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Note for clinical users

You can order glossy copies of the "patients" page, printed as a tri-fold leaflet with your clinic name, from enq@rcs-ltd.demon.co.uk .


Are x-rays harmful?

There is a lot of unjustified fear and alarm about radiation from medical procedures. These notes should put the subject into perspective.

What is radiation dose? We see images on an x-ray film because your body absorbs some of the x-ray beam, producing shadows. "Dose" is the amount of  energy absorbed from radiation, adjusted for the particular area of the body. It is measured in sieverts (Sv) or millisieverts (mSv).

What does it do? X-rays can trigger chemical reactions. We use this property to make an image on a film, or to destroy tumours in the body. But such reactions can also harm healthy tissue, so we are careful to ensure that the benefit of medical x-rays outweighs any harm that they might do.

Background dose is energy that we absorb from cosmic rays, and radioactive elements in the earth's crust, the atmosphere, and our food. The average dose is about 2.6 mSv per year in the UK, more in Scotland and the West, less in South and East England. Over 99% of background radiation is natural.

What is the risk associated with radiation dose? Background radiation has been fairly constant during the evolution of life, so our bodies naturally repair damage caused by small doses. There is no evidence of any health risk associated with normal background doses in the UK. But we know (mostly from studying industrial accidents and radiotherapy treatments) that a sudden dose of 1000 mSv can overwhelm our capacity for repair, so we keep medical doses to the minimum needed for accurate diagnosis and treatment.

What dose will I receive from an x-ray examination? This table shows some useful comparisons with exposure to natural background radiation

 examination typical dose (mSv) 

equivalent  period of natural background

 limbs & joints  <0.01  < 1.5 days
 teeth (bitewing)  <0.01  <1.5 days
 chest (1 film)  0.02  3 days
 neck  0.08  2 weeks 
 hip  0.3   7 weeks 

 thoracic spine / abdomen / pelvis

 0.7  4 months
 lumbar spine  1.3   7 months
 IVU  2.5  14 months 
 barium meal   3  16 months 
 barium enema  7  3.2 years
 head scan (CT)   2  1 year
 chest scan  (CT)  8   3.6 years 
 CT abdomen / pelvis  10  4.5 years

Note that these figures are based on the UK population average, and most people live in the south-east of England, where the natural background is lowest. A dose of 6 mSv would be equivalent to living in Cornwall or the Scottish Highlands for a year - not a prospect that would worry a Londoner!

So why do the staff use lead screens and aprons? Some radiation is scattered from the patient towards the operators. The amount from one patient is tiny, but we need to protect ourselves from the doses accumulated from the hundreds or thousands of patients that we see each year. 

Will I be radioactive after my examination? Not if you are having an x-ray, bone density or CT examination. If you are having a radionuclide test ("bone scan", some blood tests, etc) you will be given particular information and instructions. MRI and ultrasound examinations do not use ionising radiation at all.

What if I'm pregnant? You must inform your doctor or radiographer before the examination. A fetus is more sensitive to radiation at some stages of its development, particularly between 2 and 8 weeks after conception, so we need to be certain of the urgency of some tests (particularly from "IVU" downwards in the Table)  if you have missed a period, and to take special care to minimse the dose to your abdomen.

Who looks after my safety? We are governed by regulations that ensure patient safety. Your referring physician must provide enough information for us to evaluate the risk and benefit of a particular test. The practitioner (radiologist) reviews the request and considers how to best to obtain the required information. The operator (radiographer) selects the method that uses the least practicable dose. Chiropractors, podiatrists  and dentists, who take their own x-rays, have to follow the same rules as if they were referrers, practitioners and operators separately. We are advised and monitored by nationally recognised experts who review our equipment and procedures regularly.

How do you know the correct dose to use? In complex examinations, the dose is set and recorded  automatically. For simple x-ray films we use standard tables to calculate dose according to the size of your body.

I've had previous x-rays. Is this taken into account? Yes. In deciding whether an examination is necessary, we look at recent, relevant films. However there is no limit on the number of x-rays that you should have in a year: the appropriate number depends entirely on your medical condition. But we must be sure that  every x-ray examination provides useful information so that the benefit outweighs the very small radiation risk for each examination requested.




Protection of the public

The use of ionising radiation in medicine and industry inevitably exposes everybody to some radiation in addition to the natural background. The law (see "Regulations" for more details) distinguishes three broad groups:




There is an overriding requirement that any use of ionising radiation at work must be in some way beneficial, that any contingent exposure must be weighed against the benefit, and that all exposures must be as low as reasonably practicable.

The dose delivered to every patient must be individually optimised with respect to the clinical benefit that will accrue. There is no statutory limit, since a small diagnostic dose may lead to a very large therapeutic dose, and it would be harmful to abandon a lifesaving x-ray procedure (such as inserting a pacemaker) simply because we have "run out of dose". 

The dose received by employees (which includes self-employed persons, contractors, and some volunteers) is absolutely limited by statute to 20 millisievert per year. Lower limits apply to young people and women of childbearing age. The dose limit is set at a level where the occupational risk is no greater than that of working in a "safe" industry such as retailing. Employees whose remuneration does not necessarily involve their being exposed to ionising radiation (e.g. office workers in a power plant) must be treated as "others".

The dose limit for all other people, for exposure to ionising radiation generated in the course of any work,  is set at 1 mSv/yr. This is well below the level at which it can be discriminated from background, and is less than half of the lowest natural background dose rate in the UK. Simply living in the next town or county would invoke a change of more than 1 mSv/yr for many people. It is unnecessary and impracticable to set the limit any lower: for example passenger aircraft would have to carry concrete shields, or airlines would have to limit the number of flights any passenger could make in a year.

It is the responsibility of every employer who wishes to use ionising radiation at work, to cooperate in complying with the "public" dose limit. This is usually done by ensuring that the maximum likely dose in the worst case, say to a person living next door to a clinic or at the boundary of a nuclear power plant, is less than 0.1 mSv/yr. This figure,  called the "dose constraint",  is  used for designing the physical structure and working procedures of the radiation source. The employer must carry out a risk analysis before building or modifying any radiation facility; must register his intention to use ionising radiation at work, with the Health and Safety Executive; and must carry out sufficient tests before putting any new or modified facility into use, to demonstrate that it complies with the design constraint. As no person can be in two places at the same time, and it is unlikely that anyone would live or work at the boundary of more that two independent industrial undertakings simultaneously, a 0.1 mSv/yr constraint effectively prevents anyone from approaching the statutory dose limit

In practice, it is unusual for the calculated dose to any "other" person from any industrial or medical source of radiation to exceed 0.01 mSv/yr - nobody stays in the "worst case" position all day, every day. To put this into perspective, the dose you acquire from the (essential) natural potassium in your diet is a hundred times larger, and if you moved house from say Cambridge to Birmingham, the additional natural background dose would be 200 times larger, than any dose you would be likely to receive from a clinic or factory next door.










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