Biologically Effective Dose: The tendency over the history of radiation protection has been to treat whole-body irradiation with X-rays or gamma-rays as the "reference" form of exposure, and estimate biological effects from exposure by multiplying the actual dose up or down by factors that adjust either for the nature of the radiation being used or for the organ(s) actually under exposure. The definitions applied to these correction factors and their values have evolved over the years. The tendency has been to say, "when in doubt, increase the weighting factor"; that is, in the absence of firm information to the contrary, the weighting factors associated with a particular kind of radiation or with a particular biological modality of exposure are estimated from worst-case scenarios. People have gotten better at imagining the worst over the years, so many of the weighting factors (particularly for particular forms of radiation) have increased. If a committee doubles the weighting factor associated with a particular type of radiation, then the effective dose (measured in Sieverts) will double in a stroke. Is this appropriate? Perhaps not. But this approach has predominated in population dosimetry.
We are also interested in estimating the significance of an exposure, not only to an individual, but to the population as a whole. If two humans receive a high dose and a million individuals receive one-tenth as large a dose, we might expect that the social cost associated with the smaller but more widespread exposure will be greater. Much of the dosimetry we will discuss here keeps this concept in mind.
The concept of effective dose takes both the radiation weighting
and the relative sensitivities of organs into account. Thus
the effective dose, measured in Sieverts, is
E = ΣT wT
ΣR wR DT,R
i.e. it is a sum over all relevant types of radiation R
over all exposed organs T
of a product of the measured dose DT,R of radiation type R
to tissue T, multiplied by a tissue-specific weighting factor wT
and further multiplied by a radiation-type-specific weighting factor
wR. If only one organ T is being exposed to only one form
of radiation, the summations collapse to
E = wTwR DT,R.
Radiation and Tissue Weighting Factors:
By construction for whole-body irradiation wT = 1;
generally for specific organs wT < 1, but for the more
significant ones (e.g. the gonads for genetic damage, the lung and liver for
cancer), the value of wT will be larger than for less significant
organs. Table 16.2 in the text lists the ICRP values for wT:
Tissue Weighting Factors for Calculation of Effective Dose
| WT=0.01 | WT=0.05 | WT=0.12 | WT=0.20 |
Bone Surface Skin |
Bladder Breast Liver Esophagus Thyroid Remainder |
Bone Marrow Colon Lung Stomach |
Gonads |
The radiation weighting factors vary from 1 for 20, with the
low-LET forms generally close to 1 and the high-LET forms higher since
the high spatial rate of energy deposition tends to have more
drastic biological effects. The values defined by statute are:
Radiation Weighting Facotrs for Calculation of Equivalent Dose
HT
| Radiation type and energy | WR | X rays, γ rays, electrons positrons, and muons | 1 |
| Neutrons < 10 keV | 5 |
| Neutrons, 10-100 keV | 10 |
| Neutrons, 100-2000 keV | 20 |
| Neutrons, 2-20 MeV | 10 |
| Neutrons, > 20 MeV | 5 |
| Non-recoil protons, > 2 MeV | 2 |
| α particles, fission fragments,
relativistic heavy ions | 20 |
Collective Dose: The effective dose to a population is called the collective dose. This is relevant primarily for stochastic effects, e.g. cancer. The underlying paradigm of these collective-dose measurements is, as Alpen says, that "if the probability of a certain cancer is 1/100,000 for a given dose, then this probability can be equally applied to an individual or to a cohort which has received this dose." As Alpen goes on to say, the real incidence will be governed by Poisson statistics. But in general we can multiply the probability of the biological effect by the size of the population to get an estimate of the number of individuals that are expected to experience the effect.
Alpen goes into substantial detail on specific radionuclides and other sources of ionizing radiation that are present in the natural environment. As we discussed in the last chapter, the most important source of exposure to humans is radon gas, specifically 222Rn derived from 238U as a step in the cascade that leads ultimately to 206Pb, which is stable. The reason that Rn is so important is that it is gaseous, so it can enter the lung without being adsorbed onto anything. Once it does enter its decay products stick to the alveolar surfaces and can cause damage. The effective dose per member of the population from 222Rn is on the order of 38-60 mSv/yr, which is a factor of five or more larger than any other natural source. This value varies substantially with the concentration of uranium in the soil and other environmental factors, so the dose may be even larger in some areas. It's dangerous to live in the Rocky Mountains, at least if you believe that 50 mSv/yr is hazardous!
The remainder of these notes will be posted soon.