Depth of Hydrogel Spacer Rectal Wall Infiltration Was Not Associated With Rectal Toxicity: Results From a Randomized Prospective Trial.

Purpose: Rectal spacers have gained popularity as a dose-sparing material for prostate cancer radiation therapy (RT). However, the procedure can be associated with unintended rectal wall infiltration (RWI) of the spacer gel. We therefore classified RWI severity as a function of depth and explored its association with rectal toxicity using a data set from prostate cancer patients treated with RT on a prospective randomized clinical trial (RCT). Methods and Materials: Postimplant T2-weighted magnetic resonance images of 149 subjects randomized to the hydrogel spacer arm of a published multicenter RCT were assessed for the presence and depth of RWI. All implants were assigned a score of 0 (no rectal wall signal changes), 1 (rectal wall edema/signal change), 2 (partial RWI), or 3 (full-thickness RWI); RWI was defined as a score of 2 or 3. Correlations were made between RWI score and physician-reported procedure, acute, and late rectal toxicity. Results: Although 62.4% of implants had no rectal wall signal abnormalities, 24% [scores of 2 (20.1%) and 3 (4.0%)] of procedures exhibited radiographic evidence of RWI. Full-thickness RWI was associated with both a longer length (22.8 ± 7.0 mm, Conclusions: Based on data from an RCT, RWI did not contribute to increased rectal toxicity prior and up to 15 months after conventional prostate cancer RT.