12.1.1 - Iridium 192

Iridium 192 is the most commonly used isotope for high dose rate brachytherapy applications. It is produced by neutron bombardment of Iridium-191 (a stable isotope), and manufactured as a wire as an alloy with platinum. The wire is coated in platinum to filtrate the electrons produced by the iridium decay process. Importantly, 192Ir can be used as a low dose rate (LDR) treatment (as temporarily implanted wires / needles) or as a high dose rate (HDR) treatment (using remote afterloading techniques).

### Physical Properties

192Ir has an interesting decay pattern. 95% of the time, 192 Ir decays through negative beta emission to 192Pt. The remaining 5% of the time 192Ir decays through electron capture to 192Os. This process commonly releases a gamma photon with average energy of 0.38 MeV (max 1.06 MeV). 192Ir has a half life of 74 days, its biggest drawback. Specific activity varies on the concentration of 192Ir in the source; for high dose rate applications the specific activity is $2.4 \times 10^2$ TeV/g.

### Handling Issues

192Ir does not require sterilisation when used in HDR applications, as it is contained within a sealed system which is sterilised separately. Due to its high dose rate, time spent handling the source should be extremely limited. It is typically stored within a safe in the brachytherapy room, which must be securely locked to prevent accidental exposure or delibrate misuse.

### Manufacture, Supply and Disposal

192Ir is typically supplied by the source manufacturer. It is returned to the manufacturer for disposal. 192Ir is produced in a nuclear reactor by bombarding 191Ir with neutrons. This has numerous benefits, such as availability of pure 191Ir, minimal production of unwanted isotopes and a large cross section for neutron interactions. This allows high concentrations of 192Ir to be produced relatively easily and is one reason it is in such common use today.

### Construction

Low dose rate Iridium 192 is available in lengths of wire 100 - 140 mm long, 0.3 mm thick that can be cut into the desired length for low dose rate applications. The wires are flexible and non-reactive.
The HDR 192Ir source is a 3.5 mm long, 0.6 mm diameter platinum/iridium alloy with a high concentration of 192Ir, giving it a high activity. It is coated in 1 mm of platinum, which attenuates any electrons generated through decay. This capsule is welded to the end of a wire that allows it to be retracted and deployed from within a HDR remote afterloading machine.

### Summary

• High or low dose rate depending on concentration of 192Ir
• Relative ease of manufacture
• Small source size
• Stable daughter product
• Re-usable