S468
ESTRO 36
_______________________________________________________________________________________________
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.
PO-0861 Geometric validation of a 4D-MRI guided
correction strategy on the MR-Linac
T. Van de Lindt
1
, R. Koopman
1
, A. Van de Schoot
1
, I.
Torres-Xirau
1
, U. Van der Heide
1
, J.J. Sonke
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiation Oncology, Amsterdam, The
Netherlands
Purpose or Objective
Currently in radiotherapy, respiratory motion correction
strategies are performed by the use of 4D-(CB)CT.
However, moving targets in for example the upper
abdomen are not (clearly) visible on these images because
of low soft-tissue contrast. The introduction of an
integrated MRI and linear accelerator (MR-Linac) will allow
for daily MRI-guidance of the tumor. Therefore, the aim of