Abstract
Rectal hydrogel spacers are increasingly used to reduce dose and thereby limit toxicity to the rectum during prostate radiotherapy. Commercially available products require magnetic resonance imaging (MRI) for hydrogel visualization and treatment planning, with only anecdotal reports of treatment planning with computed tomography (CT) alone. MRI-incompatible hardware, patient intolerance, and added cost provide incentives for MRI-independent alternatives for treatment planning. This case series evaluates the addition of iodinated contrast to spacer gel and assesses its impact on CT visualization. Three prostate cancer patients underwent contrasted rectal spacer placement in preparation for external beam radiotherapy. Standard low-osmolar iodinated contrast was added to a commercially available rectal hydrogel. 0.5cc of contrast was added to 4.5cc of accelerator and subsequently combined with 5cc of diluent. The resultant 10cc of hydrogel was injected into each patient as per standard procedure, utilizing transrectal ultrasound guidance. Two patients were simulated with MRI; a third with a non-MRI compatible pacemaker was planned with CT alone. CT was performed on the day of hydrogel placement within 2 hours of procedure and again 7-12 days later to account for fiducial migration and dissipation of injected air. The second CT was used for treatment planning. Hydrogel visibility was compared between CT and MRI through standardized symmetry scoring, volume calculation, and measurements taken at prostate base, mid-gland, and apex. Spacer contrast was visible only on the day of hydrogel placement by CT, allowing target delineation for treatment planning. On comparison to MRI delineation of spacer one week later, minimal changes in relevant spacer measurements were observed. In one patient, a significant amount of injected air dissipated between the time of hydrogel placement and repeat CT. CT visualization of rectal hydrogel with iodinated contrast on day of placement is feasible and allows MRI-independent treatment planning with spacer gel, but requires repeat CT on a subsequent day to allow for resolution of prostatic edema, fiducial migration, and air within the hydrogel spacer.Abstract 3673; Table 1PatientSimulation Type (Day)Spacer Volume (cc)CC Dimension (mm)AP Dimension (mm)TV Dimension (mm)Base Thickness (mm)Mid-gland Thickness (mm)Apex Thickness (mm)Symmetry Score1CT* (0)16.661.416.430.815.714.715.91CT (8)15.951.417.629.519.017.716.91MR (8)15.555.817.32616.516.816.212CT* (0)12.636.613.839.215.813.811.82CT (12)14.138.812.641.512.811.502MR (12)14.748.615.343.213.212.511.213CT* (0)18.1†58.7†11†40.5†11.8†12.3†15.7†3†CT (7)11.352.41030.411.910.59.52*Spacer contrast visible, †Includes injected air Open table in a new tab
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