Accesibilidad académica: un concepto en construcción
Mito 2: “Las barreras a la accesibilidad en entornos virtuales se resuelven aplicando las normas de accesibilidad web tanto para los materiales como para los entornos
The heritable effects of radiation result from damage to the reproductive cells. This damage takes the form of alterations, known as genetic mutations, in the hereditary material of the cell.
It has already been mentioned that reproduction occurs when the ovum is fertilized by a sperm. As a result, the offspring receives a complete set of genetic material from each parent. Thus the child receives two complementary sets of genes, one from each of its parents. In general, it is found that one gene is ‘dominant’ and the other is ‘recessive’. The dominant gene determines the particular characteristic with which it is associated.
Recessive genes are only recognized when, by chance, two of the recessive-type genes come together. A considerable number of diseases are associated with recessive genes and will therefore manifest themselves only when both parents have the same recessive genes.
Spontaneous mutation accounts for the fact that an appreciable fraction of the world’s population suffers from 1 of the 500 or more defects or diseases attributable to heritable effects.
Radiation can induce gene mutations which are indistinguishable from naturally occurring mutations. It should be noted in passing that heat and chemicals can also cause mutations. Mutated genes can be either dominant, in which case their effects would manifest themselves in the first generation of offspring, or recessive, when the effect would not occur in the first generation. A recessive mutation will result in an effect only if the same mutation is inherited from both parents. It is generally assumed that all mutations are harmful, although this cannot be strictly true since man has attained his present advanced
Detriment 35
state via a series of mutations. However, this has occurred over an immense time span and the number of harmful mutations which have had to be eliminated from the species over this time is incalculably large.
Since ionizing radiation can cause an increase in the mutation rate, its use will increase the number of genetically abnormal people present in future generations. Clearly, the consequences of excessive genetic damage would be very serious indeed and strict control must be exercised over the radiation exposure of the general population.
The risks of heritable effects due to exposure of the gonads are very uncertain. Clearly, only that exposure which occurs up to the time of conception can affect the genetic characteristics of the offspring and, since the mean age of childbearing is about 30 years, only a proportion of the dose received by a typical population will be genetically harmful.
As such, the ICRP estimates (ICRP Publication 103) that the total risk of heritable disease, up to the second generation, averaged over both sexes and all ages is about 0.2 ¥ 10−2/Sv.
In a population of working age, because of the different age distribution, the risk is about 0.1 ¥ 10−2/Sv.
4.8 DETRIMENT
To assist in quantifying and combining the consequences of exposure of different organs and tissues of the body, the ICRP has developed the concept of detriment. This takes into account the relative risks and the average latency period of fatal cancers in different organs, an allowance for the ill health resulting from non-fatal cancers and for the risk of serious heritable effects in all future generations descended from an exposed individual. On this basis, the ICRP has provided estimates of what are termed detriment-adjusted nominal risk coefficients for exposure at low-dose rates, and these are shown in Table 4.1 for the population as a whole (i.e. including children) and for adults. It should be appreciated that these values are the result of calculations using data that have significant uncertainties and that, for most purposes, the use of a nominal risk coefficient of 5¥10−2/Sv is appropriate.
Table 4.1 Nominal risk coefficients for stochastic effects (10-2/Sv)
Cancer Heritable effects Total
Whole population 5.5 0.2 5.7
Adult population 4.1 0.1 4.2
In situations in which the exposure is not uniform over the body, it is necessary to know the relative contributions that individual organs make to this overall estimate of detriment, and these are shown in Table 4.2. The second column shows the probability of fatal cancer in each organ for an equivalent dose to that organ of 1 Sv. The third column gives the probability of severe heritable effects in future generations from an equivalent dose of 1 Sv to the gonads. The final column shows the relative contribution of each organ to the overall detriment, taking account of the factors discussed above.
These estimates of the relative contributions to the overall detriment from radiation exposure provide the basis for definition of the tissue weighting factors, wT, used to calculate the quantity effective dose, as discussed in Chapter 3 and further explained in section 6.3.2.
SUMMARY OF KEY POINTS
Physiology: study of functions of the body as a whole and component organs and systems.
Heart pumps blood to all parts of the body via the arteries and the veins.
Blood carries food nutrients and oxygen to cells and removes waste products.
Red blood cells transport food and oxygen.
White blood cells defend the body against infection.
Platelets are vital to the formation of clots.
Respiration: method by which oxygen is taken into the lungs and carbon dioxide is eliminated.
Digestive system converts food into a form suitable for the production of heat and energy and into molecules necessary for the growth and repair of tissues.
Stages in radiation damage process:
1. Initial physical stage (c. 10−16 s) consisting of ionization and excitation of atoms and molecules.
2. Physicochemical stage (10−8–10−5 s) consisting of dissociation of ions and formation of free radicals.
3. Chemical stage (a few seconds) consisting of the interaction of free radicals with other molecules in the body.
Table 4.2 Relative contribution of organs to total detriment (whole population) Organ or tissue Probability of fatal
cancer, 10-4 Sv-1
Probability of heritable effects, 10-4 Sv-1
Relative contribution to total detriment
Bladder 12.0 – 0.029
Bone marrow 28.0 – 0.107
Bone surface 3.2 – 0.009
Breast 33.0 – 0.139
Colon 31.3 – 0.083
Liver 28.9 – 0.046
Lung 101.5 – 0.157
Oesophagus 14.0 – 0.023
Ovary 6.0 – 0.017
Skin 2.0 – 0.007
Stomach 65.5 – 0.118
Thyroid 2.2 – 0.022
Other solid 70.5 – 0.198
Gonads – 20 0.044
Total 398 20 1.000
Revision questions 37
4. Biological stage (minutes to years) in which the chemical reactions show up as effects in individual cells.
Components of cell: nucleus, cytoplasm and outer membrane.
Nucleus contains chromosomes, which are thread-like structures made up of genes.
Genes carry the information which determines the characteristics of daughter cells.
Mitosis: the process by which single cells reproduce.
Meiosis: a stage in the formation of the reproductive cells – the sperm in the male and the ovum in the female.
Effects of radiation on cells: inhibition of mitosis, chromosome aberrations.
Acute effects: effects occurring within a few weeks of a very large exposure; owing to the depletion of cell populations.
Late effects: effects occurring at later times, typically some years after exposure; main effect is cancer induction.
Heritable effects may appear in descendants of exposed individuals.
Stochastic effects: the probability of occurrence depends on dose; mainly cancer and genetic effects.
Harmful tissue reactions: effects in tissues, the severity of which increases with dose, and for which a threshold may apply: mainly the early radiation effects plus certain late effects, such as cataract formation. Formerly known as deterministic effects.
Detriment: the harm from exposure to radiation, based on the probability of a stochastic effect weighted for lethality and life impairment.
Risk coefficient: the probability of a stochastic effect from a dose of 1 Sv. When the probability is weighted for the severity of the effect, it becomes a nominal risk coefficient.
REVISION QUESTIONS
1. Describe how radioactivity can be deposited in various organs of the body if it is (a) inhaled and (b) ingested (swallowed).
2. List the four stages in the radiation damage process.
3. Distinguish between harmful tissue reactions and stochastic effects of radiation.
4. What are the acute radiation effects? Discuss the severity of the effects over the dose range 1–10 Gy.
5. What is the major late effect of radiation and upon what assumptions are risk estimates based?
6. Using a nominal risk coefficient of 5 ¥ 10−2/Sv, calculate the risk from a dose of (a) 5 mSv and (b) 20 mSv.