12.1.2 - Iodine 125

Iodine 125 is a commonly used source for permanent implanted interstitial brachytherapy. Iodine 125 is manufactured into resin spheres which are encapsulated within a thin titanium shell.

Physical properties

125I has a half life of 59.4 days, decaying through electron capture to 125Te, a stable isotope. Gamma photons are released following the decay, with a maximum energy of 35 keV. The mean energy is 28 keV. The specific activity of 125I is $6.4 \times 10^2$ TeV/g.


125I is always supplied as an encapsulated seed, with a 0.05 mm shell of titanium around the iodine source. The iodine is located on resin spheres within the capsule or adsorbed on a silver rod.

Handling Issues

The source capsule for 125I seeds is fragile and can be damaged. It is important to perform wipe tests before use of the seeds to ensure the capsule is intact. Iodine may become highly reactive if stored at cold temperatures and it is important not to cold sterilise the seeds. It should be stored within a lead safe at least 3 mm thick.
125I sources are typically 'hot loaded', which is possible by having

Supply and Disposal

125I seeds are typically unsterile when delivered and must be sterilised prior to use. As they are a permanent implant, they do not require specific disposal, although if the patient dies within a year of insertion cremation is not recommended unless the implants are removed from the body first.



  • Permanent implant
  • Pure gamma emitter


  • Relatively long half life (when compared with 103Pd), which may complicate dose calculations over time.
  • Fragile capsule
  • Single use only

Decay Scheme

(Heavily based on the LNHB Table of Radionuclides, here)