# Iodine-125 brachytherapy seeds

Source: https://nuclide.cc/drugs/i-125-seeds/  
Nuclide record `i-125-seeds` (Agent). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

Tiny metal seeds containing a mildly radioactive form of iodine are permanently placed inside a tumour, most often the prostate, where they slowly release radiation over several months and then become harmless.

## Summary

Iodine-125 brachytherapy seeds are titanium-encapsulated radionuclide sources used mainly for permanent low-dose-rate (LDR) interstitial brachytherapy of localised prostate cancer, and also for other accessible tumours of the head and neck, lung, pancreas and elsewhere, either as primary treatment or for residual disease after surgery. In the US, seeds such as Theragenics' I-Seed, Implant Sciences' I-Plant and similar products are cleared through the FDA's 510(k) premarket notification pathway (product code KXK, 21 CFR 892.5730) as substantially equivalent to earlier cleared brachytherapy sources; brachytherapy as a procedure does not itself require separate FDA approval.

Iodine-125 has a 59.4-day physical half-life and a mean photon energy of about 28 keV. For prostate monotherapy, published series correlate a minimum dose covering 90% of the prostate volume (D90) of at least 140 Gy with improved biochemical control; when combined with external-beam radiotherapy the typical prescription is lower, around 108-110 Gy. Seeds are implanted transperineally under transrectal ultrasound (and sometimes fluoroscopic or CT) guidance, either as loose seeds or linked (so-called stranded seeds) at regular intervals, and remain permanently in place, gradually becoming inert as the isotope decays.

Iodine-125 competes with palladium-103 and caesium-131 as LDR prostate brachytherapy isotopes; comparative series have found no clear difference in outcomes between iodine-125 and palladium-103 at three years. Because this is a device/source category with many manufacturers rather than a single branded drug, the record here covers the isotope and its clinical use rather than one company's product.

## Fields

- Kind: Agent
- Status: approved
- Last checked: 2026-09-22
- Also known as: I-125 seeds; 125I permanent seed implants
- Tags: brachytherapy; prostate cancer; gamma/X-ray emitter; permanent implant
- Modality: brachytherapy source
- Mechanism: Titanium-encapsulated seeds containing iodine-125, a low-energy gamma/X-ray emitter with a 59.4-day physical half-life, are permanently implanted (typically transperineally into the prostate under ultrasound guidance) to deliver low-dose-rate radiation from within the tumour over several months.
- Dosing: Permanent transperineal interstitial implantation (typically prostate), or interstitial/surface application at other sites; Single permanent implant; monotherapy target D90 approximately 140-160 Gy, or approximately 108-110 Gy when combined with external-beam radiotherapy

## Sources

- FDA 510(k) clearance letter: I-Plant Model 3500 iodine-125 brachytherapy seeds: https://www.accessdata.fda.gov/cdrh_docs/pdf2/k023242.pdf
- Prostate brachytherapy review, Translational Andrology and Urology 2018: https://cdn.amegroups.cn/journals/amepc/files/journals/3/articles/18019/public/18019-PB1-3327-R2.pdf

## Connected records

- indications: [Localised prostate cancer, high and very high risk](https://nuclide.cc/indications/prostate-high-risk/), [Prostate cancer](https://nuclide.cc/indications/prostate/)
- technologies: [Low-dose-rate seed brachytherapy](https://nuclide.cc/technologies/ldr-seed-brachytherapy/)

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