Rectal spacers in MR-guided SBRT for localized prostate cancer very well tolerated, improve rectal sparing and target coverage

Clinical outcomes within 1 year of MR-guided stereotactic body radiation therapy (MRgSBRT) with rectal spacer for patients with localized prostate cancer (PC) show excellent tolerability, minimal gastrointestinal (GI) toxicity as well as improved rectal sparing and target coverage vs a non-spacer comparison cohort.

Ultra-hypofractionated SBRT, which entails 5–6 fractions of radiation, is gaining popularity in treatment of localized PC due to its promising clinical and quality of life (QoL) outcomes. [N Engl J Med 2023;doi:10.1056/NEJMoa2214122; Int J Radiat Oncol Biol Phys 2019;104:778-789; Lancet Oncol 2022;23:1308-1320] To alleviate its increased risk of rectal toxicity due to the substantially higher radiation dose to the rectal wall, injectable rectal spacers have been developed to expand the distance between the anterior rectum and the prostate. [BJU Int 2022;doi:10.1111/bju.15821] The spacers’ potential rectal-protective benefit is further enhanced in the MR-guided setting due to better visualization. [Br J Radiol 2021;doi:10.1259/bjr.20210521:20210521]

However, no studies have described acute and long-term toxicities and patient-reported outcomes (PROs) with rectal spacers after MRgSBRT. “Thus, the major aim of this study was to prospectively investigate the clinical use of rectal spacers for adaptive MRgSBRT in localized prostate cancer patients and to assess patient- and clinician-reported outcomes up to a median follow-up of 1 year,” wrote a group of urologists and oncologists from Hong Kong.

The study enrolled 34 consecutive patients (median age, 70 years) with low- to high-risk localized prostate cancer, who underwent five-fraction adaptive MRgSBRT with rectal spacer. Dosimetric comparison was performed against a risk- and age-matched cohort who were treated with MRgSBRT but without a spacer at a similar timepoint. [World J Urol 2024;42:97]

All inserted spacers were clearly visible on MRI. The median distance increase between the prostate midgland and rectum (at midline) after spacer insertion was 7.8 mm. “V100% >95 percent of the planning target volume was significantly higher in the spacer cohort than in the non-spacer cohort [98.6 percent vs 97.8 percent; p=0.03]. Dose exposure to the rectum in terms of V105%, V95%, V90%, and V80% was significantly smaller with spacer insertion than without [p<0.001]. Spacer use vs non-use resulted in significant improvement in rectal sparing [V95% <3 cc, 0.7 cc vs 4.9 cc],” reported the researchers.

Procedural complications following rectal spacer insertion occurred in one patient who experienced rectal bleeding approximately 1 week after spacer insertion, which completely subsided before MRgSBRT initiation.

Both GI and genitourinary (GU) adverse events (AEs) were generally mild. Grade 1 GI AEs were experienced by 26.5 percent of patients, including three cases of rectal haemorrhages and one case of rectal pain, all of which resolved at subsequent follow-ups. No grade 2 GI AEs reported. Grade 1–2 GU toxicities were reported in 94.1 percent of patients. The most common GU toxicities were urinary frequency, followed by urinary tract pain, and urinary incontinence.

Patient-reported QoL scores based on the Expanded Prostate Cancer Index Composite (EPIC) questionnaire did not change significantly throughout the entire follow-up period across the four domain summary scores (urinary, bowel, sexual, hormonal; all p>0.05). “Notably, bowel domain scores remained nearly constant and were the highest among all four domains at each follow-up phase. A slight drop in bowel function was observed in the acute phase [1–3 months after treatment], but it fully recovered in the subacute phase [4–12 months],” highlighted the researchers.

“MRgSBRT with rectal spacer for localized prostate cancer showed exceptional tolerability with minimal GI toxicities and satisfactory PROs. Rectal spacer use achieved improvements in dosimetry, rectal sparing, and target coverage,” they concluded.