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Message: <blockquote data-quote="manji" data-source="post: 1412474" data-attributes="member: 12462">From the lecture we received in 2nd year medicine ....====================EFFECTS OF RADIATION The effects of radiation are out of all proportion to the amount of energy deposited. For example a single dose of 10 gray given to the whole body will kill almost anyone who receives it, unless they are given a bone marrow transplant. However 10 gray deposits only 10 joule per kilogram of tissue (1 gray = 1 joule per kilogram). You should easily be able to calculate that 10 joule per kilogram represents a temperature rise of only 0.003 °C (remember that the body's specific heat is 3500 joule kg-1 K-1 ). Note that the heat in a cup of coffee if distributed throughout your body would raise your temperature by ten times this amount. Why is radiation so damaging? The main reason is that it produces ionisation of material that it passes through. Thus chemical changes may occur which are extremely harmful to the body. As an example, ionisation can convert extremely toxic and dangerous materials to safe ions. Na and Cl2 are extremely toxic materials however if we take a single electron from atomic Na and add one to atomic Cl we get Na+ and Cl- which are the ions present in salty water. The reverse may also occur in the body, thus superoxide (O2-) is highly toxic to body tissues although oxygen is regarded as being relatively harmless. In fact superoxide is so toxic that every single cell in your body has an enzyme called superoxide dismutase that acts to destroy superoxide about as fast as is theoretically possible. Consequently when tissue is irradiated, chemical changes occur which disrupt the normal function of the cells. This can lead to sickness or death if the damage is too high or to cancer and genetic mutations at lower levels of radiation. Radiation at sufficiently high levels may cause sickness or death within a short time after the irradiation. A dose of 2 gray will cause sickness in a substantial portion of the population (25%), while a dose of 10 gray will cause eventual death for most. Exposure over a long time to relatively low doses may result in the induction of cancer. In fact this and genetic damage are the only proven effects of very low doses of radiation. Cancer is the most likely consequence of low dose radiation and it is believed that any dose of radiation no matter how small will have a probability of inducing cancer. There is usually a delay of 5 to 10 years between the exposure and the induction of cancer. One gray of radiation given to the whole body will give a probability of cancer induction of about 4% for adults and perhaps up to twice that for children. This risk is additive so that multiple small doses over a number of years will give the same risk as from the total dose given on a single occasion (provided that the total dose and dose rate are not too high). There is also a genetic risk associated with radiation exposure. This may increase the incidence of genetic defects in the progeny of the irradiated individual. The probability of such defects is approximately 1% for 1 gray of irradiation. Note that there are a number of other interventions which may increase the incidence of genetic defects, for example, surgery undertaken to save the life of those with genetic abnormalities. These effects are summarised in Table 1. Table 1 Effects of Radiation High dose Sickness Death Organ destruction Low dose Cancer 4% per Gy Genetic effects 1% per Gy USE OF RADIATION IN MEDICINE Radiation is commonly used in medicine, in fact medical exposure is by far the largest man-made source of radiation. It far exceeds all other sources of exposure such as nuclear reactors, nuclear explosions or the industrial use of radiation. In Australia the average radiation dose from medical x-rays is about 0.8 mSv per person. Hence it is of the same order as the natural radiation dose. The largest contributor to this medical irradiation is now the use of CT scans which accounts for 45% of the total in Australia. It has been estimated that 250 people die from cancer each year in Australia as a result of exposure to the x-rays in CT scanning. Of course we can never prove that this is true because the total number of deaths from cancer is so much larger. Examples of the use of radiation in medicine are:   PROCEDURE EFFECTIVE DOSE (mSv*) Simple chest x-ray 0.05 Abdominal x-ray 1.0 Pelvis x-ray  0.7 CT scan        body        head 7 - 15 1 Angiography 5 -10 Nuclear medicine 2-10 Therapy 20-50 Gy (treated volume) OTHER SOURCES OF RADIATION The first point to note however, is that mankind has been subject to natural radiation throughout the whole of history. The earth is radioactive and so is each and every person on it. The natural radiation levels in Perth are given in Table 2. Table 2 Natural background radiation SOURCE DOSE (mSv) Terrestrial (232Th, 238U) 0.6 Cosmic rays 0.3 Radioactivity in the body         Radon 0.2       Th/U 0.2         40K 0.2 TOTAL 1.5 There are parts of the earth where the radiation level is quite high either due to the radioactivity of the earth in that environment or due to the increased cosmic radiation which occurs at altitude (see Table 3). On the Darling scarp there are areas where the natural radiation level is up to 5 mGy per annum. The beach at Minninup is capable of giving a dose near to the maximum level that is regarded as acceptable for radiation workers (20 mGy), if you were to sit on it for the whole year. If you fly in a jet aircraft the radiation level rises to about 5 mGy per hour. Thus a round trip to Europe represents an additional radiation dose of about 0.2 mGy. Concorde will give an even higher dose rate. Table 3. Areas of high natural dose Location Dose rate (mGy / year) Brazil (monazite) 5-10  Brazil (volcanic) 16-120 Kerala in India (monazite) 13 South Pacific (volcanic) 10 Nile delta (monazite) 3-4 France (granite) 2-4 Minninup (mineral sands) 2-16 OTHER RISKS The second point to note is that most common activities are not totally safe (see Table 4.). The obvious example is driving on the roads where the risk of death is approximately 1.3 in 10,000 per year, the risk of serious injury is 10 times higher. Of course this risk can be minimised by taking care etc, however some risk is always present. Likewise, most work places result in risks, as do most other activities. Thus although radiation is dangerous and it should only be used when benefit will result to the patient, there are many other activities which also introduce hazard. In particular most medical interventions have a significant risk associated with them. For example coronary angioplasty has a risk of death of approximately 0.5% per procedure. Thus we have to balance the risk against the benefit. Table 4. Activities with a risk of death of 1 in 1 million Travelling 1000 km by air Travelling 150 km by car Smoking 3/4 cigarette 2 minutes rock climbing 1 week at work (low risk occupations) 20 minutes being a 60 year old Whole body dose of 25 mGy</blockquote>

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