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
Advantages:
- High or low dose rate depending on concentration of 192Ir
- Relative ease of manufacture
- Small source size
- Stable daughter product
- Re-usable
Disadvantages:
- Broad spectrum of photon emissions (this is less of a disadvantage as it allows direct comparison with 226Ra sources, used frequently in the past)
- Frequent recalibrations due to radioactive decay
- Replacement every 3 - 4 months
Decay Scheme
(Heavily based on the LNHB Table of Radionuclides, here)
