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27 Cards in this Set
- Front
- Back
- 3rd side (hint)
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What factors determine the amount of biological damage from radiation?
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– Type of radiation
– Dose – Quality of the Radiation – Doserate (fractional or continuous) – Conditions under which it was delivered |
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Define deterministic radiation
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– The severity of the effect increases with dose
– There is a threshold below which no effect is seen – A much higher dose is required to cause the effect – Less likely to result from diagnostic radiation with the exception of fluoro or angio – At very high dose, acute radiation syndrome can occur – Example = cataracts, erythema, fibrosis, sterility, acute radiation sickness and hematopoietic damage |
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Graph deterministic radiation
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Define stochastic radiation
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– Probability of the effect increases with dose
– Do not have a threshold so even low dose exposures carry risk – Chances of these occurring increase with dose but there will be no difference in the severity – Injury to one cell or many cells could result in disease – Example = Hereditary effects, leukaemia, radiation induced cancer and genetic effect – Limitstochastic effects = ALARA, limitation via limiting repeats and dose |
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Graph stochastic radiation
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Define LNT
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– Linear non–threshold model
– Attempts to describe the risks from radiation at low and high doses – There is no safe level of radiation dose – The data from this model has been taken from high dose events such as Hiroshima and Nagasaki and nuclear accidents as well as test on animals and extrapolated for lower doses |
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Graph the LNT model
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What are modes of action?
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– All biological systems are capable of being damaged by ionizing radiation, the damage can be the result of a direct or indirect action
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What is direct action?
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– Interactions where ionization or excitation of a macro–molecules in the cell occurs and damages DNA, RNA or proteins directly
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What is the target theory?
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– Radiosensitive targets are located in the nucleus of the cell and not the cytoplasm
– DNA is a key molecule in our cells and is essential to the survival of the cell – Inactivation of the master molecule results in the death of the cell – If several molecules in the cell are damaged but they are all non DNA molecules then the cell will most likely continue to function |
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What is indirect action?
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– Damage caused in the cell by chemical by–products of ionising radiation passing through tissue
– Occurs in the cytoplasm where interacting with water molecules generates cytotoxic substances (free radicals and reactive oxygen species) – 70% of ionizing radiation damage is caused by indirect action – Primary method of indirect action is radiolysis – Example = fibroblastic changes in breast tissue |
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Describe radiolysis
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– As ionizing radiation comes into contact with the water molecules radiolysis of the water molecule occurs
– Water is abundant in all cells and so is the most likely molecule to be hit by the photon – Cytotoxic substances of solvated electrons and free radicals that are produced during radiolysis – Cytotoxic substances can diffuse through the cell exerting damage at short distances – The main reactions of concern are with DNA and protein |
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Describe free radicals and hydrogen peroxide formation
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– Hyperoxyl free radical and hydrogen peroxide are the primary cause of biological damage as a result of radiolysis
– When water is ionised during radiolysis free radicals form (free radical = **) – H20+g H+ and OH** or |
H20– g H** and OH
– They are unstable so reform to make water H20 or sometimes hydrogen peroxide H2O2 which is cytotoxic – The abundance of oxygen enhances the effect, producing more free radicals |
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Describe the bystander theory
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– Phenomenon in which unirradiated cells exhibit irradiated effects as a result of signals received from near by irradiated cells
– Damage occurs in the cytoplasm not the nucleus however can result in the nucleus of the targeted cell – Damage is from cytotoxic substances produced in the cytoplasm due to indirect effects |
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Describe laws of radio–sensitivity
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– Type of cell
– Age of cell (mature cells have a lower metabolic rate) – Stage of cell cycle – Rate of cell division – High metabolic rate, higher radiosensitivity – Increased cell maturity decreased radiosensitivity – Younger cells and tissues more radiosensitive – Increased mitotic rate, increased growth rate, increased radiosensitivity |
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Describe factors effecting cellular response to radiation
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– Totaldose
– Rateat which dose is delivered – Oxygenationof the cell – Age (youngand old more sensitive) – Qualityof radiation – LET – Typeof radiation |
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Describe risk of exposure to foetus
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– The risk is negligible for <1 mGy to foetus
– Risks relate to the stage of pregnancy and absorbeddose – Most significant risk is during organogenesis andearly foetal period – Less risk in 2nd trimester and least risk in 3rd trimester |
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What is a tetrogen?
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– Causes malformation of an embryo
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Describe conception timeline
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– Ovulation = 8–20 days, 14 days after LMP
– Implantation= 9 days after fertilization – Pre–implantation= 0–8 days – Implantation= 9–14 days – Organogenesis= 15–50 days – FoetalGrowth = 50–280 days (2nd and 3rd trimester) – First Trimester = weeks 1–13 – Second Trimester = weeks 14–27 – Third Trimester = weeks 28–40 |
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Pre–conception irradiation risks
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– Irradiation of gonads = no increase risk of cancer or malformations in children
– high dose levels = increase rate of prenatal death – Reproductive organs exposed = hereditary diseases or abnormalities passed on to future generations – Although never been seen in humans |
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Pre–implantation irradiation risks
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– Low risk = dose less than 100mGy (3 CT scans or 20 x–rays)
– These levels can be reached with fluoro pelvis procedures or radiotherapy (10–100mGy) – All–or–nothing response = spontaneous abortion or complete repair |
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Malformations in babies due to irradiation
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– Threshold = 100–200 mGy or higher
– Risk during week 3–10 – Practically will not meet threshold but protection still essential during 1stand 2nd trimester – Usually central nervous system problems, cleft palate and blindness – Cleft palate most common |
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Neuropathy due to irradiation
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– Threshold = 100 –200 mGy or higher
– Risk during week 8–25 and late pregnancy – 100 mGy = reduction of IQ – 1000 mGy = severe mental retardation, microcephaly andspina bifida |
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Leukaemia and cancer due to irradiation
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– Risk of cancer increases with foetal age
– Dose of 10 mGy = risk of induction by 1.4 |
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How can foetal dose be reduced when imaging?
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– Use a non–ionising method if possible
– Double gown in lead – Negotiate with doctor on views – Collimation, shielding – Highkv, maximum distance – If the embryo/foetus has been irradiated exceeding 5 mSv notify physicist |
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What are the effects of high doses on the foetus?
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– Doses of 100–1000 mGy = late pregnancy are not likely to result in malformations since all organs have been formed, risk of radiation induced cancer
– Decisions to terminate based on dose, magnitude, radiation type and stage of pregnancy |
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Factors effecting nuclear medicine and foetal irradiation?
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– short–lived radionuclides that do not cause large foetal doses used
– dose reduced through maternal hydration and voiding – radionuclides can cross the placenta and pose risks such as hypothyroidism – Radionuclides are excreted in breast milk so breast feeding is suspended depending on nuclide – For I131 breast feeding is completely suspended |
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