Characterization of a Novel Glue Fiducial in a Porcine and Phantom Model 
Introduction/Objectives: Glue fiducials can be delivered and imaged in a water filled bladder to then be visualised sufficiently for radiotherapy verification. An advantage is that the glue fiducials produce less artefact than standard gold fiducials. The objective was to investigate Lipiodol tissue glue mixtures with respect to deliverability and visualization in a porcine bladder and characterize and compare to gold fiducials for imaging verification. 
Materials/Methods: The fiducials consisted of Lipiodol mixed with one of two tissue glues: Histoacryl™ or Glubran2™. To simulate the clinical procedure and deliverability, the mixtures were injected into the submucosa of two fluid filled porcine bladders. The porcine bladders were imaged in a cadaver porcines pelvis to simulate human body conditions. Imaging included simulation CT, Cone Beam CT (CBCT) and kilovoltage (KV) and megavoltage (MV). 

To characterize the fiducials and artefact a CT tissue equivalent phantom study was performed. The glue fiducial mixes and controls were pipetted in a wax mold that was inserted into a torso-shaped phantom. The glue fiducials were created in two sizes: gold seed (3x0.9mm) and one similar to that produced in the porcine study (7x4mm). A simulation CT was performed and Hounsfield Unit (HU) histograms extracted for artefact analysis. HU units beyond normal tissue ranges (+150/-150HU) were indicative of fiducial/artefact. Fiducials were also scored according to visualization by a trained radiation therapist. 
Results: Discrete Lipiodol glue fiducials were successfully inserted. The Lipiodol glue combinations were visualised on CT, CBCT and KV imaging but not on MV due to insufficient contrast. Artefact analysis showed gold was brighter than all glue fiducials, producing a higher mean HU (Gold 873HU; Lipiodol/Glue 269-360HU). However, gold produced greater levels of artefact with consistently higher histogram standard deviations (Gold 110HU; Lipiodol/Glue 30-39HU) and overall broader values across all histogram percentile ranges.
Conclusion: Lipiodol glue mixtures produced deliverable fiducials in a bladder model. While not as bright as gold, they were visible on all radiotherapy imaging except MV and produced less artefact. Glue fiducials show potential as a cost effective fiducial that can possibly be used in many situations.