Saturday, January 9, 2010

X Ray Backscatter and Cancer

The NY Times wrote an article today on the potential risks of x-ray backscatter at airports. Let me take another look at this issue.

X rays are very powerful electromagnetic waves which are generated by colliding electrons. The x-ray is of very short wavelength, very high frequency and has tremendous energy.

We use strong x-rays now for over 100 years to look within the body. Namely we put the x-ray gun on one side and the file, well very few use film any more, on the other and look at what happens in between. We can see broken bones, tumors, and the like. A great advance except for one tiny problem, x-rays break DNA bonds and other such stuff and that results in cancer.

Now the powerful x-rays that go through the body are transmitted, namely they come from behind a molecule and then are absorbed or allowed to pass through. Thus the lightness and darkness. The deeper we want to go the stronger we make the scan, namely more x-ray photons. Yes, x-rays are particles, photons, but at the frequency of the x-ray. Thus the same energy per photon is used but just more photons. CAT scans use lots of photons. CAT scans are also transmission in nature, going from one side to another.

Now there is also a scattered x-ray approach using what is called Compton scattering. You see the x-ray has a wavelength similar to that of an electron, one of the electrons in the outer rings of the atom, and when it hits one of them it bounces the electron out of orbit, the x-ray loses energy and it gets scattered at a lower frequency backward and the atom is now missing an electron. Then what happens. Two things:

1. The scattered electron comes back and can be measured as backscatter

2. The poor atom is now looking to fill the electron loss and something, somewhere, will fill it. That is where the real problem comes in. If this were a cytosine on your skin DNA, say the DNA of a melanocyte in the basal layer of the epidermis it may get methylated. Then we start the process of carcinogenesis. Then we start a melanoma. You do not need a great deal of energy, you are not going through the body, you just want backscatter, and what you are backscattering off of is the skin! And if a person has dysplastic nevus syndrome, their malanocyte DNA has already had many hits, or if the person has extensive sun damage, the same. It then takes just one more hit on a single cells DNA, and off we go to the morgue!

As the Times so properly states:

The plan for broad use of X-ray body scanners to detect bombs or weapons under airline passengers’ clothes has rekindled a debate about the safety of delivering small doses of radiation to millions of people — a process some experts say is certain to result in a few additional cancer deaths.

The scanning machines, called “backscatter scanners,” deliver a dose of ionizing radiation equivalent to 1 percent or less of the radiation in a dental X-ray. The amount is so small that the risk to an individual is negligible, according to radiation experts. But collectively, the radiation doses from the scanners incrementally increase the risk of fatal cancers among the thousands or millions of travelers who will be exposed, some radiation experts believe.

Indeed the data is lacking. We do not know what the results of such continual assaults on the skins DNA will cause. There are about 2-4 billion melanocytes on a human and there are about 20-50 billion keratinocytes, the typical skin cells. It is the 2-4 billion melanocytes we are worried about. Many of them may already be primed for cancer from prior hits.

Also the issue here is that x-ray backscatter is designed to backscatter off the skin, it is not meant to go through, thus it is optimally designed to hit a melanocyte as a target.

The Times continues:

In a 2002 report on the safety of backscatter scanners, the National Council on Radiation Protection and Measurements, which is highly influential in setting regulatory standards, said it “cannot exclude the possibility of a fatal cancer attributable to radiation in a very large population of people exposed to very low doses of radiation.”

One author of that report, David J. Brenner, a professor of radiation biophysics at Columbia and director of the university’s Center for Radiological Research, said that risk might be increased as the transportation agency moves from using the scanning machines as a second-round check after metal detectors and hand searches to using them as a first-line screening system.

“When we were looking at these a few years back, it was always going to be as a secondary screening tool,” he said. “In that scenario, I don’t think there’s too much concern.” But, he said, if millions or tens of millions of passengers a year were scanned with the backscatter X-ray, he said, the risk would be higher.

The problem with all of these studies is that today we know a great deal more about the pathways associated with cancer generation. The Vogelstein model of colon cancer has now been adapted for many other forms of cancers, but there are yet to be filled in gaps. Cancers start with hits to one gene and then another. It may take 2 or even ten hits to genes before a metastatic cancer starts. However many people are genetically prone to have cancer because their genes have been primed by hits at birth. Thus it just takes a few more.

The argument made by the manufacturers is that this it is akin to the radiation on a flight. But that radiation on a flight is gamma rays not x-rays, and not x-rays aimed to scan your skin, skin deep. That is where the melanocytes are.

Frankly in my experience with radiologists, their safety standards are in many cases developed in a gross body manner and not at the molecular level. We know about the pathways at the cell level which cause cancer. We do not know everything but we know enough to set out many flashing red signs. There is a gap in our knowledge and starting an international experiment of whole body radiation is rather Mengele like in character. Or perhaps it is a part of our new health plan!