Chapter 5 Radiation Risks Flashcards
direct action
Compton and PE electrons ionize molecules
indirect actions
compton/PE electrons interact with water to produce free hydroxyl radicals that are chemically reactive
how much damage is caused dy direct vs indirect action
2/3 by indirect action
1/3 by direct action
why are mammalian cells easier to kill than bacteria?
hey have more DNA
the more DNA = the easier to inactive biological system
bacteria easier to kill than viruses
dose required for sterilization
20,000 Gy
what is radiobiology
studies effects of ionizing radiation in cells and animal models
single strand vs double strand breaks
single strand is most likely repaired, double strand more likely to result in cell death, carcinogenesis, mutation
what can damaged somatic cells do?
Induce cancer
-takes years or decades to develop
what happens to cells at lower vs higher doses
lower doses: cells more likely to undergo modification
higher doses: cells more likely to be killed
what can damaged sperm and egg cells lead to?
hereditary effects
changes in genetic code of a germ cell can affect future generations
what does cell killing depend on?
LET of radiation
dose rate (higher = more kill), repair can occur when delivery is protracted
fractionation (reduces cell kill- helps protect normal tissues)
oygenation (2-3 X more sensitive than anoxic cells)
what does law of bergonie and tribondeau state?
highest sensitivities occur when cells are undifferentiated and have high mitotic rates
therefore rapidly proliferating cells (ex bone marrow stem cells) are sensitive whereas highly differentiated and non-proliferating cells (ex neurons) are least sensitive
are peripheral lymphocites sensitive to radiation?
YES
even though they are differentiated and don’t divide
(they are an exception)
why are oxygenated cells more sensitive than anoxic cells?
oxygen prolongs lifetime of free radicals, promoting bond breaking
deterministic effects
have a treshold dose
-for doses below treshold, effect won’t occur, for doses much above, effects are expected to occur in all exposed individuals
severity may increase with increasing dose
examples of deterministic effects
skin burns
epilation
eye cataracts
sterility
stochastic effects
have no treshold dose
-pertain to carcinogenesis and induction of hereditary effects in offspring of exposed individuals
-severity is independent of radiation dose
-dose only affects probability of effect occurring
lethal uniform whole body dose
5 Gy
how do you triage the severity of acute radiation exposures?
-peripheral lymphocyte count
-immediate diarrhea, fever, hypotension
what whole body dose sterilizes stem cells
2 Gy
reduces circulating blood elements within 2-3 weeks
hematopoietic syndrome
what is LD50
uniform whole body dose that kills 50% of population
3-4 Gy without medical intervention
-at whole body doses above 8 Gy, absence of immune system means survival is very unlikely
what would whole body doses of > 10 Gy do?
lilely kill everyone in 5-10 days due to loss of epithelial lining of GI tract (GI syndrome)
what would whole body dose of 100 Gy do
kill everyone in 1-2 days from permeability changes in brain blood vessels (cerebrovascular syndrome)
peak skin dose
-usually occurs where radiation enters patient
-used to predict likelihood of skin burns
is exposed skin area taken into account when predicting burns?
No
time scales for onset of skin deterministic effects
prompt: < 2 weeks
early: 2-8 weeks
mid term: 6-52 weeks
long term: > 40 weeks
peak skin dose levels with effects
< 2 Gy : no effects
2-5 Gy: erythema possible (prompt or early)
5-10 Gy: for sure erythema (prompt or early), prolonged erythema may occur mid-term, with dermal atrophy or induration long term
10-15 Gy: prompt erythema + early dry/moist desquamation; mid-term erythema, long term dermal atrophy, induration, telangiectasia, weak skin
> 15 Gy: prompt erythema, edema, acute ulceration, mid term dermal atrophy, secondary ulceration, dermal necrosis; long term includes telangiactasia, dermal strophy or induration, skin breakdown likely requiring surgery
national cancer institute categorization of radiation-induced skin injuries
grade 1, 2-10 Gy, faint to moderate erythema
grade 2, 5-15 Gy, erythema
grade 3, > 10 Gy, moist desquamation in areas other than skin folds and creases
grade 4, > 15 Gy, skin necrosis or ulceration of full-thickness dermis
epilation
occurs temporarily for scalp dose of 3-5 Gy after 2-3 weeks
(regrowth of hair starts 2 months after irradiation, may be gray)
for dose > 7 Gy epilation likely permanent
what are cataracts
opacification of eye lens normally transparent
eye lens has no way of removing dead or damaged cells
-cataracts caused by radiation migrate to posterior pole of lens
treshold dose for cataracts
per up to 2011:
2 Gy for acute
5 Gy for chronic
now ICRP considers treshold to be 0.5 Gy for acute and chronic
latency period of cataracts
several years after eye lens doses of a few Gy
-as doses increases, latency periods may get shorter
dose tresholds for sterility
0.2 Gy- diminished sperm count
0.5 Gy- azoospermia (temporary sterility, recovery can be 3 years)
6 Gy- permanent sterility
is acute or fractionated exposure in gonads worse in males?
fractionated
permanent sterility can result from 3 Gt fractionated over a few weeks
doses for sterility in females
depend on age
10 Gy in pre-puberty
2 Gy in premenopausal women
what is main concern following exposure to radiation below the treshold of deterministic effects?
carcinogenesis
how are links between exposure and ionizing radiation established?
what do epidemiologic studies require?
-large cohorts
-control groups
-long follow-up periods
who is usually studied for epidemiological studies?
-A bomb survivors
-radiation workers
-patients exposed to high doses from therapeutic x-rays
what do excess cancer deaths depend on?
dose
age at exposure
time since exposure
gender
where have excess cancers been observed for high dose?
-lung cancer in miners
-bone sarcomas and nasopharynx cancers in dial painters
-skin cancers in radiologists, dentists, when safety was lax
-cancer following radiotherapy for lymphoma, breast cancer
-secondary cancers following radiation therapy for chilhood cancer
-breast cancer in women who underwent fractionated fluoroscopy
where have excess cancers been observed for high dose?
-thyroid cancer in kids treated for tinea capitis
-10 mGy to uterus= childhood cancer
-cancer in bomb survivors exposed to only 30 mGy
-leukemia and brain tumours in children who underwent CT
-
for whom is thyroid cancer more likely?
women, children
what organs are most susceptible to radiation-induced malignancy
bone marrow, colon, lung, breast, stomach, childhood thyroid
moderately radiosensitive organs
bladder, liver, esophagus
what is latency
time between irradiation and appearance of malignancy
latency period for leukemia
few years
at least 2 years
latency period for solid tumours
decades
at least 5 years
what model is used for cancer risk estimates in diagnostic imaging?
linear no treshold model
beneficial effect of radiation
hormesis
not accepted as an excuse for radiating someone
what is BEIR
Biologic Effects of Ionizing Radiation
US comittee that provides detail on radiation risks
-similar risk estimated to UNSCEAR and ICRP
what is UNSCEAR
UN Scientific committee on effects of atomic radiation - similar risk estimates to BEIR and ICRP
what is ICRP
International Commission on Radiological Protection
-similar risk estimates to BEIR and UNSCEAR
are males or females more radiosensitive?
females, by 70 %
-breast and lung cancer account for these differences
newborns are how much more radiosensitive than 25 year olds?
3 X
seniors are 3 X less sensitive than 25 year olds
how much are we likely over or underestimating radiation risks?
2-3 X
what is the risk of cancer for an american who isn’t exposed to radiation
40 %
how do hereditary effects occur?
-irradiation of germ cells involved in reproduction
-stochastic process with no treshold
-radiation increases incidence of mutations that occur spontaneously
is there epidemiological evidence of hereditary effects in exposed humans?
No
-children of A bomb survivors did not show effects
-all data comes from animal studies
types of hereditary effects
mendelian
chromosomal
multifactorial
ICRP hereditary risk
0.2%/Gy to the 2nd generation for the whole population
0.1%/Gy for the working population since this excludes children
what percent of spontaneous mutations in humans is due to natural background radiation?
a few percent
what is doubling dose
-absorbed dose to gonads of whole population that would double the spontaneous mutation incidence
1-2 Gy
hereditary effects account for what % of total detriment when populations are irradiated
8 %
i.e. Wt of 0.08 for gonads
how is gestation in humans divided?
pre-implantation (conception to 9 days)
organogenesis (10 days to 6 weeks)
fetal (6 weeks to term)
what can happen to fetus given exposure to radiation?
nothing if exposed before fertilization
-radiation can result in death to fetus
-congenital malformations if exposed during organogenesis
-can get growth retardation and mental retardation
-depends on gestational age and amount of radiation received
what is background incidence of congenital abnormalities in US in absence of any radiation
5 %
who offers advce on deterministic risks at different levels of radiation exposure?
ACR practice guideline for imaging pregnant women
deterministic risk to fetus for dose < 50 mGy
-no risk
deterministic risk to fetus for dose 50-100 mGy
-scientifically uncertain
-may be too subtle to be detected
deterministic risk to fetus for dose > 100 mGy
-deterministic effect may occur
-embryo death during pre-implantation
-major malformations during organogenesis
-reduced head size and mental impairment most likely during 8-15 week gestational age, with much lower risk for 16-25 weeks
dose for risk of carcinogenesis in the fetus
10 mGy, especially during 3rd trimester
plot of deterministic effect and childhood cancer incidence as a function of dose
for deterministic, x goes from 100-10,000 mGy, risk goes from 0 (100) to 50 (1,000) to 100 (10,000) in a sigmoid shape
for chilhood cancer, x goes from 0 to 100 mGy
risk increases as straight lie from 0.2 (at 0 mGy) to 1 (at 100 mGy)
0.2 % at 0 mGy is background childhood cancer incidence
what has been used for radiation protection purposes regarding carcinogenesis due to exposure of fetus
-for dose of 25 mGy, risk of 0.2% of childhood cancer
-likely reasonable during 3rd trimester but conservative during 1st and 2nd trimester
therefore, a scan that delivers 25 mGy to fetus doubles the likelihood of childhood cancer since background incidence is 0.2 %
when is radiation induced erythema likely to be observed after skin is exposed?
10 days
most lilely to occur for peak skin dose of 5 Gy or more
for a population exposed to uniform whole-body doses, genetic defects are currently estimated to contribute what % to the total detriment?
8% because wt = 0.08
