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, P = .008) and a larger circumferential percentage (35.8% ± 9.2%, P = .045) of rectal infiltration. Although 7 subjects (5%) experienced transient procedure-related rectal toxicities (most commonly perineal/rectal pain), only one had RWI (score of 2, National Cancer Institute's Common Terminology Criteria for Adverse Events grade 1). Consequently, no correlation was observed between procedural rectal toxicity and the presence/extent of RWI (P = .64). Similarly, no difference in acute (P = .64) or late (P = .85) rectal toxicity incidence or grade was detected between RWI categories; none of the 6 men with a RWI score of 3 developed late rectal toxicity by 15 months.

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.