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Dosages are calculated on the basis of the amount of energy transferred to a biological system. The risk to normal function posed by those dosages are similarly pegged to the amount of energy absorbed. From Exhibit A, it should have become obvious that this system is in need of revision. However, even within the currently adopted paradigm, a fundamental problem exists in the accepted methodology for calculating how much radiation a person receives from radioactive material deposited within the human body. As mentioned earlier in this chapter, the dose delivered to an organ is derived by averaging the amount of radiation emitted by internal contaminants over the entire mass of the organ in which they are deposited. Using external irradiation as a model, the physicists of the Manhattan Project postulated that internal emitters would produce the same biological effect for the same amount of energy deposited by radioactive decay (with consideration given to the quality factor of each type of radiation). To capture this energy transfer in their mathematical calculations, the energy transmitted by alpha and beta particles during radioactive decay was averaged over the entire mass of the target organ to yield an organ dose. The ICRP continues to this day to insist that this is the proper way of calculating dosages for internal emitters!
Alpha and beta particles emitted by radioactive contaminants create dense, localized patterns of ionization within a microscopic volume of tissue. To treat these emissions as somehow impacting and transmitting an effect to the entire mass of an organ is a gross conceptual blunder that totally distorts the reality of what actually transpires on the cellular and molecular levels. As mentioned previously, alpha particles on average traverse no more than 30 to 40 microns, approximately 3 to 4 cell diameters. Beta particles can travel as far as millimeters, depending on their energy and the density of the tissue, traversing the distance of approximately one hundred cell diameters. When emitted from an atom undergoing radioactive decay, these particles travel along discrete tracks within a small volume of cells. Biological damage is produced within individual cells along these particle tracks. While in transit, they initiate the ionization of molecules only along their path of travel, either hitting vital molecular cellular structures, such as the DNA molecule, or missing them altogether. Not all cells within the range of the particle are affected. Biological alteration occurs only in those cells that are hit by the particle. Cells that are missed by the particle suffer no injury. With internal emitters, the unit of interest for gauging biological effects is individual cells, not whole masses of tissue. This is particularly true for the induction of a cancer. Cancers arise from mutations within a single cell. This being the case, averaging the effect of a particle over an entire mass is ludicrous. Being a hit or miss phenomenon involving individual cells, how can the effect of an alpha or beta particle be averaged over the entire organ?
Chris Busby has been instrumental in highlighting the shortcomings of this physics-based averaging model which he speaks about at length in his book Wings of Death and in numerous other writings. On the website of the Low Level Radiation Campaign, a simple but effective illustration can be found that pinpoints the inadequacy of current practices for calculating the biological impact of internal emitters:
"The conventional approach of averaging the energy transfer from radioactive decay events across a whole organ or the entire body is like emptying a Colt 45 into a football stadium and averaging the effects of the 6 bullets across all the 25,000 spectators. The assumption that between them 25,000 people should be able to stop six bullets without any of them feeling more than a tap on the arm will not console the six grieving families" [1].
In this example, the 25,000 spectators are the cells of an organ. The six bullets are six alpha particles. By the averaging model, the energy from the velocity of the bullets is treated as equally distributed to all who feel no more than a tap as a result. But this model simply does not reflect the phenomenon. In reality, the full energy is absorbed by only six spectators with catastrophic consequences. In terms of biological effect, it makes no sense to speak of the impact of six alpha particles distributed over 25,000 cells. Only the individual cells hit will suffer biological damage. The remainder will escape unscathed. The dose is not received by the whole organ. It is absorbed completely by only a handful of cells.
For the incredulous readers, shaking their heads in disbelief that the international radiation protection community calculates biological impact of internal emitters by such hocus pocus averaging, rest assured that this is the way things are done.
Bibliography
[1] Quote from Low Level Radiation Campaign response to DETR consultation on the recycling of contaminated materials arising from Clearance of nuclear sites. This document plus hundreds of pages of instructive information can be found on the website of the Low Level Radiation Campaign at www.llrc.org.
