1.Overview The number of new patients with prostate cancer (PCa) in the United States in 2002 was reported to be as high as 189,000/year, with about 30,200 deaths due to prostate cancer [1]. The incidence rate of latent prostate cancer in men over 70 years of age is 25%, of which 9.51‰ can develop into clinical prostate cancer. At present, radical prostate cancer treatment and iodine 125 particle implantation are mostly used to achieve the effect of radical treatment. Iodine 125 particle implantation therapy for prostate cancer involves the implantation of iodine 125 particles into the prostate gland through the guidance device of the treatment planning system at ultrasound to treat the tumor by internal irradiation therapy, while ensuring that the radiation dose delivered to the surrounding normal structures is minimal. In the United States, only 4.2% of patients with limited prostate cancer were treated with iodine 125 implantation in 1996, but now 60% of prostate cancer patients have been treated with iodine 125 implantation because of its efficacy, minimally invasive nature and few complications. 2. Advantages of iodine 125 particle implantation (1) Higher dose than external irradiation therapy for localized prostate; (2) Lower dose than external irradiation radiotherapy for bladder and rectum; (3) Continuous low dose irradiation therapy, more effective than external irradiation therapy, suitable for treatment of long-term slowly progressing prostate cancer; (4) Iodine 125 has the effect of penetrating into local tissues, with good efficacy and less damage. 3.Principle of iodine 125 particle implantation therapy Iodine 125 particle implantation therapy destroys DNA double strands through the direct effect of radiation or through the indirect effect of the generated free radicals. When tumor cells divide, they are unable to undergo cell division and die because the integrity of their DNA is damaged; while tumor cells that do not divide can survive for a longer period of time. Generally, cells in M and G2 phases are more sensitive to radiation, while cells in S phase are not sensitive. 4. Indications and contraindications (1) Indications Indications for brachytherapy alone: ① clinical stage T1-T2a; ② Gleason score 2-6; ③ blood PSA 20μg/L; ④ peripheral nerve invasion; ⑤ multi-point biopsy with positive pathological results; ⑥ bilateral biopsy with positive pathological results; ⑦ MRI examination clearly shows extra-prostatic envelope invasion. Gleason score of 7 or blood PSA of 10-20 μg/L: depending on the case. Indications for iodine 125 particle implantation therapy (including as a complementary treatment to external radiotherapy) combined with androgen blockade: preoperative prostate volume >60mL, androgen blockade is required to shrink the prostate. (2) Contraindications: ① expected survival less than 5 years; ② large post-TURP defect or poor prognosis; ③ poor general condition; ④ distant metastases. Relative contraindications: ① gland greater than 60mL; ② previous TURP; ③ middle lobe protrusion; ④ severe diabetes; ⑤ history of multiple pelvic radiotherapy and surgery; ⑥ those with high American Urological Association (AUA) score. 5. Steps and Methods The standard mode of particle implantation is trans-perineal under the guidance of template and TRUS. The required equipment includes a three-dimensional treatment planning system, ultrasonography and particle implantation equipment. Brachytherapy should include three steps: (i) acquisition of ultrasound images of the prostate and treatment planning; (ii) iodine 125 particle implantation; and (iii) assessment of postoperative dose distribution. 5.1. Preoperative preparation, administration of antibacterial drugs, and cleansing enemas are routine preoperative preparations for iodine 125 particle implantation therapy. Patients with a large prostate or a narrow pubic arch should have their prostate volume and pubic arch evaluated before the procedure. The ABS recommends a peri-implantation correction of less than twice the prescribed dose, especially when implanting particles after transurethral resection of the prostate (TURP), and recommends a dose of 144 Gy for 125I for brachytherapy alone. The prescribed dose should cover the prostate and its surrounding area of 3-8 mm, so that the prostate target area is approximately 1.75 times the actual prostate volume. Because some patients have the possibility of extraprostatic envelope invasion or because of deviations in particle implantation, the uncertainty in both is 5 mm and 3 mm, respectively [9?10]. 5.2, Iodine 125 particle implantation, a catheter is left in place and contrast is injected into the bladder, then prostate puncture is performed from the perineum under transrectal ultrasound guidance according to the developed treatment plan. When it is determined that the puncture needle has reached the designated location, the seed is then implanted into the designated location by means of a seed implantation device. The distribution of the seeds can be understood and adjusted by X-ray fluoroscopy during the operation. When all the seeds are implanted, cystoscopy is performed and the seeds that have fallen into the bladder are removed. Intravenous antibacterial and hemostatic drugs should be administered on the first postoperative day. The duration of postoperative indwelling catheterization is determined by the preoperative prostate volume and urination. 5.3. Assessment of postoperative dose distribution. Due to uncertainties such as prostate edema after treatment, prostate movement during treatment, and the operating procedure, it is necessary to understand how the radiation dose is distributed in the prostate through postoperative dose distribution assessment, which is closely related to the efficacy and complications of brachytherapy. Postoperative dose distribution assessment is commonly done by CT. CT scans reveal an enlarged prostate that may originate from edema or bleeding [11]. Therefore, premature CT scans often overestimate prostate size and underestimate the dose of radiotherapy in the prostate. CT scans performed several weeks after implantation more accurately reflect the size of the underlying prostate. Studies have found that CT scans are performed at 4 weeks postoperatively, while in practice they are often performed earlier. 6. Efficacy and complications and follow-up A large number of studies have shown that there is no significant difference between the efficacy of iodine 125 particle implantation therapy and radical surgery and external radiotherapy for prostate cancer. Complications of iodine 125 particle implantation therapy include short-term complications and long-term complications. Short-term complications (within 1 year) are related to puncture trauma and acute radiation injury. Most patients have urinary tract irritation such as urinary frequency, urinary urgency and pain after surgery, and some patients show difficulty in urination and increased nocturia. Most studies suggest that urinary tract symptoms can return to normal in 90% of patients after 1 year [17?18]. The incidence of acute urinary retention ranges from 1 to 34% [20?21] and is mostly seen in patients with higher IPSS scores and longer prostate lengths. Short-term rectal complications are rectal irritation such as increased stool frequency and urgency, which are mostly self-limiting and usually treated symptomatically. Long-term complications (occurring after 1 year) are common with chronic urinary retention, mainly related to scarring due to radiation injury to the bladder neck and urethra. The incidence of urinary incontinence is 1-24%, and up to 20%-85% in those with a history of TURP surgery [20]. Approximately 12% of patients present with urethral stricture, which may be related to excessive radiation dose to the urethral bulb and may be resolved by periodic urethral dilatation. Proctitis develops within 3 years after implantation. It mostly manifests as mild blood in the stool and is often self-limiting, but in severe cases, rectal ulcers or even prostate rectal fistula can occur. 7, radiation protection The radiation emitted during the decay of 125I and 103Pd seeds is very short, and the radiation dose of individual particles is also very low. smathers et al [22] study confirmed that the radioactivity carried by the patient is much lower than the standard specified by NRC (Nuclear regulatory comission). 8.Conclusion Brachytherapy, as a treatment modality with positive efficacy, small trauma and few complications, will have a broad development prospect in China.