Answer to Question #3552 Submitted to "Ask the Experts"

0. Considering that two views of the chest were taken, would both of these tests impact the same cells, since there are cross sections at each point, or would the radiation have affected different cells each time?
    
0. I have read that radiation can damage DNA, forever altering it. Could the amount of radiation she received from these two x rays have damaged her cells' DNA?
    
0. I have often heard that the amount of radiation in x rays can be compared to flying in an airplane. Is this an accurate comparison though, since the amount of exposure from a chest x ray occurs in a split second, whereas the same total amount of radiation from flying in an airplane would occur over many hours. Doesn't the time period of the dose come into play in determining the cell damage and, if so, then wouldn't these two exposures not really be comparable?
    
0. If the body's cells can repair themselves after low doses of radiation, then why are the effects of radiation considered accumulative?
    
0. I read that it was recently discovered that the cells repair themselves faster from higher doses of radiation than lower doses. Does this mean it is actually worse to get a chest x ray now than it was decades ago?
    
0. I have seen on your Web site some comparisons of the increase in cancer risk from x rays versus the general population. How are these comparisons made though when the general population that is used in the comparison I would assume has had x-ray exposure as well? In other words, a person with (x) amount of x rays is not being compared to a person having had NO x-rays to see what the difference in cancer rates are, correct?
    
0. I have read that the effects of radiation damage are more severe to young children than adults. Is this due to the rapid cell division. If so, then does this mean that many more cells are actually effected by the radiation than just those exposed?
    
0. Lastly, can you please tell me the approximate difference in radiation exposure from a chest x ray done now versus one done 30 years ago.

The answers to your questions follow:

0. The two views were obtained with different geometries (back to front and right to left). They exposed the same tissues (same cells) but not to the same distribution of dose. Radiation level decreases as the beam passes through the body, with the largest dose being at skin entry and the smallest at skin exit.
    
0. All available evidence points to DNA damage as the major mechanism of radiation injuries. However, the dose to the patient's chest from two films was extremely small. Radiation interacts with tissues in a purely random distribution. It is therefore theoretically possible that one or a very few cells could have been damaged. We have no concrete proof that such damage occurs from such small doses. In the event that it does, there are two possible outcomes: (1) the damage could be repaired or (2) damaged cells might die. In the first case, there would be no remaining damage. Cells die all the time for a variety of causes, and the death of one or a few more would make no detectable difference to the patient.
    
0. The dose to the patient from a common diagnostic x-ray procedure (for example, chest) is approximately equivalent to the extra radiation dose one receives from a typical flight in a jet airliner from cosmic radiation, which is more intense at high altitude. It further is approximately equivalent to the dose that every person gets in a couple of days from natural background radiation. In general, radiation effect is proportional to the rate at which the radiation is received. The dose-rate effect from large doses (for example, radiation therapy, nuclear accidents) is about four. That is, radiation administered at a high dose rate is about four times more effective than the same dose spread over time. However, this factor disappears as doses get small. Very small doses at a high dose rate have about the same effect as they do at low dose rate. We have no concrete proof that any of this is harmful. A few studies show that these small doses may even be beneficial.
    
0. Part (generally most) but not all radiation damage is repaired. The unrepaired portion accumulates.
    
0. Speed of repair of radiation injury is not the major factor. Completeness of repair is.
    
0. You are correct. Nowadays it is virtually impossible to find people who have not been exposed to diagnostic x ray. What is being compared is the diagnostic medical dose versus the natural background dose that everybody gets and cannot avoid.
    
0. Adverse effects from small doses of radiation are stochastic in nature. The most common are cancer and mutation. These effects are rare and are all-or-nothing; either a cancer occurs or it does not. Frequency of occurrence in an exposed population is a function of dose. Radiation-induced cancers occur years to decades after exposure; the period at risk is essentially the remainder of the life span. Mutations are transmitted genetically and occur in offspring (perhaps many generations later). As a person ages, several things happen that change this picture. Likelihood of having additional children decreases. Remaining life span, or the opportunity for radiation-induced cancer to appear, grows shorter, and the probability of death from unrelated causes grows larger. Therefore, risk of radiation injury from low doses (medical, environmental) decreases with age. Cellular effects, such as variation of radiosensitivity with cell division, are more related to high doses of radiation. These cellular injuries lead to deterministic effects, such as acute radiation sickness, hair loss, or cataracts, in which the severity of the injury is a function of the number of cells killed or injured.
    
0. There has been a rapid decline in radiation exposure per medical procedure due to technological advances. Speed of x-ray film has increased in the past 30 years or so by a factor of at least four. Refinements in x-ray beams have reduced patient exposure by at least tenfold. Current patient dose is about 2 to 5% of what it was in 1975.

S. Julian Gibbs, DDS, PhD