Radiation and Patients
Each and every one of us is constantly surrounded by radiation (mostly gamma radiation) that is very similar to the x-rays used for medical diagnosis and treatment. Radiation is an essential part of everyday life. This radiation comes not just from cosmic rays in our surroundings, the air we breathe, building materials, and the food and drink we ingest, but also from our own bodies. We all emit thousands of gamma rays every minute because of the radioisotopes, primarily potassium-40 but also isotopes of uranium and thorium, which are present in all of us. So radiation itself should not be feared, although too much of it can have ill effects.
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It is necessary to point out that there are different types of radiation; ionising and non-ionising. We are not talking here about radiation from cell phones, microwave ovens, ultra-violet radiation, infrared radiation and radiofrequency waves used in TV and radio; these are all types of non-ionising radiation. X-rays and radiation emitted from radioactive substances are ionising radiations which are powerful enough to break molecular bonds.
We derive benefits from radiation. Without non-ionising radiation, we would not have many of the tools of modern life, and without ionising radiation we would not have the medical tools for diagnosing diseases and disorders. The key issue in the use of radiation for diagnosis is the balance between benefit and risk. That balance is assessed by ‘appropriateness’, which is referred to as the justification principle in radiation protection. When a particular x-ray examination or procedure is justified by the health professional qualified to do so, based on an individual patient’s medical needs, patients can be reassured that any radiation risk has been considered in the appropriateness assessment and that the potential benefits outweigh the risks. There are well-established appropriateness or referral criteria from professional societies like the Royal College of Radiology (UK), the American College of Radiology, the European Commission, the French Society of Radiology, the Canadian Association of Radiologists, and the Royal Australian and New Zealand College of Radiologists.
There is a tendency to overplay the risks of radiation and compare them with radiation from an atomic bomb, which is unrealistic. Ignoring the risks when they are likely to be present is also unreasonable. There are some situations where risks are speculative (based on extrapolation of data), and there are disputes among scientists on their likelihood. Patients and the public have a right to be appropriately informed about radiation risk, and one of the key issues is how much radiation will I, as a patient, receive from a radiological examination. However, the answer may not stay the same over time. As technology changes, the radiation dose associated with a given examination may change. Generally, improved technology should result in a lower radiation dose to the patient of more than 50%, but that is not always true as it depends on the required image quality. Modern CT scans have much greater image quality than was available in the 1970’s to 1990’s. Better image quality leads to more information but comes with the cost of a higher radiation dose. Moreover, national surveys indicate a wide variation in radiation dose for the same examination. Thus the tables below are representative but not universally applicable.
Table 1: Classification of major radiological examinations in broad category of radiation dose (Adapted from RPOP website of IAEA).
|Procedure||Effective Dose mSv||Equivalent number of PA chest radiograph (each 0.02 mSv)||Increased Risk of Cancer||Equivalent Period of Natural Background|
||Not defined/applicable||Not applicable||Not known||Not equivalent|
|One in a million||Few days|
||1 – 5||50 – 250||1 in 10,000||Few months to a few years|
||5 – 20||250 – 1000||1 in 2,000||A couple of years to several years|
As is apparent from Table 1, plain radiographs like chest x-ray and indeed all plain x-rays of any part of the body (lumbar spine, abdomen, head) involve low to intermediate radiation doses somewhat close to the radiation dose one receives from natural background over the course of a year or two. Higher dose examinations are the ones that need to be looked at more seriously, particularly when higher dose examinations are to be repeated. Table 2 gives radiation doses in CT examinations.
Table 2: Mean effective doses from CT examinations *
|CT examinations||Mean effective dose (mSv)||Equivalent number of PA chest radiographs (each 0.02 mSv)|
|Chest (pulmonary embolism)||15||750|
|Nuclear medical **
* With the use of technology and good technique it is possible to perform many of the examinations listed in the Table above with almost half the radiation dose.
** Mettler F et al. Radiology 248; 254-263, 2008. [Note: The dose will depend upon activity administered]
What are the guidelines for avoiding more radiation than necessary?
The following principles are helpful:
- Justification: Every examination involving radiation should be duly justified to indicate that the benefits outweigh the risks and alternative investigations that do not use ionising radiations have been considered.
- Optimisation: Once justified, the examination should be performed with the minimum radiation dose. The imaging facility should use the As Low As Reasonably Achievable (ALARA) principle while generating images with diagnostic image quality, avoiding an unnecessarily high, or low, image quality. Higher image quality than needed for diagnosis may result in unnecessarily high radiation dose, whereas low image quality may require repeating the examination.
- Comparison with reference: The radiation dose imparted by the facility for a particular examination should be compared with regional, national or international reference levels.
- Tracking: Keep track of all radiological examinations whether it involves ionising radiation or not. Tracking can help avoid another examination or identify an inappropriate dose associated with examination.
Is there a maximum dose for patients that should not be exceeded?
No. There is clear agreement among international organisations with responsibility for radiation protection that specifying a limit can hinder useful application of radiation and can be detrimental. Not performing a needed examination can often be more harmful than performing it. No dose of medical diagnostic radiation is considered too much for a patient when the procedure is justified by a qualified person who has carefully weighed the risks and benefits to the patient.
What are the risks from radiation?
This should be considered in two parts: what is certain and what is uncertain. Under all reasonable circumstances, it is not possible for a patient to be exposed to enough radiation from x-ray machines to cause death from acute exposure. This has never occurred. The doses necessary to obtain skin reddening (erythema), skin injuries and hair loss are reasonably well established. Skin injuries have been observed in patients undergoing interventional procedures that replace surgery (like angioplasty), mostly when performed by repeatedly exposing the same body part to radiation. The occurrence of skin injuries is estimated to be about 1 in 10,000 interventional procedures, which makes them very rare. Skin injuries and temporary hair loss caused by CT have been recorded only in recent years with the first report appearing in 2005, though CT has been in use since 1972. It should also be borne in mind that these injuries occurred mostly through error, rather than through normal use of CT. Guidelines developed to introduce safeguards by manufacturers provide further assistance in avoiding radiation injuries in CT. Genetic defects from radiation exposure, passed on to offspring, have not been seen in humans, and so the main area of concern is the cancer risk. The field of radiation risks is well studied, much more than many other areas of risk in daily life. The current estimates of probability of cancer from radiation exposure to dose of 10 mSv is 1 in every 2,000 people, a dose that is on the higher end of the dose from an abdominal CT scan (though a well optimised CT scan is possible with a much lower dose than this). This implies that if 2,000 people were to receive a radiological examination that imparts 10 mSv, there would be a probability that one of them may have cancer over and above the background cancer rate. The ‘normal’ rate of cancer is fairly high, up to almost 1 in 3 of the whole population, but it ranges between 14% and 30.1% depending upon sex, country and region [Global Cancer Statistics, 2011]. This translates to about 280 to 602 ‘expected’ cancers per 2,000 people. Therefore exposing all 2,000 to 10 mSv each would increase this by one to 281 to 603 per 2,000.
Viewed in this light, the additional risk is put into perspective. Nevertheless, the importance of radiation risk should not be underestimated when repeated examinations are performed, and doses should be kept as low as possible. There may be ways to perform the same examination with a much lower dose. If dose is halved, the risk is also halved, which is why the ALARA principle is considered so important.
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