An unusual fluid collection between prostate and rectum

69-year-old, otherwise healthy, male with a rising prostate-specific antigen (PSA). Transrectal prostate biopsies showed prostatic adenocarcinoma, Gleason score 7 (3+4) in 4 of 12 biopsy cores. MRI showed no extracapsular invasion, iliac or inguinal lymphadenopathy, or bone metastases. Patient anticipating radiation therapy (refused surgery). What explains this MRI appearance (Figure 1)?

High-resolution prostate MRI with dedicated protocol (Figure 1) shows a T2-hyperintense, homogeneous, well-delineated fluid collection between the prostate and the rectum. No pathologically enlarged locoregional lymphadenopathy or bone metastases identified.

Endorectal ultrasound (Figure 2) shows a heterogeneous, mostly hypoechoic collection between the prostate and the rectum.

Background Traditional treatment options for low and intermediate risk prostate cancer patients include active surveillance, external beam radiation therapy (EBRT) or surgical resection (radical prostatectomy). The main side effects of radiation therapy are fatigue, urinary, rectal and cutaneous toxicity, including sexual dysfunction and secondary pelvic neoplasm. Rectal toxicity remains a potential concern with all types of EBRT, including proton therapy [1].

Clinical Perspective Injection of a synthetic polyethylene glycol hydrogel to increase the distance between the prostate and rectum has been shown to reduce rectal toxicity following irradiation [2]. It is performed as an outpatient procedure under local anesthesia with or without moderate intravenous sedation (IV midazolam and fentanyl in divided doses). Under real-time endorectal ultrasound guidance, the procedure usually starts with insertion of 3 fiducial gold markers laterally in the prostate. This is followed by a final (4^th) needle pass to bring the needle tip along the midline into Denonvillier’s fascia (between prostate and rectum). After confirming proper position with hydrodissection, the hydrogel is injected continuously over 12 seconds with immediate polymerization and solidification in the rectoprostatic space. Rectal discomfort during injection is not uncommon and can last from a few minutes to 48 hours.

Imaging Perspective MRI plays a key role in the work up of prostate cancer patients to rule out regional (iliac and inguinal) lymphadenopathy and/or bone metastases (mainly of the axial skeleton) [3]. High resolution prostate MRI with dedicated protocol is obtained within days after transperineal spacer injection to confirm adequate thickness (10-12 mm) of the hydrogel, before starting radiation therapy. The fiducial markers appear as low-intensity signal/artifact; the hydrogel shows homogeneous T2 hyperintensity (i.e., fluid-like) and gets absorbed within 12 weeks, thus limiting the time window of opportunity for EBRT and demanding good coordination between IR and radiation oncology teams.

Outcome Percutaneous transperineal injection of a hydrogel spacer between the prostate and rectum reduces rectal irradiation, decreases the incidence of late (3-15 months) rectal toxicity from 7% to 2%, and improves bowel quality of life [2]. Complications from hydrogel spacer injection are rare (1.6%), mostly consisting of needle transgression of the rectal wall (managed conservatively and postponing – but not precluding – subsequent treatment) [4-5].

Take Home Message / Teaching Points  Hydrogel spacer injection is indicated before radiotherapy for histologically confirmed low or intermediate risk prostate cancer, without extracapsular invasion [5]. Its homogeneous T2-hyperintense fluid signal and well-delineated appearance should be considered on prostate MRI in this clinical context.