Ionizing Radiation in Medical Diagnosis and Treatment
Overview:
“The dose makes the poison,” Paracelsus said some 500 years ago. Paracelsus was a 16th century Renaissance physician who is often credited as the father of toxicology. A doctor’s duty is to diagnose and treat but, if mishandled; the treatment can often do as much damage as the disease. Nowhere is this truer than in the field of ionizing radiation, particularly in medical procedures that use of x-rays, as in computed tomography (CT) scans, or in interventional procedures such as cardiac catheterization for angioplasty. What happens if a patient receives a very high dose of radiation during a radiological procedure? There are two general types of risks he/she might face the first one is readily visible and the symptoms can come relatively early, such as skin reddening, or erythematic and hair loss. The second effect might manifest itself slowly and take years to appear, such as an increased risk of cancer. Radiation effects on skin have been reported primarily in patients undergoing interventional procedures such as angioplasty. This may happen in 1 in 10,000 cases and are not possible in simple examinations such as plain x-rays of chest or any part of the body. Some skin injuries in computed tomography (CT) examinations have only recently been reported and again are rare. With this, the main concern is a long-term risk of cancer.
Background:
The use of ionizing radiation has transformed medicine in the last 100 years. Many of the discoveries that advanced our understanding of human anatomy and function happened in the last few years of the nineteenth century. The invention of x-rays by C.W. Roentgen1 in 1895 was an impressive discovery followed by the discovery of radioactivity by H. Becquerel, who noted in 1896 that uranium was emitting radiation without the need of exposure to sunlight. In 1898, Marie S. Curie announced the discovery of radium, which is more radioactive than uranium2. Curie was instrumental in ensuring that her discovery found applications in helping others, for example, equipping mobile radiography units often referred to as petite Curies. Curies died in 1934 from plastic anemia, a disease most likely due to her lifelong exposure to ionizing radiation.
Nowadays, ionization radiation is used in medicine for two main purposes:
- Diagnostics
- Therapy
Diagnostics:
The largest source of man-made radiation exposures to humans stems from diagnostic procedures. These can be broadly divided into radiology and nuclear medicine. In radiology, an external radiation source is used to generate photons (typically x-rays) of sufficient energy to penetrate human tissues. A detector system at the exit site of the beam determines the transmitted photons, which provide a projection image of all structure in the body. This information can be used as a single projection image (radiograph), for example, chest x-rays. The fluoroscopic imaging allows the clinician to follow anatomical movement (e.g. breathing), surgical interventions (e.g. fluoroscopy-guided biopsy), or search for structures in the patient by moving the imaging apparatus. Finally, multiple projection images of the same anatomy can be acquired from different directions and a computer used to reconstruct three-dimensional information, this process called computed tomography (CT), is usually done in several sections to reduce the influence of scatter in the reconstructed image.
In nuclear medicine, a radioactive isotope, most commonly 99m-technetium is administered to the patient. The isotope is used to label a substance of interest that will follow a physiological pathway of interest. As such, nuclear medicine provides information about not just anatomical features in the patient but also metabolic activity and physiological pathway. The emitted radiation can be detected from outside the patient using mostly gamma cameras, sophisticated arrangements of collimator and detector systems that allow determination of the location of the isotope.
Therapy:
Most uses of radiation for therapy are concerned with cancer treatment. In cancer treatment, radiotherapy aims to deliver a very high dose to the tumor while trying to minimize the dose to surrounding normal tissues. There are two possible ways to deliver radiotherapy:
- External beam radiotherapy where the radiation is directed to the cancer from outside of the patient’s body. As the distance of the radiation source is typically of the order of a matter, this type of therapy is also sometimes referred as tele-therapy.
- Brachytherapy (brachys is Greek for “close by”, “near”) is the use of radioactive isotope brought into close contact with the tumor to deliver the radiation.
The Radiation Warning Sign:
Radiation is a natural part of our lives. Visible light is the most common type of radiation that we use for seeing things every day. There are also forms of invisible radiation in our environment that come to us from outer space and from the small amount of natural radioactive substances that are in the earth, the air we breathe, the water we drink, the food we eat, as well as in our own bodies. Whereas, ionizing radiation cannot be seen, heard, smelled by humans. However, as radiation is considered a hazard, it is important to make people aware of its presence. The internationally recognized symbol indicating radiation hazards is the black trefoil on a yellow background. (This is shown in figure 1).
External and Internal Exposure:
“The International Commission on Radiological Protection, ICRP, is an independent Registered Charity, established to advance for the public benefit the science of radiological protection, in particular by providing recommendations and guidance on all aspects of protection against ionizing radiation.” (www.icrp.org) 3
Hazards from ionizing radiation can broadly be divided in two groups:
External exposure: Radiation reaches a person from outside through the skin is typically from a radiation source that can be turned off like an x-ray unit. The radiation from this source may cause damage in an organism while it is turned on. In the case of external exposure, nothing radioactive left in the body. Hazards of this type may occur in radiology or radiotherapy departments.
Internal exposure: This occurs most commonly after the incorporation (e.g. breathing in, consuming with food, absorbing through the skin) if radioactive isotopes. The radioactivity remains in the organism until the isotope has decayed or until it is excreted. These hazards may be present in nuclear medicine departments, research laboratories or in hospital environment. If you, a loved one or someone you know need to undergo x-ray examination here are some suggestions:
- Try and fine out if the health facility has a program for quality assurance and certification in which the patient-doses are comparable with international standards;
- Never refuse a needed examination. Despite the risks associated with x-rays, you should beat in mind that the benefits of x-ray examinations outweigh the risks. What is most important is that the examination should have been duly justified by the doctor for you;
- Do not expect the health-care providers to give information on the exact figure of the radiation dose. It is important to know that there is no internationally prescribed “upper limit” for a radiation dose; and
- Carry records of all previous radiological examinations.
References:
- Kron, T., Wilhelm Conrad Rontgen, Australas. Phys.
- Macklis, R.M., Portrait of science. Scientist, technologist, proto-feminist, superstar, Science, 295, 1647-48, 2002.
- http://www.icrp.org/
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