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78 Cards in this Set
- Front
- Back
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Cataractogenesis
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Another late effect of radiations
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Cataract
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describes any detectable change of the normally transparent lens of the eye
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The lens of the eye functions like a camera lens. It is a self-renewing system with cell divisions continuing throughout life.
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However, there is no blood supply and no system for removal of dead or damaged cells
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There is a transparent layer of epithelial cells on the interior side of the capsule that covers the lens.
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The layer maintains the function of the lens by slowly growing toward the center, achieved through cell division at the periphery ( called the equator) of the lens.
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New cells are produce in the
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lens of the eye
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Because radiation is especially harmful to dividing cells,
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exposed cells at the equator are most prone to damage. For unknown reasons, damaged cells move toward the back or rear of the lens before converging on the center.
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Such cells at the back of the lens or rear;
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prevent light from traveling straight forward, resulting in opacity ( cloudiness)
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15 rem/yr eye limit
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1.5 rem/yr ALARA
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Radiation can cause cataract formation presenting as a partial opacity (cloudiness) in the
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crystalline lens.
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Symptoms are usually observed after months of latency;
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two or three years on average of radiation exposure
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The dead cells moves to the
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posterior pole of the eye
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Senile cataract looks different then
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radiation induced cataracts
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Unlike senile cataracts, a condition common of old age
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few radiations cataracts advance, and visual impairment is infrequent
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Latent Period
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time between irradiation and formation of the cataract. May be as long as 30 years.
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Decreased latent period
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with increased dose
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Threshold Effect:
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At least 200 rads acute exposure is required to cause formation of a cataract.
- Need a certain amount |
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Cataract Formation is Dose Dependent
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At 250 to 650 rads, patients develop cataracts in about 8 years
- These cataracts are usually stationary |
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About 650 to 1150 rads,
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the latent period is about 4 years and some cataracts become progressively worse.
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250 to 650 rads
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patients develop cataracts in about 8 years.
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Radiation induced cataracts can be distinguished from other cataracts.
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This is unlike other health effects of radiation, like cancer, which are indistinguishable from cancers caused by other initiators.
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Radiation induced cancers begin as a dot usually at the posterior pole. As the cataract enlarges it may resemble a donut with a clear area in the center.
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If the cataract continues to worsen it may become indistinguishable from other types of cataracts.
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Diagnostic Radiology & Nuclear Medicine: Benefit vs. Risk
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In an attempt to determine the health effect of radiation on the population, several different units which describe dose have been developed. These are known as dose descriptors
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Dose equivalent:
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Is the absorbed does times a quality factor (Q) If the absorbed dose is in rads, the dose equivalent is in Rem. If the absorbed dose is in Gray, the dose equivalent is in Sievert
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In general, Q for X and gamma radiation is 1.0,
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Q for high energy neutrons is 20, and Q for alpha radiation is 20. There is no unit attached to quality factor
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Effective Dose
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The weighted sum of the dose equivalent for each body tissue. This unit attempts to account for the different radio sensitivities of tissues in the body
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Genetically Significant Dose (GSD)
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The equivalent dose to the gonads weighted for the age and sex distribution of the expected population.
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An index of the presumed genetic impact on the whole population.
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It attempt to average the effect of radiation on the whole population.
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Also, attempts to account for those people exposed who will actually bear children
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GSD is very age and sex dependent
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Collective Effective Dose:
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Sum of the product of the effective dose and the number of persons exposed
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Per person dose from diagnostic procedures for the US is about
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130mrem. THIS IS AN AVERAGE
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Not an even distribution based on age. Older people received more radiation
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due to increased health problems.
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Background Radiation
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Composed of radiation from.......
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Cosmic radiation: charged particles and photons from outer space. ( Approx. 26 mrems/yr at sea level, about 50 mrems/yr in Denver)
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from out of space
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Earth's crust; naturally occurring radioactive materials in the soil, and rocks
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contribute to background dose. Varies with location
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Internal exposure
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caused by radioactive material from foods and water which we ingest, Largest contribution is Potassium-40
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Radon
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-Largest component
- Radon daughters are dangerous |
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Varies widely with location and type of house.
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On the average, radon makes the largest contribution to the background radiation humans receive.
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The bronchial epithelium receives about 500 mrem/yr from radon and its daughter products.
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Since only the lung is irradiated it is multiplied by a weighting factor.
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Man made radiation, medical X-rays, CRT's, nuclear fuel cycle, radioactive waste, air travel, fallout
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from weapons testing, consumer products.
- Medical use is the largest category. |
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Doses from diagnostic imaging; X-Ray & nuclear procedures
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make up the largest fraction of man made background dose.
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Average Annual Dose From Background Radiation =
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360 mrem or 3.6 mSv
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In the U.S, average exposure of radon =
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200mrem
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Exposure from food & water
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40 mrem
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Few products available to the public contain radioactive material. One of the most common is the smoke detector which contains a small amount of Americium-241.
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The dose to the public from a smoke detector is essentially zero, because such a small amount is used and the energy if emission is low.
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Am-241 is an alpha emitter and a low energy gamma emitter. The alpha emission causes air in a chamber within
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the smoke detector to be ionized generating a very low current. If smoke enters the chamber, it interrupts this current setting off the alarm.
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High Natural background areas
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No definite increase in cancer or genetic mutations in areas with high natural background.
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This may be confounded by the small populations living in high background areas and their life style.
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Approximate Exposure from Natural Sources Different US Locations
- See P.P - No specifics |
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Ok really let's go
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Benefit vs. Risk
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First of all, some independent studies estimate that up to 1/3 of all X-rays are performed to
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generate income or as a defense for legal practices.
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If we compare the Average GSD (Genetically Significant Dose 30mrem) for diagnostic radiography with the
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genetic doubling dose 100rem, ( no specifics) there is no genetic effects.
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(All at once) Another way of looking at risk from DI, is to compare the average background radiation of 360mrem with the dose the patient receives.
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This is based on the absent increase in negative health effects observed in people living in high natural areas. (Getting gradually)
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US population exposed to diagnostic X-rays based on risk estimates, we may see in 1 year
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2320 fatal cancers
464 non fatal cancers 464 serious heritable defects |
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This risk is small when we think of the benefit of quick, accurate diagnostic examinations
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which improve health and save lives.
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Isn't it ironic that there are paradoxes like mammography, it's used as a screening procedure to detect small cancers.
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The majority of women receiving mammograms have negative results so they received a radiation dose with no real benefit. It could turn our that screening mammography induces cancers than it detects!
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Dental X-Rays can expose the thyroid which is very radiosensitive,
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Make sure they cover your thyroid when they take an X-Ray, a full mouth series can add up tp 5 rads to the oral cavity
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Risk to the fetus from DI
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In short, the risk is low and therapeutic abortions are extremely rare, not even considered unless the dose exceeds 10 rem to the fetus
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10-12 million nuclear medicine procedures performed each year
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Only about 60,00 of these are therapeutic procedures.
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Critical organ is different then
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target organ
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Of interest for risk assessment are....
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1. The total body dose, since this will determine the risk of leukemia
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2. The dose to the critical organ, since this may be many times larger then the total body dose and radio sensitivity varies with the tissue type. (Critical organ: organ with greatest concentration of RM)
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3. The gonadal dose since this is an indication of the genetic hazard.
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Nuclear Medicine dosimetry
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dose measurement
Depends on...... |
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1. The distribution of the radiopharmaceutical and its uptake in the critical organs
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2.Inhomogeneous distribution even within the critical organ
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3. The biological h 1/2 of the nuclide, which may vary with the patients age and condition.
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( How long does it stay in body)
bone scan = Lower biological half life vs. higher physical half life. - You usually secrete it before it physically decays |
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Nuclear Medicine
Specific risk from nuclear medicine procedures |
For the total US population exposed to NM procedures, based on risk estimates, we may see in 1 year.
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540 fatal cancers
108 non fatal 108 serious heritable defects |
These health effects must be weighed against the benefit that patients receive from greater than 10 million exams performed annually in the country.
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According to the NRC, the Average Dose Equivalent to the US population in NM is about 14mrems/year and
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about 39mrems year for diagnostic imaging X-rays
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As we have studied, children are more sensitive to radiation because of rapid cell growth. Their long life expectancy also gives them more time to develop cancer.
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A study of children treated for tinea capitis with X-Rays showed an increase in thyroid cancers. The children's thyroids received about 6 rads.
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It is a good practice to limit pediatric cases in NM.
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This is largely up to the radiologist and referring M.D. Doses should also be reduced according to weight in children.
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It is also good practice to interrupt
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breast feeding after having a NM procedure.
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For studies done with Tc-99m, 60 hours is more then enough to reduce
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the infants dose to a negligible amount
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Other isotopes may require an interruption long enough that breast feeding
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will be terminated.
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In Utero risk from NM
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Many factors effect the dose to the fetus from radiopharmaceutical including
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The critical organ
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the age of the fetus
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if the radiopharmaceutical crosses the placenta
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biological half life
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The fetal thyroid takes up iodine from the 10th week onward.
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It is wisest to avoid any procedures in pregnant women which uses radio-iodine
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You generally don't do any procedures on
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pregnant women.
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Genetic Risk
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The genetic risk on the US population is low from NM procedures.
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This is because of the relatively small fraction of the population that have a NM exam.
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The genetic burden from NM exams is also low because not many young people who may bear children have the procedures.
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