D McClatchy*, K Remillard, A Saraf, J Efstathiou, J Pursley, Massachusetts General Hospital, Boston, MA

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  D McClatchy

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Rectal spacer hydrogels are used in prostate cancer patients to create space between the prostate and the rectum. Typically, hydrogels are water-equivalent, and an MRI is required for delineation. Iodinated hydrogels are being commercialized to allow visualization on CT. We aimed to simulate and investigate the dosimetric effects of density variations, setup errors, and spacer shifts for patients with iodinated spacers compared to water-equivalent spacers.

Methods: A clinically deployed RayStation 8A planning system was used to create a standard-of-care prostate VMAT plan for a cadaver with an implanted iodinated spacer (SpaceOAR Vue, Augmenix). RayStation’s built-in dose deformation tools were used to simulate setup errors of 5mm, spacer shifts of 2mm, and changes in spacer density. The resulting changes in average dose and D1 were analyzed for all targets and OARs. The simulated setup error and spacer shift were repeated for n=4 clinical patients with water-equivalent spacers and the first n=2 clinical patients with iodinated spacers (ntot=3 with iodinated spacers).

Results: The iodinated hydrogels had a significantly higher average HU value on the planning CT than water-equivalent hydrogels ( 113 vs. 20, p<0.003), and could be delineated on CT alone. With a simulated doubling of iodinated HU in the cadaver, D1 and Davg changed by <1.5% across all targets and OARs. For both hydrogels, dosimetric changes of >20% were observed for some OARs after simulated 5mm set-up errors, but target D1 and Davg exhibited less than a 5% change. For 2mm spacer-shifts, all OAR/target dosimetric changes were <5% except for the rectum. For both setup and spacer-shift analyses, all dosimetric changes were insignificant between water-equivalent and iodinated spacers (p>0.01).

Conclusion: This work has provided initial evidence showing iodinated spacer hydrogels provide significant increase in CT contrast to aid in spacer delineation while causing minimal dosimetric changes compared to water-equivalent hydrogels.

Keywords

Deformation, Prostate Therapy, Polymer Gels

Taxonomy

TH- External Beam- Photons: Dose reconstruction over deforming anatomies

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