The patient was a 78-year-old prostate cancer patient. He had no medical history other than surgery history of the goiter and nasal hemangioma. A prostate-specific antigen (PSA) level was 13.89 ng/mL in a routine evaluation. Clinical stage was T3a. An ultrasound-guided transperineal prostate biopsy was performed, and revealed Grade Group 4 adenocarcinoma in 1 of 24 specimens. Two months after the biopsy, administration of leutinizing hormone-releasing hormone (LHRH) agonist was initiated. He opted for external beam radiation therapy (EBRT) 46 Gy in 23 fractions combined with high-dose-rate (HDR) brachytherapy 15 Gy in 1 fraction as a definitive treatment. Four months after the initiation of hormone therapy, SpaceOAR was inserted into the space between the prostate and rectum and fiducial markers were inserted into prostate under local anesthesia with lidocaine (day 0). The insertions were completed without any side effects, and magnetic resonance imaging (MRI) confirmed that the SpaceOAR was inserted into the correct position (Fig. 1, left). Three weeks after the insertion of SpaceOAR (day 21), EBRT with computed tomographic (CT)-linac was initiated. Clinical target volume (CTV) was prostate, all seminal vesicles and whole pelvic lymph node regions. Planning target volume (PTV) margin of whole pelvis was 3 mm, 7 mm, 8 mm in the RL, SI, AP directions, respectively. Six days after the initiation of EBRT (day 27), he started to feel perineal pain. Owing to increased perineal pain, administration of antibiotics was initiated for a diagnosis of urinary tract infection on day 40. Perineal pain gradually subsided, and he finished the oral antibiotics administration in one week (day 47). Four days after that (day 51), he felt perineal pain and frequent urination again, and restarted the antibiotic treatment. On day 60, oral administration of opioid was initiated due to increasing perineal pain. The next day (day 61), he had a high fever, thus MRI was performed for detailed examination. Peri-SpaceOAR abscess was detected on the MRI (Fig. 1, middle), thus HDR brachytherapy was postponed, administration of antibiotics and opioid via intravenous drip started, and transperineal drainage was performed. The pain gradually improved and the abscess was found to have shrunk slightly on MRI, therefore administration of antibiotics changed to oral administration (day 76). Three weeks later (day 97), shrinkage of the abscess and improvement in inflammatory change were confirmed by MRI. Therefore, on day 112, an additional radiation therapy was initiated. At our conference, every other day EBRT of 20 Gy in 4 fractions was recommended as an additional radiation therapy instead of HDR monotherapy to decrease the dose per fraction. MRgRT with 60Co MRIdian under a magnetic field of 0.345 T (ViewRay Inc.) was selected to assess intrafractional and interfractional motion of prostate, seminal vesicle and OARs such as rectum and bladder accurately. CTV was the prostate and seminal vesicles. PTV margin was 5 mm, 4 mm, 3 mm in the RL, SI, AP directions, respectively. On the last day of MRgRT (day 119), he experienced perineal pain again, and MRI was performed. On the MRI of day 120, peri-SpaceOAR inflammation aggravated and penetration to rectum was suspected (Fig. 1, right). Colonoscopy was subsequently performed, and penetration of SpaceOAR into the rectum was detected (Fig. 2). To treat rectal perforation, he was kept nothing per os and administration of antibiotics via intravenous drip and intravenous hyperalimentation started. Besides, he was transported to another hospital for receiving hyperbaric oxygen therapy (HBOT) (day 131). HBOT was initiated on day 131. After 24 times of HBOT for 5 weeks, the recovery from rectal perforation was confirmed by colonoscopy, and administration of antibiotics was seized. Ten weeks after the termination of HBOT, disappearance of a peri-SpaceOAR abscess was cofirmed on MRI (day 243).