Iodine-125 Permanent Seed Implant (Low-Dose-Rate Brachytherapy) for Prostate Cancer — Overview, Diagnosis & Treatment Options | MyMedicPlus

Iodine-125 (I-125) permanent seed implant brachytherapy — also called low-dose-rate (LDR) brachytherapy — is a curative treatment option for localized prostate cancer in which 60–120 radioactive iodine-125 seeds (each approximately 4.5 mm in length) are permanently implanted into the prostate gland under transrectal ultrasound (TRUS) guidance. The seeds emit low-energy gamma radiation (average energy 28 keV, half-life 60 days) that irradiates the prostate to a prescribed dose of 145 Gy (monotherapy) or 108 Gy (as a boost with external beam radiotherapy). The radiation dose is delivered gradually over approximately 8–10 months as the seeds decay. LDR brachytherapy is a well-established, guideline-endorsed treatment option for low- and favorable intermediate-risk prostate cancer, endorsed by AUA, ASTRO, and EAU guidelines. It offers equivalent cancer control to radical prostatectomy and external beam radiotherapy for appropriate candidates, with favorable urinary and erectile function preservation compared to surgery. The procedure is performed as a day case under spinal or general anesthesia.

LDR brachytherapy is indicated for histologically confirmed, localized (organ-confined) prostate cancer in men who are appropriate candidates. Optimal candidates for LDR monotherapy per ASTRO/AUA guidelines: low-risk prostate cancer (PSA ≤10 ng/mL, Gleason score ≤6 [Grade Group 1], clinical stage ≤T2a) and favorable intermediate-risk (one of: PSA 10–20, GS 7 = 3+4 [GG2], or T2b). LDR brachytherapy boost combined with EBRT is appropriate for unfavorable intermediate-risk and selected high-risk cases. Absolute contraindications include prior TURP (transurethral resection of the prostate) leaving a large surgical defect, very large prostate volume (>60 cm³ without hormonal downsizing), severe lower urinary tract symptoms (IPSS >20), and pubic arch interference (measured on ultrasound or CT planning). Relative contraindications: prior pelvic radiotherapy, coagulopathy, and limited life expectancy. IPSS (International Prostate Symptom Score) >15 predicts higher risk of post-implant urinary retention.

Urinary symptoms are the most common side effects following I-125 seed implant. Acute radiation urethritis develops in the first weeks after implantation: frequency, urgency, dysuria, nocturia, and decreased urine flow, reaching peak severity at 3–6 months. Post-void residual increases transiently; urinary retention requiring catheterization occurs in 5–15% of patients and resolves in most by 6–12 months. Alpha-blockers (tamsulosin, alfuzosin) are prescribed pre-implant and maintained for 3–6 months to mitigate obstructive symptoms. Rectal symptoms — proctitis, rectal urgency, and rectal bleeding — affect approximately 10–15% of patients and are generally mild. Erectile dysfunction develops in 30–50% of patients by 5 years — lower than after radical prostatectomy (~80%) but higher than for many other patient populations. Urinary incontinence is uncommon (1–3%) in men with intact sphincters and no prior TURP. Seed migration to the lung or bloodstream is detected radiographically in up to 20% of patients but is clinically insignificant.

Pre-implant evaluation includes a thorough urological assessment. Transrectal ultrasound (TRUS) of the prostate is performed to determine prostate volume (optimal 20–50 cm³ for LDR monotherapy), assess for pubic arch interference, and plan seed distribution. PSA, Gleason biopsy grade (ISUP Grade Group), clinical T-stage, and MRI pelvis are used for risk stratification per NCCN 2024 guidelines. MRI prostate (multiparametric MRI, mpMRI) is recommended before biopsy and implant to identify extracapsular extension or seminal vesicle invasion that would change management. Baseline IPSS questionnaire and IIEF-5 (erectile function) score are documented. Dosimetric planning is performed by a medical physicist using TRUS volume study to calculate seed number, strength (activity), and spatial distribution necessary to achieve target D90 (dose covering 90% of the prostate volume) of ≥145 Gy. Post-implant dosimetry CT scan at 4 weeks confirms adequate dose distribution.

Seed implantation is performed under spinal or general anesthesia as an ambulatory procedure. The patient is placed in the dorsal lithotomy position. A TRUS probe is inserted rectally for real-time imaging guidance. Using a brachytherapy template grid fixed to the perineum, 15–25 hollow needles are inserted transperineally into the prostate under TRUS and fluoroscopic guidance. I-125 seeds (stranded or loose) are deposited at 1 cm spacing according to the pre-planned dosimetric distribution. Seed positions are verified in real-time using fluoroscopy and TRUS imaging. The procedure takes 60–90 minutes. Patients are discharged the same day with a urinary catheter (removed next morning) and antibiotic coverage. Post-implant instructions include: avoid close proximity to pregnant women and young children for 2 months (radiation precaution), use a urinary strainer for 2 weeks to detect any passed seeds, and avoid strenuous exertion for 1 week. PSA nadir should reach <0.5 ng/mL by 24–36 months and is monitored every 6–12 months. The biochemical failure definition (PSA nadir +2 ng/mL) is the Phoenix criterion.

LDR brachytherapy achieves excellent long-term biochemical disease-free survival for low- and favorable intermediate-risk prostate cancer. For low-risk disease, 10-year biochemical disease-free survival (Phoenix criterion — PSA nadir +2 ng/mL) is 85–90%, equivalent to radical prostatectomy and external beam radiotherapy. Favorable intermediate-risk patients treated with LDR monotherapy achieve 10-year BDFS of 75–85%. Fifteen-year data from Seattle (Sylvester et al.) demonstrate sustained cancer control with minimal late mortality from prostate cancer-specific causes. Prognostic factors for treatment success include pre-treatment PSA level, Gleason grade group, T-stage, and post-implant dosimetric quality — D90 (dose covering 90% of prostate volume) ≥145 Gy is strongly associated with biochemical control. Late urinary complications — urethral stricture, persistent urinary obstruction — occur in 2–5% of patients and typically resolve with intervention. Late rectal complications (rectal fistula) are rare, occurring in less than 1% of patients. Erectile function is preserved in 70–80% of men treated with LDR monotherapy at 5 years — significantly better than radical prostatectomy outcomes. Second primary malignancies from radiation are extremely rare given the low-energy, localized nature of I-125 radiation. Biochemical failure after brachytherapy is managed with salvage focal therapy, HIFU, cryotherapy, or systemic hormonal therapy depending on the clinical context and patient fitness.

Careful patient selection per AUA/ASTRO guidelines is the primary determinant of optimal outcomes and minimal toxicity. Pre-treatment alpha-blocker therapy (tamsulosin 0.4 mg/day initiated 2–4 weeks before implant and continued 6 months after) significantly reduces post-implant urinary retention. Hormonal therapy (LHRH analogue for 3–6 months) can downsize glands larger than 60 cm³, enabling implant in otherwise ineligible patients. Meticulous intraoperative dosimetry — real-time planning with intraoperative adjustments — reduces dose hotspots in the urethra and rectum. Post-implant CT/MRI dosimetric analysis at 4 weeks with quality assurance assessment of D90, V100, and rectal dose constraints guides early detection of underdosage, which may prompt salvage EBRT. Avoidance of NSAIDs and anticoagulants peri-procedurally reduces hematoma risk.

Patients who develop acute urinary retention (inability to void) after seed implantation should contact their urologist immediately or attend the emergency department for catheterization — do not attempt to wait beyond 4–6 hours of inability to void. Severe rectal pain, hematochezia (rectal bleeding), or rectal fistula symptoms should be evaluated urgently by a colorectal surgeon and radiation oncologist. Pulmonary symptoms (cough, hemoptysis) are rarely caused by seed migration but should be evaluated with chest X-ray if they develop. Any new onset of back pain, leg weakness, or urinary incontinence in the first year after brachytherapy requires MRI to exclude spinal cord compromise from vertebral metastases, as these symptoms may indicate disease progression rather than treatment toxicity. Rising PSA on three consecutive measurements after reaching nadir (biochemical failure) requires prompt urological oncology review to plan salvage therapy.