Radiological evaluation of an iodised hydrogel for prostate radiotherapy applications

Purpose: Physical separation of healthy tissue and target volumes in prostate radiotherapy through the insertion of hydrogel can improve patient toxicity rates. An iodised hydrogel may provide anatomical separation of prostate and rectum while being easily visualised through radio-opacity. The aim of this study was to characterise SpaceOAR VueTM in kilovoltage (kV) images and megavoltage (MV) radiotherapy treatment planning. Methods: Two cassettes were 3D-printed, one filled with water and the other with SpaceOAR VueTM. Transmission dose through each cassette was measured in slab phantom geometry and compared for 6MV and 10MV photon energies. The SpaceOAR VueTM slab phantom setup was simulated using computed tomography (CT) and a treatment plan created. The plan was calculated with the hydrogel segmented and material assignment set to water, and the resultant dose compared to corresponding measurement doses. The first 5 patients treated with SpaceOAR VueTM were assessed with the volume and Hounsfield units (HU) of the hydrogel evaluated in CT and cone beam computed tomography (CBCT) imaging. Results: Transmission through Water and SpaceOAR VueTM agreed to within 0.5% for both photon energies. Furthermore, the segmentation of SpaceOAR VueTM and material assignment to water, resulted in a plan dose that agreed to measurement to within 0.5%. Clinically, the SpaceOAR VueTM volume and HU did not vary over patient treatment course, however was found to display differently on different kV imaging modalities. Conclusions: SpaceOAR VueTM was found to be radio-opaque on kV images, but dosimetrically behaved similarly to water in MV treatment beams, making it suitable for clinical use.