SpaceOAR® is used in definitive radiation therapy, particularly intensity-modulated radiation therapy (IMRT), for prostate cancer to reduce the absorbed radiation dose to the rectum. Although rare, SpaceOAR®-related infections have been reported, and radiation therapy during an active infection has been suggested as a potential cause of rectal perforation. Here, we report a case of a patient who developed a peri-SpaceOAR® abscess during IMRT for prostate cancer, which subsequently led to rectal penetration as a result of abscess drainage.

A 74-year-old male patient with localized prostate cancer classified as intermediate risk underwent definitive radiation therapy with intensity-modulated radiation therapy (IMRT). Prior to treatment, SpaceOAR® was implanted. Post-procedural digital rectal examination revealed no leakage of the hydrogel or hematochezia, and plain MRI confirmed appropriate placement of SpaceOAR®. IMRT was initiated with a total dose of 70 Gy in 28 fractions. During a positioning plain CT at the 26th fraction, air was observed around SpaceOAR®; however, IMRT was completed with prophylactic antibiotic administration. The patient remained asymptomatic with no significant inflammatory response upon discharge. Two weeks after completing IMRT, the patient presented with lower abdominal pain. Laboratory tests showed a mild increase in inflammatory markers, with WBC 10,100/μl and CRP 1.61 mg/dl. MRI revealed a T2-hypointense structure with restricted diffusion around SpaceOAR®, raising suspicion of SpaceOAR®-related infection. Although no apparent rectal perforation was observed on MRI, lower gastrointestinal endoscopy revealed a penetration in the anterior wall of the lower rectum. The rectal mucosa showed no signs of inflammation, and radiation-induced mucosal damage was not suspected. The patient was diagnosed with SpaceOAR®-related infection and rectal penetration, and treatment with antibiotic therapy a low-residue diet was initiated.

Although antibiotic therapy temporarily alleviated lower abdominal pain and inflammatory markers, the symptoms recurred each time antibiotics were discontinued. Follow-up MRI confirmed that SpaceOAR® had been almost completely absorbed, after which no further inflammatory response was observed even in the absence of antibiotic therapy.

In this case, the rectal penetration was considered to have functioned as a drainage duct for the peri-SpaceOAR® abscess. Since the penetration may serve as the drainage duct, careful consideration is required before closing the penetration.