3. Case Presentation

A 70‐year‐old Caucasian male was diagnosed with favorable intermediate‐risk prostate cancer, characterized by adenocarcinoma, Grade II, Gleason score of 7 (3 + 4) and a PSA of 9.9. A multiparametric MRI in May 2022 revealed a suspicious lesion in the prostate (PI‐RADS 5), leading to a transrectal ultrasound and biopsy in June 2022, which confirmed prostate adenocarcinoma involving 50% of a single core. His cancer was staged as T1c N0 M0, indicating no nodal or metastatic involvement. Overall, these findings were consistent with favorable intermediate‐risk localized prostate adenocarcinoma. Androgen deprivation therapy was not administered, given the favorable intermediate‐risk features and the plan for definitive moderately hypofractionated external‐beam radiotherapy alone.

The principal clinical challenge was determining how to deliver definitive prostate cancer treatment while minimizing the risk of fistula progression, recurrent infection, impaired wound healing, or disruption of subsequent fistula repair.

In addition to prostate cancer, the patient has a significant medical history of rheumatoid arthritis (RA) and hypertension, both of which are managed with a comprehensive medication regimen, including prednisone, hydroxychloroquine, and abatacept. However, abatacept was discontinued for the patient until after his radiation therapy.

The patient also has a history of alcohol use and tobacco abuse. These lifestyle factors have likely contributed to his chronic health issues, including degenerative joint disease, alcoholic liver disease, and chronic pain in his right ankle.

The patient also had a history of multiple perineal abscesses, likely involving the left ischiorectal fossa, which may have contributed to the development of an intersphincteric fistula. Clinical examination demonstrated an external fistula opening in the left posterior anal canal with a posterior midline defect at the dentate line, without masses or other abnormal findings. Pelvic MRI demonstrated a left‐sided intersphincteric fistula extending cranially from the 3:00 position to caudally at the 5:00 position and continuing to the skin (Figure 1). The fistula measured approximately 5 cm in length. At the apex of the fistula, there was a 1.8 × 1.7 × 2.0 cm heterogeneous collection containing fluid and air with peripheral enhancement, consistent with an abscess at the level of the anorectal junction. The fistula was comma‐shaped in the axial plane, traveled within the intersphincteric space, and measured 1.4 cm in maximal transverse dimension. He considered his treatment options and ultimately refused surgery in favor of radiation therapy.

Figure 1: MRI scans showing positions of prostate and perianal fistula. (A) Coronal MRI image demonstrating perianal fistula (marked by arrows) of 47.6 mm in length. (B) Sagittal MRI image showing perianal fistula, marked by yellow arrows.

Before drainage and seton placement, the patient had a history of multiple perianal/perineal abscesses, indicating active or recurrent fistula‐associated infection risk prior to definitive prostate cancer treatment.

As the first step in the staged management plan, a silastic vessel loop seton was passed through the fistula tract and secured to itself with 0‐silk ligatures before radiotherapy and was maintained throughout the radiation course. The purpose of seton placement was to preserve drainage of the fistula tract, reduce the risk of recurrent abscess or uncontrolled local infection during prostate radiotherapy, and provide a clinically visible marker of the fistula region during treatment planning.

Given the proximity of the fistula tract to the prostate treatment region, radiotherapy planning was designed to maintain definitive target coverage while limiting incidental dose to the perianal tissues whenever feasible. To support reproducibility of the treatment approach, key planning and dose parameters are summarized below:

The perianal fistula and seton were contoured as a dedicated avoidance structure based on direct visualization on fused MRI and CT images obtained at the time of simulation (Figure 2). All organ‐at‐risk (OAR) dose constraints were met, including rectal doses of V5900 cGy 24.397%, V6400 cGy 21.037%, and V6900 cGy 16.682%, and bladder doses of V6400 cGy 25.135%, V6900 cGy 21.113%, and V7400 cGy 0.013%. Femoral head dose constraints were satisfied, with D0.035 cc of 2665 cGy to the left femur and 2879.1 cGy to the right femur. The bowel space received a maximum dose (D0.035 cc) of 1585.8 cGy, and the penile bulb received a mean dose of 4595.5 cGy. Dose–volume histogram analysis of the contoured fistula/seton demonstrated a minimum dose of 244.4 cGy, a maximum dose of 2644.3 cGy, and a mean dose of 697.9 cGy. Pretreatment IMRT quality assurance was performed prior to delivery, with gamma analysis demonstrating 99.9% of sampled points within 3.0%/2.0 mm of the predicted dose distribution. Daily image‐guided radiation therapy (IGRT) was conducted following immobilization using kV imaging with fiducial marker matching; three gold fiducial markers had been placed in the prostate under ultrasound guidance prior to simulation.

Figure 2: Multipanel figure indicating various aspects of radiotherapy planning. Green organ represents prostate. Small pink region represents the position of the perianal fistula marked by seton placement. (A) Coronal CT scan with color wash indicating positions of the prostate and perianal fistula. (B) Dose‐volume histogram (DVH) illustrating the relative dose percentages received by the prostate (green curve) and perianal region (pink curve) during radiotherapy. (C) Coronal CT scan showing the perianal region. (D) Sagittal CT scan showing positions of the prostate and perianal fistula.

The patient was treated with definitive radiation therapy, receiving 70 Gray in 28 fractions using intensity‐modulated radiation therapy (IMRT) with daily IGRT. He tolerated radiation well, with a good biochemical response, and his PSA levels began to decline within 3 months.

Three months after completing radiation treatment for prostate cancer, he underwent surgical treatment for the perianal fistula. At the time of repair, exam under local anesthesia using a perianal block showed the seton was intact, with no signs of an active abscess or fistula in the left posterior quadrant. Digital examination of the anal canal showed normal tone, no masses, and no blood. Anoscopic inspection revealed normal mucosa.

The surgical treatment for this perianal fistula involved the following steps. The seton was divided with scissors and removed. A probe was passed from the external opening of the fistula to the internal opening at the anal verge. There was minimal muscle involvement, as the fistula appeared to be predominantly superficial. It was carefully divided with cautery, and hemostasis was ensured. The tract was gently debrided, and the perineum was then dressed with fluff gauze, ABD pads, and mesh briefs.

This fistulectomy was successful, and the patient tolerated it well without complications. Following fistulectomy, the patient was managed with sitz baths twice daily, short‐course oral analgesics, and local hygiene measures. No antibiotics were required, and the wound healed by secondary intention over several weeks. At the last follow‐up, 26 months after completion of radiotherapy, the patient was clinically well, with no wound‐healing complications, no recurrent infection, no fecal incontinence, and no fistula recurrence. His most recent PSA, measured in April 2026, was 0.45 ng/mL. He remained free of biochemical recurrence by the Phoenix definition, defined as PSA nadir +2 ng/mL.