure 1. Proportion of intra-CBCT (solid lines) and intra-
treatment (dashed lines) motion in SI direction larger than
the motion represented in the reconstructed 4DCBCT for
reconstruction with a) PICCS Belt, b) PICCS Calypso, c) FDK
Belt and d) FDK Calypso.
Conclusion
All 4DCBCT reconstruction methods failed to rep resent
the full tumour motion range , but performed similar.
Thus, the belt as an external surrogate is sufficient for
4DCBCT reconstruction. For a safe treatment in spite of
motion exceeding the motion range from the images,
adequate ITV-to-PTV margins or a real-time treatment
adaptation directly tackling motion peaks and
unpredictable motion need to be chosen.
While the 4DCBCT is not able to capture and predict the
whole motion range of a treatment fraction, it serves as a
valuable tool for accurate patient setup.
PO-0860 Characterization of a novel liquid fiducial
marker for organ motion monitoring in prostate SBRT
R. De Roover
1
, W. Crijns
2
, K. Poels
2
, R. Peeters
3
, K.
Haustermans
1,2
, T. Depuydt
1,2
1
KU Leuven - University of Leuven, Department of
Oncology, Leuven, Belgium
2
University Hospitals Leuven, Department of Radiation
Oncology, Leuven, Belgium
3
University Hospitals Leuven, Department of Radiology,
Leuven, Belgium
Purpose or Objective
Stereotactic body radiotherapy (SBRT) for prostate is a
cost-effective treatment option with improved patient
comfort and maintained excellent clinical outcomes.
However, to ensure low levels of toxicity very accurate
delivery is imperative, especially when combined with
integrated focal boosts as in the Hypo-FLAME clinical trial
methodology. Within this context, intra-fraction organ
motion management becomes even more relevant. The
novel BioXmark® (Nanovi A/S) biodegradable radio-
opaque liquid fiducial marker was studied as alternative
for current markers used in prostate motion management.
The marker can be injected with very thin needles (down
to 25G) and the injection procedure allows to vary the
marker-size by altering the injected volume. In this study
the automatic detectability of BioXmark® in 2D kV X-ray
imaging was determined. Additionally, as Hypo-FLAME
involves a multi-modality delineation of the boost foci,
visibility/artefacts in different types of volumetric
imaging was investigated.
Material and Methods
BioXmark® consists of sucrose acetate isobutyrate (SAIB),
iodinated-SAIB and ethanol solution. Upon injection,
ethanol diffusion out of the solution causes a viscosity
increase and formation of a gel-like marker.
A total of 8
markers (size 5-300 µL) organized in a rectangular grid
were injected into a gelatin phantom.
X-ray projection images using the Varian TrueBeam STx
OBI were obtained by putting the gelatin phantom on top
of an anthropomorphic pelvic phantom. A total of 120
images of each marker were acquired varying the positions
of the marker relative to pelvic bony structures and using
24 clinically relevant X-ray kVp/mAs settings. Volumetric
imaging was performed with CT, CBCT and MRI using a CIRS
pelvic phantom.
Automated marker detection was based on the normalized
cross-correlation (NCC) of the projection image with a
marker template retrieved from the CT image. Prior to
detection, single markers were artificially isolated to
minimize interference between detection of the different
markers. Reference marker positions were manually
determined on the image with highest exposure settings.
A detection was successful if the optimal NCC value lied
within a 1 mm (3 pixels) tolerance of the reference
position. The tolerance was extended to 4 pixels to deal
with the uncertainty of manual delineation.
Results
Detection success rates augmented with increasing
marker-size obtaining a maximum for intermediate size
(25-75 µL) markers (Figure 1). Larger marker sizes (>75 µL)
had decreased detection success rates due to higher
susceptibility for interference with the bony structure
edges. Volumetric image artefacts were minimal whilst
the markers itself were clearly visible (Figure 2).
Conclusion
Intermediate size (25-75 µL) BioXmark® liquid fiducial
markers showed high detectability and minimal image
artefacts making them a patient friendly alternative (thin
needles) for the current markers used in fiducial-marker-
based intra-fraction organ motion monitoring in prostate
SBRT.