Patients have become more aware of the risks associated with exposure to any form of radiation. It is important to remember than any dose of radiation has the potential to induce malignant changes. Even small doses can cause tissue damage and currently there is no threshold below which no risk occurs. When asked, " Doctor, is this x ray really necessary", the challenge for us as clinicians is to explain the risks to patients in a way that they can understand. This is part of informed consent. The days where we could say: "Trust me. The exposure levels are nothing to worry about" are over. But what are the risks? How can we give patients a true sense of the relative safety of x ray images? Dr. Paul Abbott recently published an excellent article Are Dental radiographs Safe? Australian Dental Journal 2000:34:(3):208-213. This month's Endo Fax will highlight the findings from this paper so you can use these explanations in your practice.
A Quick Review of X ray Physics Radiation Affects - Direct vs. Indirect
X Rays disperse their energy as they pass through tissues. Soft tissues are more susceptible to damage than hard tissues because they are weak aqueous solutions. Radiation damage to tissue can be classified as Direct or Indirect. Direct damage is caused by a photon or electron hitting a DNA or RNA tissue molecule and disrupting the bonds. The affected molecules cannot pass on important genetic information. This can result in carcinoma or genetic abnormalities. Indirect damage occurs when electrons hit other atoms in their pathway causing further ionization and excitation. These highly reactive ionized molecules can cause disruption in enzymes and nucleic acids. Cell function and cell reproduction can be affected.
Radiation Affects - Somatic vs. Genetic
Somatic effects concern the individual person being exposed. Genetic effects can influence the progeny of the irradiated individual. Different parts of the body have different levels of mitotic activity that determines their relative level of "radiosensitivity". The effects can be acute or long term. Acute effects can be erythema, pigmentation and ulceration. Long-term effects can be induced leukemia, cancer and genetic damage. The problem with x ray films is that it is impossible to show a direct "cause and effect" relationship of any particular exposure because of the latency period of symptoms.
Sources of Radiation
75% of people's exposure is provided by Natural background radiation. Cosmic rays account for about a third of the natural background radiation while terrestrial (natural radioactive substances on earth) make up the rest. The amount of natural background radiation also depends on geographic location. It can range from 3 microSv per year to as much as 10-15 times that amount depending on the country that you live in. Man made radiation accounts for about 25% of the Annual Collective Dose (ACD) to which people are exposed. For this paper, exposures are measured in Seivert units (Sv). Most of us are more familiar with REMs(Radiation Equivalent Man) 1 Sv = 100 REM.
One of the best ways to compare radiation doses is to use the BERT (Background Equivalent Radiation Time). This unit tells us how many hours, days or weeks of exposure to Natural Background Radiation would be equal to the same dose that your patient would get from a Dental X ray. The medical chest X ray is generally the type of x ray procedure most familiar to patients.
The figures below can give us some relative values to compare with this generally acknowledged standard.
Investigations Effective Dose (microSv) Equivalent Chest Films BERT
1 PA or BW - D Speed round collimator 4 .13 16 hours
Endo (4-5 PAs) 16-20 0.6-0.8 3.3 days
FMS (10-15 films) 40-60 1.3-2.0 6.7-8 days
Panoramic Film 7 0.2 28 hrs
Chest Film 30 1.0 4 days
One Transatlantic Flight 37.5 1.2 5 days
One Flight - Europe to Australia 112.5 3.7 15 days
The risk of the risk of induction of fatal cancer or serious hereditary ill health from radiation has been calculated to be 1 in 80 per Sv. (Note- the table above is in MICRO Sv ie/ 1PA = .004 Sv) This equates to an estimated risk of 1 in a million per from a panoramic film and 1 in 10 million for a periapical film. While these figures show that the risks are very low for each film exposed, it is important to consider that the effects of radiation are also cumulative. Clinicians should always carefully consider the potential harm for every film proposed. The number of previous medical and dental radiographs should always be considered and patient's concerns about radiation must not be dismissed or ignored.
X Rays During Pregnancy
Many dentists refuse to take x-ray images on pregnant patients even though they have little justification in doing so. The most sensitive time for radiation affects to a fetus is between the 32nd and 37th day of gestation (4 ½-5 ½ weeks). Very few women realize they are pregnant at this time. The general consensus is that for significant risk to occur more than 10 microSv of radiation is required and the fetus must be in the direct pathway of radiation. Both of these requirements are unlikely to occur during dental x radiography. Therefore radiographs during pregnancy are not contraindicated if the potential benefits are significant (e.g. emergency endo treatment). For medico-legal and psychological reasons, use of a lead apron in pregnant patients is advised. There appears to be no need, on radiation grounds, to defer dental radiography during pregnancy.
Reducing the Risks to Patients All dentists have an obligation to their patients, their staff and themselves to minimize the risks by the use of the ALARA rule (As Low as Reasonably Achievable). This applies to all aspects of radiography including what films should be taken, what dose to use, how many films should be taken and the best technique to use. The risk/benefit ratio should be considered and justified for each exposure.
1. Use of Lead Aprons
Use of lead aprons with a minimum thickness of .25 mm of lead has been recommended, while others question the merit of such devices. Lead aprons help reduce the amount of primary radiation reaching the areas of the body that are in the direct pathway of the primary beam. BUT some sources say that they may potentiate the effects of scatter radiation because some of the indirect scatter beams may be trapped between the apron and the body, reflecting the beam back to the tissues that they were meant to protect. Since the risk of malignancy from the primary beam of a Dental X ray is 1 in 10 million, from the scatter of the same beam without an apron is - 1 in 100 million, and since collimation produces such a narrow unscattered beam in most modern machines, the use of an apron would seem unnecessary. However, the lead apron provides psychological security for patients who believe they are helping to protect their distant organs.
2. Good Radiographic Technique
Studies have shown that many radiographs need to be re-taken for reasons related to poor technique and radiation practices. One study showed 34 % of retakes were necessary due to processing problems, 28 % due to incorrect x ray technique, 3 % due to exposure problems and 1% due to lost films. In an informal survey of his own endodontic practice, Dr. Abbott claimed that as many as 65% of films supplied to him were not of diagnostic quality and almost 90 % had not been adequately processed. Use of paralleling devices can lower the numbers of retakes and improve image quality.
3. Equipment, Design and Maintenance
Well maintained modern equipment is essential for diagnostically acceptable radiographs and for radiation safety. All aspects of every machine- including filtration, beam size, timer etc. should be checked regularly for proper function. High-speed film is preferred to lower speed film even though some diagnostic compromise in images has been noted. Accurate processing techniques, proper fixation and wash ensure that films that are stored can be read in the future. Operatories should be designed with the proper shielding to protect adjacent patients and staff.
4. Digital Radiography
Digital radiography and imaging has the potential to drastically reduce exposure to patients by as much as 80% (for periapical and bite-wing images). Unfortunately, there is some sacrifice in image quality and although the images are good, they are not up to film quality as of yet. They are getting close. Because of the need to have a networked office, operatory computer workstations, image management programs and high cost, these systems are still not in widespread use by dentists. There is no question that film use will eventually be replaced by these systems in most modern offices.
OK, What's the bottom line? What do I tell my patients?
1. The chances of you contracting a malignancy or risking serious hereditary ill health from one dental periapical radiograph are about 1 in 10 million.
2. The chances of you contracting a malignancy or risking serious hereditary ill health from one extra-oral panoramic radiograph are about 1 in one million.
3. Pregnant patients should not avoid X ray images when they are indicated.
4. Dental X ray images are one of dentistry's most important diagnostic tools. Only the minimal required images will be taken. While there is a measurable risk to the patient, the potential benefits of dental X ray images for diagnosing oral disease far outweigh the risks.