The question of the relationship between radiation, X-rays, cosmic rays, (radioactive) radon gas, and cancer is a murky one, and there are many unanswered questions. In some cases, exposure to radiation gives the patient an increased lifetime risk for cancer, and for reasons unknown to medical science, the risk for women is greater than the risk for men. Because of the very long time delays that can be involved (up to 45 years has been recorded), it would appear that radiation is a cancer potentiator, rather than a direct inducer.(But radiation itself can be an inducer for a cell that has already been potentiated.) In the cases (Hiroshima, Chernobyl) where a direct and short time correlation has been observed, it is probable that the intensity of the radiation is also important. We know that high energy, high intensity radiation is lethal to humans, and lower intensity radiation is lethal to cancer cells. We also know about radiation sickness. We have a rough idea of the lethal dose of radiation (which is different for alpha rays, beta rays, photons (gamma rays), neutrons, and cosmic radiation). We know how much radiation shielding astronauts need to keep them alive.
We do not know, and will never know, the LD50 dose for humans for any type of radiation. We cannot even measure human radiation exposure. The unit called the Sievert is used, but the Sievert depends on the type of radiation, the energy, and, most important of all, the amount of radiation energy absorbed by various tissues, and this latter term can only be estimated (and very poorly, at that) from animal studies. We also know that we are missing some scientific factor: The residents of Denver dwell 5,000 feet higher than the residents of NYC, and therefore have one mile less of atmospheric shielding from cosmic rays. How is it then that Denver residents do not have a significantly higher rate of cancer than do Manhattanites?
There was a recent article published in JAMA detailing the increased amount of radiation we are all getting over our lifetime from X-ray studies. In fact, the U.S. gov't has officially classified radiation as a carcinogen. Why is it then that no patient is ever given a release form to sign than delineates the estimated immediate and lifetime personal carcinogenic risk from the proposed X-ray study? (I once tried to generate and have my ER patients sign such a form, and I was immediately told by the chairmen of Radiology and Medicine to stop.)
Thirty years ago, when I was a physics professor, I started to do some experiments with a colleague of mine from Princeton. Since he had a joint appointment with Princeton Physics Dept. and the Princeton Plasma Physics Lab, which meant that he also worked on the Stellarator, our research was classified (and still is, for all I know). We then co-opted a physicist at Oak Ridge Nat'l Labs for good radioactive sources. We were trying to see if there is a minimum radiation dose below which there is NO cancer risk. The belief now as well as then was that there is not, and that there is no minimum safe dose of radiation. We were unable to come to a definite conclusion. We could not determine whether or not a certain straight line on a graph passed through the origin.
The universal fear of radiation is so great that MRI was initially called by its correct name NMR (Nuclear Magnetic Resonance), but NMR was changed to MRI so as to not scare patients away from the MRI. We could greatly decrease food poisoning and contamination with germs and bugs by subjecting wheat, fruit, etc., to killing levels of radiation, but again, the public is so fearful of the possible consequences of radiation that Congress has never passed a food-radiation enabling law, and we can therefore expect future lethal food outbreaks similar to the E. Coli in hamburgers. The one time we should have been fearful, we were not, and many women who licked camel's hair brushes to make a finer point before painting the numbers on a wristwatch with a radioactive chemical developed mouth and jaw cancers.
I guess this blog is written with mixed emotions, and no firm purpose. As a physicist I have the greatest respect for the dangers of radiation, and as a physician, I know its many beneficial uses. I wish we all knew more!