Protection of the Rectum during Prostate Radiation

Prostate cancer is the most common malignancy in men, with an estimated 191,900 new diagnoses and 33,330 deaths in 2019. ¹ Treatment modalities range from radical prostatectomy to radiation therapy to close observation and are standardized by national guidelines. ²

Radiation therapy for prostate cancer can take the form of placement of brachytherapy seeds into the prostate or external beam radiation therapy. Scatter radiation from these techniques, however, provides a significant risk to adjacent organs. As radiation therapy technology and administration techniques have improved, a significant limiting factor to dose administration is the risk of rectal toxicity from scatter radiation. ³ Depending on the dose delivered and sensitivity of the organ, in this case the rectum, there may be long-lasting radiation injury.

As radiation dose is reduced by the square of the distance, separating the prostate from the rectum is a very effective means of decreasing radiation exposure. The inverse holds true as well, as some studies have suggested dose modulation in patients with decreased prostate to rectum distances. ⁴ By definition, this makes the rectum an “organ at risk.”

Increasing the distance between the prostate and rectum serves as a protective mechanism for the rectum and may allow for increased and more effective dose to the prostate. Hydrodissection with saline to increase this space, while technically feasible, proves ineffective as saline quickly dissipates and is resorbed. However, placement of a hydrogel into the peri-rectal fat with ultrasound guidance can create up to 1.5 cm of separation between the prostate and rectum. This hydrogel remains in place for approximately 3 months. Use of rectal spacer material has been established to be safe and effective in limiting rectal toxicity. ^{5 6 7} Herein, we review the technique and considerations for placement of a hydrogel between the prostate and rectum in patients undergoing radiation therapy for prostate cancer.

Patient Selection, Materials, and Technique

Patient Selection

Any patient scheduled to undergo radiotherapy of the prostate, whether brachytherapy or external beam radiation therapy, should be evaluated for placement of a hydrogel for radiation protection. Patients with an active infection should not undergo hydrogel placement. Patients should also be screened for a prior history of pelvic surgery, as hydrogel injection in these patients may be contraindicated depending on surgery type and anatomy.

Anatomy

Anatomical knowledge of the prostate and its surrounding structures is crucial to procedural success. The prostate is situated between the bladder and the muscles of the pelvic floor, with the urethra passing through the central portion of the gland. The seminal vesicles sit posterosuperior to the prostate and drain via the ejaculatory ducts into the urethra. Immediately posterior to the prostate lies the rectum.

The prostate lacks a true capsule; rather it is surrounded by an incompletely adherent fibromuscular band, often described as a pseudo capsule. This is defective anteriorly and adherent to the surrounding pelvic fascia. The prostate is surrounded by a total of three fascial layers: the prostatic fascia, Denonvilliers' fascia, and endopelvic fascia.

Denonvilliers' fascia, also named the posterior prostatic fascia or the seminal vesicle fascia, is a firm membranous structure between the rectum and the prostate. It lies at the posterior and lateral angle of the prostate and essentially separates the prostate from the rectal wall.

The neurovascular bundles supplying the prostate approach from posteriorly and enter the prostate at the five o'clock and seven o'clock positions. By maintaining a straight midline approach, these structures are avoided, decreasing the likelihood of neurological and hemorrhagic complications.

When visualized on ultrasound, these structures are quickly identified ( Fig. 1 ). From a procedural standpoint, identification of the rectal anatomy is critical, particularly identification of the rectal hump. Guiding the needle over the rectal hump avoids inadvertent contamination with rectal contents or misadministration of the hydrogel into the rectal wall.

Fig. 1.

Transrectal ultrasound images demonstrate the standard view for injection of the hydrogel. Specific anatomical landmarks are identified prior to injection including the prostate (A), Denonvilliers' fascia (B), rectal mucosa (C), and the rectal hump (D).

Materials

A side firing endorectal ultrasound probe is required. In addition, the hydrogel kit comes with the necessary materials for administration of the hydrogel. Currently, the SpaceOAR system (Boston Scientific, Marlborough, MA) is the only Food and Drug Administration (FDA)–cleared absorbable hydrogel intended for this use. The hydrogel packaging typically includes the injection needle, deconstructed hydrogel components, and administration set. A table for placing the patient into the dorsal lithotomy and a stand for the ultrasound probe play a crucial role in improving operator and patient comfort.

Patient Preparation

Patients should be scheduled in a timely manner to minimize the delay in starting treatment. Many radiation oncologists choose to wait up to 1 week after hydrogel placement to start therapy to allow any inflammation in the area to resolve. As the procedure is performed with a guidance from a transrectal ultrasound, the patient is asked to do a Fleets enema the night before. Patients are not asked to do on the morning of the procedure, unless the procedure is later in the day as this can impact patient comfort and convenience during transit to the hospital. Patients are also instructed to remain NPO for 8 hours prior to the procedure to facilitate safety during moderate sedation. While some advocate the use of antibiotics, in our experience there has been no need for antibiotic prophylaxis. Moderate sedation is initiated in the preprocedural holding area to help with patient anxiety.

The patient is brought to the procedure room where he is placed in the dorsal lithotomy position. A rectal examination is performed with lidocaine jelly to gently dilate and relax the anal sphincter, and to identify any other confounding pathology such as anorectal hemorrhoids or perianal fissures. The perineum is prepared with a chlorhexidine scrub and draped with sterile technique.

A side firing endorectal ultrasound probe prepared with a probe cover and gel is introduced into the rectum. The side fire probe has two transducers that allow visualization in both the axial and sagittal planes. Care should be taken so that contact is not lost between the ultrasound probe and the rectal wall. Anatomical landmarks are always reidentified ( Fig. 1 ). In particular, the prostate gland, rectal hump, seminal vesicles, and perirectal fat are specifically noted. Lidocaine 1% is administered in the midline and laterally to each side through a perineal puncture, approximately 1.5 cm above the rectum.

Hydrogel Preparation

The hydrogel is supplied as a dry powder that is reconstituted with a diluent ( Fig. 2 ). Once the powder is dissolved, 5 mL is withdrawn into a 10-mL syringe and any excess is discarded. Then 1 mL of air is withdrawn into the syringe. Similarly, 5 mL of accelerant is withdrawn into a 10-mL syringe, excess is discarded, and 2 mL of air is withdrawn into the syringe. Both syringes are connected to the supplied “Y” connector and syringe holder. Both syringes should be held in the upright position henceforth, until administration to prevent inadvertent mixing of the two solutions and occlusion of the delivery apparatus.

Fig. 2.

The hydrogel kit (SpaceOAR shown here) includes the necessary equipment for administration of the hydrogel, including the deconstructed hydrogel components, 18-gauge needle, “Y” connector, and syringe holster ( a ). The deconstructed hydrogel comes in the form of a powder ( b ) and is readily dissolved in the supplied diluent ( c ). The reconstituted hydrogel is then placed into a syringe and, alongside the supplied accelerant, connected to the holster and “Y” connector ( d ). Care is taken to keep the individual components from prematurely mixing in the administration apparatus by propping the unit up against gravity ( e ). The hydrogel readily forms into a clear, firm, gelatinous spacer ( f ).

Needle Placement

An 18-gauge needle is flushed with saline to expel any air and advanced under ultrasound guidance to the perirectal fat via a transperineal approach. Care should be taken to ensure the bevel is pointing down (toward the rectum). The needle should be advanced in the midline, over the perirectal hump and then directed down to avoid the prostate and is advanced into the perirectal fat. Care should be taken to avoid entering the rectum with the needle. If the rectum is violated, the procedure should be aborted and reattempted after the patient has undergone a course of antibiotics. If the perirectal hump is too prominent, the patient's legs can be brought down, flattening the angle of the pelvis and redistributing the tissue of the rectal hump to allow a better trajectory. Needle position is checked in the axial and sagittal planes to ensure it is not in the prostate or the rectum. Axial plane should show the needle tip in the midline behind the prostate. Manipulation of the needle anteroposteriorly should not pull the prostate down or pull the rectum up. Aspiration via the needle should not produce any blood. Injection of 1 to 2 mL of saline should fill the target zone and quickly dissipate.

Hydrogel Injection

The air lock in syringes is expelled to the level of the shoulders of the syringe. The hydrogel is injected through the 18-gauge needle over 10 to 12 seconds with visualization under ultrasound ( Fig. 3 ). The needle is then withdrawn. Overall, as the technique becomes streamlined, procedure room procedural time decreases to approximately 10 minutes.

Fig. 3.

Preinjection imaging demonstrates placement of the needle into Denonvilliers' fascia (open arrow). Postinjection of the hydrogel demonstrates increased spacing between the rectum and prostate. Solid arrow denotes the hydrogel in Denonvilliers' fascia.

Postprocedural Imaging

After placement of the hydrogel, a treatment planning CT or MRI is performed. If performing CT, it is helpful to do it as soon as possible after placement of the hydrogel because air in the hydrogel will assist in visualization. If an MRI is performed, a localizer sequence and a T2 sequence are sufficient ( Fig. 4 ). We schedule the patient for this cross-sectional imaging immediately following hydrogel placement for patient convenience and to prevent delay in treatment planning. Additionally, if there are concerns of claustrophobia, related to cross sectional imagining such as MRI, sedation has already been administered, obviating the need for extra medication. Future iterations of hydrogels are reported to contain contrast agents compatible with CT for improved visualization, though these are not currently on the market.

Fig. 4.

( a ) Preprocedural axial contrast-enhanced CT demonstrates the proximity of the rectum and prostate, immediately adjacent to each other (arrow). ( b) Post procedural axial T2 FSE MRI demonstrates the placement of the hydrogel and the increased spacing between the prostate and rectum (bracket).

Potential Risks and Complications

Overall, placement of the hydrogel spacer is well tolerated and with minimal risks. Rare complications are reported in the literature via the Manufacturer and User Facility Devise Experience (MAUDE) database and are summarized by Aminsharifi et al. ⁸ The most common reported symptoms are localized and self-limited, such as pain and inflammation. Interestingly, a small number of unique major complications have been reported in the literature. These include acute pulmonary embolism, severe anaphylaxis, prostatic abscess and sepsis, rectal wall erosion, and rectourethral fistula formation. While rare, these reported complications highlight the necessity for strict adherence to procedural technique. In our experience, there have been no major or minor complications.

Coding and Billing

Currently, as the only FDA approved, commercially available hydrogel on the market is SpaceOAR (Boston Scientific, Marlborough, MA), their coding and billing guide serves as the most readily available reference for reimbursement. ⁹ As of January 2019, this procedure has a CPT code: 55874 “Transperineal placement of biodegradable material, peri-prostatic, single or multiple injection(s), including image guidance, when performed.” This is typically paired with the ICD-10-CM code “C61-Malignant neoplasm of the prostate.” Per the manufacturer coding and reference guide, the procedure has a total relative value unit of 4.80.

Discussion

Implementation of a dedicated clinical pathway of care for patients undergoing hydrogel placement is easily achieved. The process is best streamlined with trained staff contacting patients prior to the procedure with preparation instructions and setting expectations for the procedure. Additionally, the patient is scheduled for cross-sectional imaging for treatment planning on the same day. Setting appropriate expectations for the patients improves patient experience. Not uncommonly, patients will compare the procedure to their biopsies, and note the hydrogel placement to be better tolerated.

Although some patients are anxious about having any procedure performed, most patients tolerate the procedure very well with moderate sedation. Starting sedation early in the preprocedural area, prior to bringing the patient to the procedure room, markedly reduces anxiety. Placement of a transrectal probe is not alarming to this population, as most patients have already had a transrectal prostate biopsy and colonoscopy in the past. Placement of a Foley catheter is unnecessary and adds to morbidity and complications by increasing the risk of urinary tract infection.

Some patients have mild discomfort with injection of lidocaine and are particularly sensitive at the perineal muscles. Injection of the gel in Denonvilliers fascia is not painful, but some patients describe a sensation of pressure or mild discomfort. Injection of the lidocaine not only in the midline but also laterally on each side helps prevent the sensation of pressure or discomfort.

In our small series of 63 patients, we have not utilized antibiotics and have not had infectious complications. If perforation of the rectum were to occur, the procedure should be stopped, and a short course of antibiotics should be administered. The procedure can be rescheduled once any chance of infection is cleared. The gel is resorbed over 3 to 4 months with no residual effects. Utilization of the hydrogel for organ displacement has the potential for other treatment arenas, such as pancreatic cancer treatment or during renal or hepatic percutaneous ablation.

Utilization of equipment with high-resolution probes significantly improves needle visualization and leads to efficient and precise hydrogel placement. Our initial experience with a dated imaging unit was technically feasible, although imaging quality slowed procedural times significantly. Conversion to a new, high-resolution unit with a sophisticated transrectal ultrasound probe immediately and drastically improved procedure times to less than 10 minutes.

Conclusions

Scatter radiation poses a significant risk to the rectum during radiation therapy of the prostate. Placement of a hydrogel spacer between the prostate and rectum presents a safe and technically feasible option for radiation protection of the rectum. Performed as an outpatient ultrasound-guided procedure, placement of the hydrogel spacer allows for a more than 3-month window for radiation therapy. Interventional radiologists are uniquely poised to learn and perform the procedure, with brief learning curve required to become proficient.