The ALARA principle
ALARA is an acronym for "As Low As Reasonably Achievable".
The rationale behind ALARA is:
- Radiation may be harmful
- Radiation has uses in diagnostic and therapeutic medicine
- In order to receive the benefits of radiation, the public must accept the possibility of harmful effects from radiation
- To protect the public and employees, the potential and amount of exposure should be reduced
- This reduction should be as low as economically feasible.
There are three ways to implement the ALARA principle.
The amount of radiation produced over a period of time is relatively constant (particularly with brachytherapy sources). Dose is therefore reduced if the time spent exposed to the radiation is reduced.
Radiation intensity is inversely related to the square of the distance (inverse square law). Therefore, by doubling the distance the dose of radiation is reduced by four times.
Radiation may be attenuated by shielding material. The amount of shielding required is dependent on the penetration of the dose. Electron emitters require minimal shielding, whereas megavoltage x-rays may penetrate several metres of concrete.
ICRP Recommended Dose Limits
The latest guidelines from the ICRP are summarised in the table below (Report 103)
|Type of Limit||Occupational||Public|
|Effective Dose||20 mSv/year*||1 mSv/year|
|Lens of eye||150 mSv/year||15 mSv/year|
|Skin||500 mSv/year||50 mSv/year|
|Hands and Feet||500 mSv/year||N/A|
Practical measures for reducing dose
The amount of time spent handling radioactive sources can be reduced by performing dummy runs with non-radioactive equipment. Time is also a factor when placing rooms near a radioactive source - preferably rooms with low occupancy rates (eg storerooms) should be placed next to radiotherapy bunkers.
The distance from radioactive sources is important both when handling sources (using long handled tools) and in bunker design. Waiting areas should not be directly opposte the entry into the linear accelerator bunker, but preferably some distance away. Distance must not be too great, which would increases costs and time to treat patients.
Personal shielding should be considered for electron and kilovoltage x-ray treatments. A small lead shield over the body is sufficient to attenuate most of the radiation. For megavoltage x-rays, lead garments may simply increase the surface dose received by the worker.
Shielding of bunkers is done to reduce the dose by 3 or 4 tenth-value layers (ie: reduce transmission dose by a factor of 1000). For a primary bunker in a linear accelerator, this equates to 260 cm of concrete. Denser materials can be used, but increase the cost of shielding by a huge amount. It is therefore important to plan radiotherapy departments carefully to avoid the need to purchase expensive shielding.