Ionising radiation in medicine

Ionising radiation provides medicine with some of its most important diagnostic tools, as well as being an important component of the armoury for treatment of cancer.

X-rays
X-rays, which don’t make the patient radioactive, form the main use of ionising radiation in medicine. X-rays are transmitted through tissues, but some, such as bone, transmit fewer of the X-rays than others.  A plain X-ray film or radiograph essentially provides a shadow image of the structures within the body.  X-rays can also be used to investigate problems in the arteries, the digestive system or the bowel by introducing liquids, called contrast media, which transmit fewer of the X-rays than the surrounding tissue. This allows blockages, narrowing or other abnormalities in the vessels to be viewed on a computer monitor while the patient is being examined.  In the past all these techniques used media such as film and fluorescent screens, but the digital age has now taken over many aspects of medical imaging. The recording of images in digital format has been one of the major developments in the last 10 years.  Doctors can now view the results of X-ray examinations on their office computers via the internet without waiting for film processing and delivery.

The development of computer processing technology has allowed images to be reconstructed in different formats.  By placing an array of detectors to record the image produced by an X-ray beam transmitted through the body, and then repeating the process at many different angles, it is possible to reconstruct images either of slices through the body or organ surfaces represented in three dimensions which provide the clinician with a better way of looking at disease processes in the patient.  This technique, known as computed tomography (CT scans), was first used in UK hospitals in the 1970s to provide images of tissues within single slices, and can now provide detailed anatomical images in many different forms. CT scans can give better diagnostic information than a conventional X-ray examination.

Nuclear Medicine
Radioactive materials may be used as tracers in a technique called nuclear medicine.  This uses radionuclides, fixed to various pharmaceuticals which are involved in important physiological functions.  These tests use radionuclides which have a relatively short half-life i.e they lose their radioactivity quite rapidly which minimises the radiation dose to the patient. The radiation emitted from these radiopharmaceuticals can be detected and used in the diagnosis of diseases of the bone, heart, lung, kidney and many other organs.  The radioactivity will still be inside your body for a little while after the test and your therapist will advise you on what precautions to take after you leave the hospital.

Radioactivity is also used to treat a number of illnesses. Hyperactive thyroid glands and prostate cancers are just two of many examples.

Doses from diagnostic procedures
The safety of the patient, hospital staff and anyone else who might be nearby is paramount when using ionising radiation. Medical staff, radiographers, engineers and radiation protection professionals all work together to achieve this. Imaging is carried out in a way that minimises any risk whilst obtaining high quality results. As long as there is good clinical need, the very small risk from radiation exposure is far outweighed by the benefits of a diagnostic X-ray. You should however tell your doctor if you have been X-rayed for the same condition recently so that unnecessary exposure can be avoided. In this context it should be pointed out that children are about twice as radiosensitive as adults and unborn babies even more so. If you are possibly pregnant you should inform the radiographer so that special precautions maybe used.

The radiation doses received from all these diagnostic applications are relatively low.  Common examinations such as a chest X-ray give an effective radiation dose (0.02 mSv (millisieverts)) similar to that which an average person in the UK would receive from background radiation every three days or the extra dose received from cosmic radiation during a return flight from London to Scotland.  The majority of both X-ray and nuclear medicine examinations give doses similar to those that would be received from natural background radiation in several months (0.2 mSv – 2 mSv). The highest doses are from CT examinations, and these equate to the dose that might be received from natural background radiation in 3-4 years (8-10 mSv).

Radiotherapy
One of the most vital applications of ionising radiation is in radiotherapy for the treatment of cancer.  The radiation dose needs to be high enough within the tumour itself to kill all the cancer cells, but the dose to the surrounding healthy tissue must not be high enough to cause serious damage.  Treatment may be carried out using external beams of radiation which are directed towards the tumour site from different angles around the body in order to give a higher dose level at the tumour. An alternative method is to implant sources within or next to the malignant tumour to irradiate it directly..  A delicate balance must be achieved in which sufficient radiation is administered to kill all the tumour cells but the dose to the surrounding tissues is kept at a level where serious damage is minimised.  This is achieved through careful planning of the dose delivery and dose distribution and by giving the doses in a series of fractions, allowing the healthy tissue to recover in between exposures.  The doses received from radiotherapy are hundreds or thousands of times greater than those from diagnostic procedures and so the risks to healthy tissue are much greater and the planning process and the clinical justification for each patient requires extensive input from experienced clinical oncologists and medical physicists. .

Thus ionising radiation is becoming an ever more important component of medical diagnosis and treatment. Applications are advancing more and more rapidly with the availability of new technology.

Colin J Martin

Reviewed and revised February 2007