SpaceOAR hydrogel complications in prostate cancer

Abstract

SpaceOAR hydrogel, a novel biodegradable spacer, is increasingly used in managing prostate cancer patients undergoing radiation therapy to minimize rectal radiation dose and associated complications. However, its use has raised new concerns regarding its potential complications and impact on subsequent imaging interpretation. This article provides a pictorial review of the imaging complications of using SpaceOAR hydrogel in prostate cancer patients. We present multiple examples demonstrating the types of complications that can occur, potential underlying mechanisms, and their impact on patient outcomes and imaging interpretation. This review aims to provide radiologists and oncologists with an updated understanding of these complications, guiding better patient management and interpretation of imaging studies.

Introduction

Prostate cancer is males’s most common non-cutaneous malignancy, with approximately 160,000 new cases each year. ¹ Radiation therapy is one of the primary treatment modalities for prostate cancer. ² One of the key challenges in delivering effective radiation therapy for prostate cancer is to maximize the radiation dose to the prostate while minimizing the dose to the surrounding healthy tissues, particularly the rectum, which is in close anatomic proximity. This challenge has led to the development and use of various strategies, one of which is the application of hydrogel spacers. ³

SpaceOAR hydrogel, a Food and Drug Administration (FDA) approved biodegradable spacer, is injected between the prostate and rectum. ² It temporarily creates a greater distance between these structures during radiation treatment, reducing rectal radiation exposure and associated complications. ⁴ Despite the reported benefits, SpaceOAR hydrogel has led to new challenges, particularly in radiological imaging. ⁵

As radiologists play a critical role in prostate cancer diagnosis, staging, and follow-up, understanding the complications related to SpaceOAR hydrogel and their impact on imaging interpretation is crucial. This pictorial review aims to present relevant examples depicting complications of SpaceOAR hydrogel in prostate cancer patients, emphasizing their impact on patient outcomes and imaging interpretation.

Ideal placement and basic technique

The SpaceOAR hydrogel is introduced into the body through a transperineal injection of precursor fluids into the adipose tissue plane between the anterior rectal wall and Denonvilliers' fascia. Upon administration, the liquid rapidly expands the naturally existing space, solidifying it into a hydrogel spacer within approximately 10 s of injection. The chemical makeup of the SpaceOAR hydrogel, predominantly consisting of water and polyethylene glycol (PEG), shares similarities with other medically used products employed for surgical hemostasis and sealing. ⁶ The hydrogel is engineered to maintain its dimensions for a period of 3 months, after which it undergoes hydrolysis, transitioning into a liquid state within 6 months of placement. This is followed by absorption by the body and final elimination by kidneys. ⁷ Figures 1 and 2 demonstrate the MRI image of ideal gel placement.

Figure 1.

Sagittal and axial T ₂ weighted images demonstrate the ideal placement of the gel in the rectoprostatic fossa (straight arrow).

Figure 2.

Sagittal and axial T ₂ weighted images demonstrate the ideal placement of the gel in the rectoprostatic fossa (straight arrow).

Complications

While the application of SpaceOAR hydrogel in prostate cancer patients undergoing radiation therapy generally enhances patient outcomes by reducing rectal radiation dose, ⁸ it is not without potential complications. The minor infiltration in the rectal wall usually may not be of any apparent clinical consequence to patients. ⁸ However, it is important for the interpreting radiologist to be aware of these complications, such as rectal perforation, periprostatic venous intravasation or prostatic abscess for timely and correct diagnosis. ^9,10

Rectal wall infiltration

Infiltration of the rectal wall with the gel during the procedure is one of the common complications that can occur with spacer hydrogel placement. ¹¹ However, more severe infiltration into the rectal wall with delamination into the muscle layer causes an increased risk for radiation-induced rectal injury, especially with dose escalated and hypofractionated regimens. ¹¹ Figures 3 and 4 demonstrate the extension of radiodense SpaceOAR gel into the anterior rectal wall.

Figure 3.

Axial CT and axial T ₂ weighted FAT SAT images demonstrate radiodense SpaceOAR gel in the rectal wall, in a semi-circular manner along the anterior wall (white straight arrows). No extension into the rectal lumen.

Figure 4.

Axial CT and axial T ₂ weighted FAT SAT images demonstrate radiodense SpaceOAR gel in the rectal wall, in a semi-circular manner along the anterior wall (white straight arrows). No extension into the rectal lumen.

A rare complication of SpaceOAR gel insertion secondary to mechanical injury during the procedure can occur. Figures 5 and 6 reveal the extension of the gel into the rectal lumen due to rectal wall perforation during the procedure, which would increase the risk of clinical symptoms (such as increased bowel frequency, urgency, mucus discharge, and visible blood in the stools) and radiation-induced proctitis. Usually, radiotherapy is delayed in such cases, and the rectal injury is allowed to heal. ¹²

Figure 5.

Sagittal T ₂ weighted and axial T ₂ weighted imaging shows the focal rectal perforation, with the extension of the SpaceOAR gel into the rectal lumen (straight arrows).

Figure 6.

Sagittal T ₂ weighted and axial T ₂ weighted imaging shows the focal rectal perforation, with the extension of the SpaceOAR gel into the rectal lumen (straight arrows).

Gel extending to the base of the penis

Figure 7 demonstrates gel at the base of the penis. It is an important finding to detect, given the potential for rectourethral fistula formation in such cases. ¹³ Although radiotherapy is an independent risk factor for rectourethral fistula formation, the risk is higher in patients with gel extending up to the base of the penis. ¹⁴

Figure 7.

Axial non-contrast CT images demonstrate the mildly radiodense SpaceOAR gel at the base of the penis (straight arrow).

Old gel seen as nodular recurrence

The hydrogel, a biodegradable spacer, undergoes hydrolysis, transitioning into a liquid state within 6 months of placement. Despite this, it can occasionally persist as a nodular region within the recto prostatic fossa. While this typically carries no clinical implications, it is critical to accurately identify this phenomenon, as it may be misconstrued as an area of recurrence. Radiologists should always consider this possibility during evaluation. Correlation with CT scans can be beneficial in complex cases, given the radiodense nature of the gel. Figures 8 and 9 demonstrate low signal on T ₂ weighted images in the region of the prior SpaceOAR gel placement. Axial CT image (Figure 10) shows a corresponding faint hyperdensity in the rectoprostatic fossa, confirming the presence of old SpaceOAR gel in that region.

Figure 8.

Axial T ₂WIs demonstrate the old SpaceOAR gel in the rectoprostatic fossa with low signal on T ₂WI (straight arrows) and mild adjacent fibrosis and scarring. T ₂WI, T ₂ weighted image.

Figure 9.

Axial T ₂WIs demonstrate the old SpaceOAR gel in the rectoprostatic fossa with low signal on T ₂WI (straight arrows) and mild adjacent fibrosis and scarring. T ₂WI, T ₂ weighted image.

Figure 10.

Axial CT images show faint hyperdensity in the corresponding region in the rectoprostatic fossa, confirming the signal to be old SpaceOAR gel (straight white arrows).

Gel intravasation into the periprostatic venous complex

Direct puncture of the periprostatic venous complex during the procedure results in the extension of the gel into the periprostatic venous (Santorini) complex. ^15,16 Despite the usual clinical insignificance of this complication, it remains essential to correctly interpret this finding due to the minimal gel volume involved. This allows for confirmation of its resolution in follow-up imaging studies. In addition, ensuring the gel quantity remains adequate for providing necessary rectal shielding during radiation therapy is also crucial. ¹⁵ Figures 11 and 12 demonstrate an extension of the SpaceOAR gel into the periprostatic venous complex, predominantly on the right side. Some amount of gel is also observed in the expected area of the rectoprostatic fossa.

Figure 11.

Axial CT images show faint hyperdensity in the periprostatic venous plexus, more so on the right side (straight arrows).

Figure 12.

Axial CT images show faint hyperdensity in the periprostatic venous plexus, more so on the right side (straight arrows).

Conclusion

SpaceOAR hydrogel, a pioneering biodegradable spacer, has emerged as an effective tool in managing prostate cancer patients receiving radiation therapy, successfully mitigating rectal radiation dose and concomitant complications. Nevertheless, its use may have potential complications and implications for subsequent imaging interpretation. This pictorial review has presented a broad array of complications associated with SpaceOAR hydrogel usage, ranging from rectal wall infiltration and perforation to periprostatic venous intravasation, prostatic abscess formation, and the rare persistence of the gel in a nodular form that may mimic recurrent disease. As the utilization of SpaceOAR hydrogel increases in clinical practice, the importance of radiologists being familiar with these complications cannot be overstated. This familiarity allows for timely and accurate diagnoses and the ability to distinguish between benign post-treatment changes or probable complications or disease recurrence.