Reducing Rectal Injury in Men Receiving Prostate Cancer Radiation Therapy: Current Perspectives

Endorectal balloons also effect the dosimetry of radiation therapy plans, and many groups of investigators have studied this dosimetric impact. In 3DCRT plans, smaller endorectal balloons (40 cc air-filled) reduced the high-dose exposure to the posterior rectal wall.⁵⁵ This finding was largely attributed to the increased distance between the prostate and the posterior rectal wall. However, when the seminal vesicles (SVs) were included in the target volumes, only intermediate-sized balloons (60 cc) led to reductions in intermediate and high posterior rectal wall doses.^56,57 These results were reproduced even when SVs were included in the treatment field.⁵⁸

van Lin et al performed a unique study comparing endorectal balloons of various sizes. They studied 40, 80, and 100 cc balloons, versus no balloons, in patients undergoing four-field 3DCRT and IMRT treatments.⁵⁹ They analyzed 284 treatment plans and examined the rectal wall mean dose, the rectal wall NTCP, and the absolute rectal wall volumes exposed to ≥50 Gy (V50) and ≥70 Gy (V70). In patients receiving 3DCRT, the endorectal balloons significantly reduced the measured parameters and the 80 and 100 cc balloons performed better than the 40 cc balloons. In IMRT patients, no significant reductions in rectal wall dose parameters could be demonstrated for any of the balloons.

Because IMRT has become the standard of care for prostate cancer, these results pertaining to 3DCRT are less relevant. In contrast to the van Lin et al analysis,⁵⁹ the previously mentioned study by Patel et al⁵⁸ found that the 100 cc balloons reduced the absolute rectal wall V60, V65, and V70 in patients receiving IMRT. The authors compared rectal dose sparing with endorectal balloons in patients treated with 3DCRT plans to those with IMRT plans, both with and without balloons. They found that the rectal dose sparing in a 3DCRT with a balloon was as conformal as an image-guided IMRT approach without a balloon. However, inclusion of a balloon with IMRT produced further rectal sparing.

The majority of published studies focus on the dosimetric effects of endorectal balloons, but data on patient- and physician-reported toxicities with the use of these balloons is scarce. van Lin et al⁶⁰ compared 24 patients treated with versus 24 patients irradiated without a 40 cc endorectal balloon. These patients were treated with 3DCRT to 67.5 Gy and subsequently underwent rectosigmoidoscopy at multiple time points after radiation therapy. A total of 146 endoscopies and 2,336 mucosal areas were analyzed. The endorectal balloon significantly reduced the rectal wall volume that was exposed to doses >40 Gy, and late rectal toxicity (grade ≥1) was reduced significantly. Telangiectasias were most frequently seen and appeared after 6 months. At 1 and 2 years, there were significantly fewer high-grade telangiectasias observed in the endorectal balloon group in mucosal areas exposed to doses >40 Gy. The authors found that grades 1–3 late rectal bleeding occurred in 33% of the patients without the balloon versus 13% with the balloon. However, given the small number of patients in the study, this difference did not reach statistical significance. While these results are promising, larger prospective trials, especially with IMRT techniques, are needed to confirm these reductions in the rectal toxicity with the use of the endorectal balloons.

Tissue spacers

Given the close proximity of the anterior rectal wall to the prostate, many investigators have begun studying the use of bioabsorbable spacers to physically increase the distance from the prostate to the rectum. The rectoprostatic fascia (or Denonvilliers’ fascia), is a membrane that separates the prostate and bladder from the rectum. This potential space is accessed to inject bioabsorbable material directly posterior to the prostate, thereby increasing the distance from the prostate to the anterior rectal wall. To date, various bioabsorbable materials have been injected between the prostate and rectum and have successfully increased the separation between the rectum and prostate.⁶¹ This procedure is typically done prior to starting a course of external beam radiation therapy (EBRT) and can also be undertaken simultaneously with a BT implant or fiducial marker placement. Hyaluronic acid,⁶² human collagen,⁶³ interstitial biodegradable balloons,⁶⁴ and synthetic polyethylene glycol (PEG) hydrogels^65,66 have been the main materials studied in small trials evaluating safety and its subsequent degradation over time.

Synthetic PEG hydrogels are now FDA approved for used in prostate cancer. Strom et al⁶⁶ treated 100 patients of prostate cancer with high dose rate (HDR) BT with or without IMRT. The patients received a transrectal ultrasound (TRUS)-guided transperineal injection of 10 mL PEG hydrogel DuraSeal™ (Covidien, Mansfield, MA, USA) in their anterior perirectal fat immediately prior to the first HDR BT treatment and 5 mL PEG hydrogel prior to the second HDR BT treatment. They compared this to 100 patients treated without a tissue spacer. PEG hydrogel significantly increased the mean prostate–rectal separation (mean ± SD, 12 ± 4 mm with gel vs 4 ± 2 mm without gel, P < 0.001) and significantly decreased rectal D2cc (47% ± 9% with gel vs 60% ± 8% without gel, P < 0.001). There were no significant differences in pain between patients who did and did not receive PEG hydrogel, and 3% of gel patients developed prostatitis/epididymitis. However, no patients who received gel and HDR BT experienced infections when ceftriaxone and gentamicin were prescribed. While this report focused on EBRT in combination with HDR BT, other investigations of synthetic hydrogels have been published with other radiation therapy modalities, including low dose rate BT^67,68 and stereotactic body radiation therapy (SBRT)⁶⁹ also with promising results.

Whereas the aforementioned studies focused primarily on the safety and efficacy of spacer placement, rectal toxicity rates have been reported with various RT modalities. Uhl et al⁷⁰ performed a phase II trial that included 52 men who received transperineal injection of SpaceOAR (Augmenix, Waltham, MA, USA) prior to IMRT to 78 Gy. Six patients (12%) experienced acute GI Grade 2 toxicity, with no patients experiencing Grade 3 or 4 toxicity. In addition, no patient had GI toxicity Grade ≥2 after 12 months. A separate prospective analysis was published by Eckert et al⁷¹ in which 11 patients had undergone SpaceOAR injection prior to IMRT to 78 Gy. Radiation treatment planning showed low rectal doses despite dose escalation to the prostate. This resulted in mild acute rectal toxicity without Grade 2 events and they revealed complete resolution of the GI rectal toxicities within 4–12 weeks. Last, a matched-pair analysis investigating QOL after prostate IMRT found that patients treated after SpaceOAR injection had lesser changes in their baseline Expanded Prostate Cancer Index Composite bowel bother scores when compared with their matched-pair cohorts.⁷²