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S468

ESTRO 36

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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