General management of sealed sources
ALARA principles should be adhered to at all times.
To reduce dose while handling:
- Distance is accomplished by using long handled tools and not handling the sealed source directly if possible
- Time is accomplished by training with dummy sources prior to hot sources. This speeds up the treatment process.
- Shielding involves working with mirrors or with transparent plates that attenuate the lower energy photon beams. Lead gowns are also advisable when dealing with kilovoltage emissions. Shielding is more difficult with high photon energy sources (such as 198Au or 60Co) and may increase dose to the person by acting as skin bolus.
Sources should be handled carefully as some types (particularly seeds) have thin, fragile capsules.
Before insertion of any sealed source, a 'wipe test' should be conducted to detect any leakage from the source capsule.
Cleaning has many of the same principles as handling, although care must be taken with some sources (125I) during sterilisation to prevent formation of reactive species.
Storage is important to prevent unintentional exposure or deliberate misuse of sealed sources. Sources should be kept in a locked safe that attenuates most of their radiation. The safe should be alarmed in the event of unauthorised persons trying to gain access to the source.
Transport has many of the same issues as storage.
Disposal of short lived temporary implants or HDR sources is usually handled by the company that provides the source.
Disposal of long lived, resuable implants is more troublesome, and they should be handled at a nuclear waste site (which the ACT is lacking).
Choice of sealed sources
High activity sources are best used for high dose rate brachytherapy. This type of brachytherapy can be done as an outpatient (particularly intracavity brachytherapy) as treatment can be carried out within 10 - 15 minutes once the patient is set up. The most commoly used high activity source is 192Ir.
Very low dose rate sources are best used for permanent implants. The low penetrating power of the photons released by 103Pd and 125I mean that patients can be discharged immediately following insertion. Patients do not need to remain in hospital while the radiation has its effect.
Temporary low dose rate brachytherapy implants, such as low dose rate 192Ir and 137Cs, are used in situations where HDR is not available or unfeasible. LDR treatments require the patient to remain immobile, typically for days, while the dose is delivered. This is associated patient discomfort and complications from prolonged bed rest such as deep venous thrombosis.
Electron sources such as 90Sr/Y are used when the treatment volume is very thin and localised, such as in endovascular brachytherapy or pterygium treatment. The rapid dose fall off limits dose to neighbouring structures (eg. the lens of the eyeball) which is not possible with gamma releasing sources.
- 12: Brachytherapy