Direct action or damage

The X-ray photons, or high-energy ejected electrons, interact directly with, and ionize, vital biologic macromolecules such as DNA, RNA, proteins and enzymes, as shown in Fig. 7.1A. This ionization results in the breakage of the macromolecule’s chemical bonds, causing them to become abnormal structures, which may in turn lead to inappropriate chemical reactions. Rupture of one of the chemical bonds in a DNA macromolecule may sever one of the side chains of the ladder-like structure. This type of injury to DNA is called a point mutation. The subsequent chromosomal effects from direct damage could include:

If the radiation directly affects somatic cells, the effects on the DNA (and hence the chromosomes) could result in a radiation-induced malignancy. If the damage is to reproductive stem cells, the result could be a radiation-induced congenital abnormality.

What actually happens in the cell depends on several factors, including:

Indirect action or damage

This process, which is shown in Fig. 7.1B, involves the ionization of the water molecule producing both ions and free radicals which can combine to damage the vital biologic macromolecules such as DNA. The sequence of events involved is summarized in Fig. 7.2. The free radicals can recombine to form hydrogen peroxide, a cellular poison, and a hydroperoxyl radical, another toxic substance. Both of these substances are highly reactive and produce biological damage. By themselves, free radicals may transfer excess energy to other molecules, thereby breaking their chemical bonds and having an even greater effect. As about 80% of the body consists of water, the vast majority of the interactions with ionizing radiation are indirect.

Tissue reactions (deterministic effects)

These are the non-cancer damaging effects, to the body of the person exposed, that will definitely result from a specific high dose of radiation. The severity of the effect is proportional to the dose received, and in most cases a threshold dose exists below which there will be no effect. They were previously referred to as deterministic effects, but are now referred to as tissue reactions by the International Commission on Radiological Protection (ICRP) because it now recognizes that some of these effects are not determined solely at the time of irradiation but can be modified after radiation exposure. They are further subdivided into:

Stochastic effects

Stochastic effects are those that may develop. Their development is random and depends on the laws of chance or probability. These damaging effects may be induced when the body is exposed to any dose of radiation. Experimentally it has not been possible to establish a safe dose – i.e. a dose below which stochastic effects do not develop. It is therefore assumed that there is no threshold dose, and that every exposure to ionizing radiation carries with it the possibility of inducing a stochastic effect. The lower the radiation dose, the lower the probability of cell damage. However, the severity of the damage is not related to the size of the inducing dose. This is the underlying philosophy behind present radiation protection recommendations described later. Stochastic effects are further subdivided into:

Cancer induction

If a somatic (body) cell is irradiated a radiation-induced malignancy cancer may develop. Quantifying the risk is complex and controversial. Data from groups exposed to high doses of radiation have been analysed and the results used to provide an estimate of the risk from the low doses of radiation encountered in diagnostic radiology. The high-dose groups studied include:

• The survivors of the atomic explosions at Hiroshima and Nagasaki

• Patients receiving radiotherapy

• Radiation workers – people exposed to radiation in the course of their work

• The survivors of the nuclear disaster at Chernobyl.

The problem of quantifying the risk is compounded because cancer is a common disease, so in any group of individuals studied there is likely to be some incidence of cancer. In the groups listed above, that have been exposed to high doses of radiation, the incidence of cancer is likely to be increased and is referred to as the excess cancer incidence. From the data collected, it has been possible to construct dose–response curves (Fig. 7.3), showing the relationship between excess cancers and radiation dose. The graphs can be extrapolated to zero (the controversy on risk assessment revolves around exactly how this extrapolation should be done), and a risk factor for induction of cancer by low doses of radiation can be calculated.

After reviewing all the available evidence, the International Commission on Radiological Protection suggest there is a 1 in 20,000 chance of developing a fatal cancer for every 1 mSv of effective dose. Using this estimate, a broad estimate of risk from various X-ray examinations may be calculated and these are shown in Table 7.1.

Table 7.1

A broad estimate of the risk of a standard 30-year-old adult patient developing a fatal radiation-induced malignancy from a variety of dental and medical X-ray examinations

X-ray examination	Estimated risk of fatal cancer
Bitewing/periapical radiograph (70 kV, round collimation, D-speed film)	1 in 1,000,000
Bitewing/periapical radiograph (70 kV, rectangular collimation, F-speed film)	1 in 10,000,000
Panoramic radiograph (average)	1 in 1,000,000
Upper standard occlusal	1 in 2,500,000
Lateral cephalometric radiograph	1 in 5,000,000
Skull radiograph (PA)	1 in 1,000,000
Skull radiograph (lateral)	1 in 1,250,000
Chest (PA)	1 in 1,430,000
Chest (lateral)	1 in 540,000
CT head	1 in 14,300
CT chest	1 in 3000
CT abdomen	1 in 3500
CT mandible and maxilla	1 in 80,000 to 1 in 14,300
Barium swallow	1 in 13,300
Barium enema	1 in 9100
Dento-alveolar cone beam CT	1 in 2,000,000 to 1 in 30,000
Craniofacial cone beam CT	1 in 670,000 to 1 in 18,200

Risk is age-dependent, being highest for the young and lowest for the elderly. The risks shown in Table 7.1 are for a 30-year-old adult. The 2004 European Guidelines on Radiation Protection in Dental Radiology recommend that these should be modified by the multiplication factors shown in Table 7.2, which represent averages for the two sexes. In fact, at all ages risks for females are slightly higher and risks for males slightly lower.

Table 7.2

The multiplication factor for risk for different age groups based on the 2004 European Guidelines on Radiation Protection in Dental Radiology

Age group (yr)	Multiplication factor for risk
<10	×3
10–20	×2
20–30	×1.5
30–50	×0.5
50–80	×0.3
80+	Negligible risk

This epidemiological information is being updated continually and recent reports suggest that the risk from low-dose radiation may be considerably greater than thought previously. However, the present figures at least provide an idea of the comparative order of magnitude of the risk involved from different investigations. Dental radiology employs low doses of radiation and hence the risk of stochastic cancer-induction is very small. However, the total number of intraoral and extraoral dental radiographs taken is very high – estimated at around 20 million per year in the UK alone. It is thought that diagnostic radiology (medical and dental) is responsible for about 700 cases of cancer per year in the UK of which about 10 cases are attributable to dental radiology – hence the need for the practical radiation measures outlined later.