WE‐EF‐303‐01: FEATURED PRESENTATION: Hydrogel Fiducial Markers for In‐Vivo Proton Therapy and Range Verifications Using PET

Abstract

Purpose:Currently there are no clinically used techniques for in‐vivo proton treatment/range verification. Our aim was to develop patient implantable markers that can be visualized in CT/x‐ray for treatment‐planning/beam‐positioning, and also in PET for proton treatment/range verification.Methods:Biocompatible/biodegradable hydrogel polymers were immersed in O18‐enriched water and O16 water, respectively to create O18‐water hydrogels (0.5 cm3) and O16‐water hydrogels (1 cm3) (both >99% water and <1% polymer). Also, 5–8 µm Zn powder was suspended in O16 water and O18‐enriched water and cross‐linked with hydrogel polymers to create Zn/16O‐water hydrogels (30%/70% mass ratio, <1% polymer) and Zn/18O‐water hydrogels (10%/90%). A block of extra‐firm “wet” tofu (12.3×8.8×4.9 cm, ρ⁼1) immersed in water was injected with Zn/O16‐water hydrogels (0.9 cm3 each) at four different depths using an 18 gauge needle. Similarly, Zn/18O‐water hydrogels (0.9 cm3) were injected in a different tofu phantom. As a reference, both 16O‐water and O18‐water hydrogels in petri‐dishes were irradiated in a “dry” environment. The hydrogels in the “wet” tofu phantoms and “dry” petri‐dishes were CT‐scanned and treatment‐planned. Then, they were positioned at the proton distal dose fall‐off region and irradiated (2 Gy) followed by PET/CT imaging.Results:Significantly high PET signals were observed only at O18‐water hydrogels in the “dry” environment. Zn/O16‐water hydrogels injected in the tofu phantom showed outstanding CT visibility but provided no noticeable PET signals. Zn/O18‐water hydrogels in the “wet” tofu showed excellent CT visibility and moderate PET visibility, however, weaker PET signals than the “dry” environment possibly due to O18‐water leaching out.Conclusion:The developed hydrogel markers can be used as universal fiducial markers due to their CT/PET/MRI/US visibility. Their PET visibility (possibly contributed more by activated O18‐water than Zn) after proton irradiation can be utilized for proton therapy/range verification. More investigation is needed to slow down the leaching of O18‐water.