2369 - Spectral CT Imaging for the Visualization of Hyaluronic Acid Based Rectal Spacers prior to Radiation Therapy

R. J. Megahed, C. L. Chien, F. Kalantari, S. Maraboyina, and P. Sabouri; Department of Radiation Oncology, UAMS Winthrop P. Rockefeller Cancer Institute, University of Arkansas for Medical Sciences, Little Rock, AR

Purpose/Objective(s): Hyaluronic Acid (HA) based rectal spacers are often placed prior to radiation therapy to provide a physical barrier between the rectum and prostate to reduce dose to the rectal wall. While these spacers are seen clearly on transrectal ultrasound which helps with placement, visualization on computed tomography (CT) remains less optimal. Magnetic resonance imaging (MRI) is often necessary for volume delineation. Spectral CT imaging can enhance the visualization of these spacers and possibly reduce the necessity of an MRI. A spectral CT acquires high and low energy data which can be recomposed into virtual monoenergetic (MonoE) CT at different kiloelectron volt (keV) levels. In this research we evaluated the potential of spectral CT imaging for enhancing the visibility of HA rectal spacers. Materials/

Methods: MRI and Spectral CT were acquired and registered for volume delineation for six patients who underwent HA rectal spacer insertion prior to radiotherapy. The spacer for each patient was contoured by the physician on the MRI, the MonoE 40 keV and the conventional 120keV CT from the same spectral CT scan. The contour on the MRI was considered as the reference, and the overlaps between the contours of the MRI and MonoE CT (or the conventional CT) were assessed using the Dice Similarity Coefficient (DSC) and the Hausdorff Distance (HD). The contrast to noise ratio (CNR) was calculated to quantify the visualization on both MonoE and the conventional CT. All three metrics were evaluated using a paired t-test to check if the improvement of visualization is statistically significant (p<0.05) in MonoE 40 keV CT as compared to the conventional 120 kVp CT.

Results: The overlap of the contours between the MRI and the monoE 40 keV CT was significantly better than the one between the MRI and the conventional CT based on the results of DSC and HD (p<0.05). The result of CNR showed that MonoE 40keV CT has better image contrast between the spacer and prostate than the conventional CT (p<0.05).

Conclusion: We show that contours created from a recomposed monoenergetic 40 keV CT better resemble those created from MRI imaging. This shows that Spectral CT imaging can significantly improve visualization and delineation accuracy of rectal spacers.