Addition of Iodinated Contrast to Rectal Hydrogel Spacer to Facilitate MRI-Independent Target Delineation and Treatment Planning for Prostate Cancer

Abstract Purpose/Objectives Hydrogel spacers reduce rectal dose toxicity during prostate cancer radiotherapy. Current products require MRI for visualization during treatment planning. MRI-incompatibility and cost argue for alternatives utilizing CT. This case series evaluates the addition of iodinated contrast to hydrogel as such an alternative. Materials/Methods Three patients underwent rectal hydrogel spacer placement with iodinated contrast modification. CT was performed within 1 hour of injection and again 1 week later. MRI was obtained at the time of second CT. Hydrogel delineation was compared between CT and MRI and between paired CTs. Results Spacer enhancement was visible on CT immediately after hydrogel placement (mean HU 122, range 52-193) but not at second CT 1-week later [mean HU 8, range (-)8-29]. Delineated spacer volume did not significantly differ between immediate post-procedure CT and MRI ≥1 week later in two patients (Patient 1: 16.6 vs 15.5cc; Patient 2: 12.6 vs 14.7 cc; paired t-test P =0.81). Conclusion CT visualization of rectal hydrogel admixed with contrast is feasible, allowing delineation of interface with rectum/prostate.