Ionising radiation - dose
By quantifying the radiation delivered or received, safe work practices are ensured through monitoring.
Dose is a generic term which may refer to absorbed dose, equivalent dose or effective dose. Effective dose is meansured in Siverts (Sv).
Absorbed dose (Gy) energy absorbed by unit mass.
Equivalent dose (Sv) is radiation weighted.
Effective dose (Sv) is tissue and radiation weighted.
Throughout this module, 'dose' will mean effective dose.
Doses typical of common medical imaging examinations
ARPANSA RPS 14.1 Annex A: Approximate effective doses arising from common radiological examinations in adults.
Ionisation radiation - dose versus risk
What does a dose mean in real terms? Compare the dose received to the time it would take to accumulate the same dose due to background radiation. The table shows a given dose, the time equivalent of background radiation and the lifetime risk of cancer due to that dose.
The risks arising from most medical imaging examinations do not pose a substantial increase in risk compared to the 1 in 3 chance of being diagnosed with cancer in a lifetime.
Ionisation radiation - risks in perspective
To help put radiation risks in perspective the table below compares scenarios which represent a 1 in a million chance of death. It can be seen from the above table that, under normal circumstances, the risk is relatively small compared with the risk of death from other causes.
The risk from a medical imaging procedure though small, cannot be described as being nil and current policy is to keep the radiation exposure as low as is possible. Often the benefit of the diagnostic information far outweighs the radiation hazard and the risk associated with a patient's illness may be such that any additional hazard due to radiation will be insignificant.
Ionisation radiation - dose limits
Dose limits are applied to both radiation workers and members of the public. By having dose limits in place we may avoid deterministic effects altogether and minimise the likelihood of stochastic events later.
- Radiation workers have a dose limit of 20mSv per year averaged over 5 years.
- Non radiation workers and the general public have a dose limit of 1mSv per year.
- Pregnant Radiation workers have a dose limit of 1mSv for the duration of the pregnancy.
- Dose limits do not apply to the exposure of patients as part of their diagnosis or treatment.
The table below hows the occupational and public dose limits (effective and equivalent) which are used by the radiation safety legislation.
Those who work with ionising radiation closely, such as Radiographers, wear dose-meters to show that if radiation safety legislation is complied with it is very effective at minimising the occupational radiation dose. Most Radiographers return doses of less than 2 mSv. Radiation workers who work in Nuclear Medicine, the DSA suite and Cardiac Catheter lab often return higher doses that the general Radiographer.
It has been proven that those staff who don’t work directly with radiation (i.e. administrative staff, operational services officers etc.) do not need to wear dose-meters. We are required to only monitor those who are likely to return a dose of 1mSv or greater.
Dose limits and monitoring
To monitor our exposure levels, we wear dose-meters. Dose-meters provide proof that safe radiation practices are being undertaken and determine a dose received as a result of occupation exposure or radiation incident. Wearing of dose-meters can also be used to determine a dose received if it is believed a staff member was exposed to ionising radiation during a radiation incident. In the case a staff member only has one badge, the badge should be worn under lead.
- Dose-meters are worn at chest or waist height.
- If wearing a lead apron they are worn under the apron.
- Some staff have two dose-meters
- - ‘IN’ worn under lead apron
- - ‘OUT’ worn outside the lead apron
- Badges should be worn for the duration of your shift and returned to the control board at the end of your shift.
- All badges are sent for assessment quarterly with the resulting report being displayed in the general x-ray area. The reports are kept tin the QRMS folder maintained by the RSO.
- Pregnant staff changed to monthly monitoring for the duration of the pregnancy.
- Monthly monitoring of nuclear medicine staff, catheter lab and DSA staff.
Radiation and pregnancy
Pregnant staff are very safe as long as they follow radiation safety guidelines and safe work practices. The pregnant radiation worker must notify the RSO regarding her pregnancy. The workers dosimeter will be sent for reading on a monthly basis for the duration of the pregnancy. If you have any concerns regarding possible exposure as a pregnant worker please speak to the RSO.
It is not possible to accurately measure the dose to the foetus and so it must be inferred from the exposure to the mother. Radiation protection principles limit exposure to the mother in order to achieve minimum risk to the foetus.
Due to the possible effects, dosimetry during pregnancy differs from the usual protocol.
The level of risk to pregnant women is dependent on the stage of foetal development.
It is well known that the foetus is more sensitive to the effects of radiation than the adult human. The radiation-related risks throughout pregnancy are related to the stage of pregnancy and the absorbed dose. Radiation risks are most significant during organogenesis and in the early foetal period, somewhat less in the 2nd trimester, and least in the 3rd trimester.
The medical irradiation of pregnant patients should be justified on an individual basis. If a diagnostic radiological examination is medically indicated in a pregnant patient, the risk to the mother from not performing the examination is usually greater than the risk of potential harm to the foetus. Most properly performed plain film and low dose examinations pose no substantial increase in risk to the mother or foetus compared to other risks throughout the pregnancy. However interventional radiology procedures involving extended fluoroscopy times and CT scans of the Abdomen/pelvis may result in significant doses and therefore an increased risk to the foetus.
All females of reproductive age, who are to have a radiological examination or required to assist with the examination, should be asked about the possibility of being pregnant. General radiographic examinations remote to the foetus can be undertaken of the pregnant patient without concern. Proper collimation and suitable radiation protection should be applied. The Radiographer shall apply all relevant dose minimisation techniques, consistent with the radiologic diagnosis sought.
Direct x-ray exposure of the foetus must be avoided unless the authorised person* for the examination has given a specific direction in relation to the irradiation of the foetus. All pregnant patients should be counselled regarding the level of risk due to them undergoing an x-ray examination while pregnant. This should be done by the referring doctor/authorised person.
Other radiological examinations with likely high doses, that could result in increased risk to the foetus, should have foetal dose estimates. Pregnant staff carers are not to hold patients during an x-ray examination.
Benefits must outweigh the risk.