RADIATION ISSUES :AN OVERVIEW
1.Deterministic and stochastic effects
Detreministic effects-------
- Deterministic effects occur at high dose levels, such as those given in radiotherapy treatments and are due to radiation-induced cell death.
- Deterministic effects are characterized by having a threshold dose below which the effect is not observed. The severity of the effect increases with dose and dose rate.Cataract formation and skin damage are examples of deterministic effects. Normally, in diagnostic procedures, doses are well below the threshold where deterministic effects are observed .
- The stochastic effects are of a random statistical nature and the probability of occurrence (but not the severity of the effect) is related to dose. In this effects, the probability of an effect occurring increases with dose up to a maximum, above which the curve flattens off .Cancers and genetic effects are such effects.
- The timing of the appearance of radiation-induced cancers varies, with a mean incidence for leukaemia at about 7 years postirradiation, about 5 years for thyroid and bone cancers and 20 or more years for most other cancers.
- In case of damage to the germ cells, genetic effects may occur in future offspring. To date, no hereditary effects have been demonstrated convincingly in humans; however, based on animal experiments, it is concluded that hereditary effects are a possibility.
2.Fetal irradiation
- Cell killing and cancer induction may occur as a result of in utero radiation and it is recognized that the fetus is highly radiosensitive during prenatal development.
- There are radiation-related risks throughout pregnancy, associated with the stage of pregnancy at which irradiation occurs and the fetal absorbed dose.
- Risks are most significant during organogenesis and the early fetal period, less in the second trimester and least in the third.
- Irradiation of a fetus can result in a reduction in IQ (if irradiation takes place 8–25 weeks postconception when the central nervous system is developing), an increased risk of cancer in later life, an increased incidence of congenital abnormality, or a genetic risk to the next generation.
- However, a recent ICRP report on pregnancy and medical irradiation concludes that, for most well-conducted diagnostic procedures, there is no measurable increase in risk over the background incidence of such effects.
- So.exposure of the fetus to diagnostic levels of radiation should not be a reason to terminate a pregnancy, but efforts should be made to minimize the irradiation of known pregnant or potentially pregnant women.
3.PRINCIPLES OF RADIATION PROTECTION
- The aim of radiation protection practice is to restrict radiation dose so that, with the exception of radiotherapy treatments, doses to staff, patients and the public remain below the level at which deterministic effects occur and the probability of stochastic effects occurring is limited to an acceptably low level.
- To achieve this aim, the ICRP recommends the application of three principles: justification, optimization and limitation.
- Justification implies that no practice resulting in exposure to ionizing radiation should be adopted unless it results in sufficient net benefit to exposed individuals or society to offset the detriment. The use of radiation in healthcare is a justified practice
- Optimization requires that the individual dose, the number of people exposed and the likelihood of inadvertent exposure should be kept as low as reasonably achievable (ALARA), economic and social factors being taken into account. In UK legislation this is translated into as low as reasonably practicable (ALARP).
- Limitation: The exposure of individuals should be subject to dose limits designed to ensure that no individual is exposed to an unacceptable radiation risk.
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Effective whole body dose (mSv)
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Individual organs or tissues (skin, hands, forearms,
feet, ankles) (mSv)
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Lens of eye limits (mSv)
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Employees aged 18 years and over
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20
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500
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150
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Trainees aged under 18 years
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6
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150
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45
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Any other persons
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1
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50
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15
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Pregnant staff
A. ECTIVE DOSES FOR COMMON RADIOLOGICAL
EXAMINATIONS EXPRESSED IN TERMS OF THE EQUIVALENT NUMBER OF CHEST X-RAYS AND
LENGTH OF EXPOSURE TO BACKGROUND RADIATION THAT WOULD GIVE THE SAME DOSE
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Examination
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Typical effective dose (mSv)
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Equivalent number of chest X-rays
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Equivalent length of background
exposure
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Limbs and joints (except hip)
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< 0.01
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< 0.5
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< 1.5 d
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Chest PA
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0.02
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1
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3 d
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Skull
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0.06
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3.0
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9 d
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Thoracic spine
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0.7
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35
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4 months
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Lumbar spine
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1
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50
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5 months
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Hip
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0.4
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20
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2 months
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Pelvis
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0.7
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35
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4 months
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Abdomen
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0.7
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35
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4 months
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IVU (intravenous urogram)
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2.4
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120
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14 months
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Barium swallow
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1.5
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75
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8 months
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Barium meal
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2.6
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130
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15 months
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Barium follow-through
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3
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15
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16 months
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Barium enema
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7.2
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360
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3.2 years
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CT head
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2.0
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100
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10 months
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CT chest
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8.8
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400
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3.6 years
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CT abdomen or pelvis
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10
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500
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4.5 years
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Lung ventilation 133Xe
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0.3
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15
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7 weeks
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Lung perfusion 99mTc
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1
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50
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6 months
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Kidney 99mTc
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1
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50
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6 months
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Thyroid 99mTc
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1
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50
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6 months
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Bone 99mTc
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4
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200
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1.8 years
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Dynamic cardiac 99mTc
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6
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300
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2.7 years
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PET head 18FDG
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5
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250
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2.3 years
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B.FETAL DOSES FROM A SELECTION OF COMMON DIAGNOSTIC PROCEDURES
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Procedure
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Mean fetal dose (mGy)
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Maximum fetal dose (mGy)
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AP abdomen
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1.4
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4.2
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Barium enema
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6.8
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24
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PA chest
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< 0.01
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< 0.01
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Pelvis
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1.1
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4
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CT abdomen
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8.0
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49
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Bone 99mTc
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3.3
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4.6
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Dynamic cardiac 99mTc
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3.4
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3.7
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GFR (EDTA) 51Cr
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<0.01
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<0.01
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