Radiation Biology Flashcards
What is “exposure”?
Ability of x-rays to ionize air, measured in Roentgens (R).
The concentration, in air, of radiation at a specific point - and is the ionization produced in a specific volume of air.
Unit of exposure?
Roentgens (R)
What is “Absorbed Radiation Dose” or “Radiation Dose”?
Amount of energy absorbed per unit mass at a specific point.
Measured in Gy or Rads (1 Gy = 100 rads).
How much energy from ionizing radiation has been absorbed in a small volume.
Units of absorbed radiation dose?
Gy or Rads
1 Gy = 100 Rads
What is Equivalent Dose?
Absorbed dose of different types of radiation creates different levels of biologic damage.
Weighting factor is used to adjust the value - alpha particle does more damage than electron
EqD = Dose x Weighting factor
Units for equivalent dose?
Sieverts
What is Effective Dose?
Takes into account whether radiation has been absorbed by the specific tissue.
Taking into account the type of radiation and the variable sensitivity of the organ/body part. Use a “tissue weighting” conversion factor.
If all the dose is absorbed, then 1 Gy = 1 Sv.
EfD = EqD x Tissue factor
Units for Effective Dose?
Sievert
Units for Exposure, Absorbed Dose, Equivalent Dose, and Effective Dose?
Exposure = Roentgen
Absorbed Radiation Dose = Gray or Rads (1 Gy = 100 Rads)
Equivalent and Effective Dose = Sievert
What does KERMA stand for?
Kinetic energy released per unit mass
What is Kerma?
A measurement to help estimate how much PRIMARY energy actually gets transferred when you expose a person to x-rays (or any type of radiation).
Primary radiation, not scatter.
Kerma is based on photon energy:
Low energy - Kerma is going to be the same as the absorbed dose - all the dose gets absorbed - photoelectric domination
High energy- Kerma is going to be more than absorbed dose - going to have photons shooting right through - not contributing to dose - but still counted as Kerma.
What is Air Kerma?
Estimating Kerma in air prior to it interacting with tissue as a “pure” measurement of energy.
When it interacts with the body, can’t separate all the secondary stuff, plus have trouble estimating the kinetic energy lost as heat.
What is Kerma based on?
photon energy
Low energy - Kerma is going to be the same as the absorbed dose - all the dose gets absorbed - photoelectric domination
High energy- Kerma is going to be more than absorbed dose - going to have photons shooting right through - not contributing to dose - but still counted as Kerma.
Difference in Kerma for low and high energy photons?
Low energy - Kerma is going to be the same as the absorbed dose - all the dose gets absorbed - photoelectric domination
High energy- Kerma is going to be more than absorbed dose - going to have photons shooting right through - not contributing to dose - but still counted as Kerma.
How does Kerma compare to tissue dose?
Kerma is estimated dose
Tissue doses are higher than air kerma - usually around 10%.
X-ray interaction with tissue is going to create scatter and secondary electrons which will contribute to dose, but by definition, won’t be included in kerma.
What is “Kerma-Area Product (KAP)”?
Also called Dose Area Product
Amount of Kerma (potential dose) multiplied by the cross sectional area of the x-ray beam.
Better way to measure the total radiation potentially incident on the patient - Measurement of total radiation used in the exam, more than the actual dose in the patient.
What will happen if you increase the kVp but concentrated it in a smaller area what would happen to Kerma-Area Product?
Not change KAP
Is Kerma-Area Product dependent on distance?
No.
It is the “whole beam” - independent of the source distance. Can measure it at any point along its path and it will be the same.
Farther away from the source the dose decreases via inverse square law, but the distance increases via divergence also by the square - cancels out.
How does distance affect entrance skin Kerma and Kerma Area Product?
Closer will increase geometric magnification.
Will increase entrance skin kerma (greater risk of burn), but NOT change KAP.
What does collimation do to Kerma Area Product?
Decreases it by decreasing cross sectional area.
What does collimation do to entrance air kerma?
Increases entrance air kerma b/c the automatic brightness monitor is going to call for an INCREASE in juice.
What are Kerma Area Product and Air Kerma good indicators for?
KAP is good indicator for stochastic risk
Air Kerma is good for deterministic risk
What does Kerma Area Product represent?
Total energy incident on the patient
Does NOT indicate the risk of skin burns.
What is Deterministic Risk?
Cause and effect - there is a threshold.
I get “x-amount” or radiation, I get ____. Cross threshold.
More dose after threshold = more severe that thing gets.
What is Stochastic Risk?
“Shit happens”
No-threshold, by chance model. Don’t know if you need a little or a lot of dose, but there is a relationship between dose and chance. More dose = more likely, but there isn’t a threshold.
Chance based and follow a Linear Quadratic No Threshold Model - risk increases with dose but doesn’t automatically result in a bad outcome.
What model does Stochastic risk follow?
Chance based and follow a Linear Quadratic No Threshold Model - risk increases with dose but doesn’t automatically result in a bad outcome.
Differences between Deterministic and Stochastic Effects?
Deterministic - Has a threshold; Severity is dose related; Does Not include cancer risk
Stochastic “Random” - Has NO threshold; Severity is NOT dose related; Probability of effect increases with dose; Includes heritable effects and carcinogenesis (NOT cell killing)
What is included in Stochastic Model but not Deterministic?
Stochastic has heritable effects and carcinogenesis (NOT cell killing)
Deterministic does not include cancer risk
What causes the actual damage done in tissue by radiation?
Result of ionization produced by x-rays/gamma ray photons giving energy to orbital electrons and alpha/beta particles interacting electromagnetically with orbital electrons.
What determines the biologic effects from ionizing radiation?
Primary variables include those inherent to the cells and the conditions of the cells at the time of irradiation.
Also variables related to the radiation (absorbed dose, dose rate, type of radiation, and energy of radiation).
Damage to biologic systems occurs in this order:
Molecular –> Cellular –> Organic
In what order does damage to biologic systems occur?
Molecular –> Cellular –> Organic
Molecular is always first. Ionized atoms do not bind properly to other molecules. Loss of function in the molecule leads to loss of cellular function.
What is Linear Energy Transfer?
Average amount of energy deposited per unit path length of the incident radiation. Important for assessing potential tissue and organ damage.
High LET: Neutrons, protons, alpha particles, and heavy ions - much more damaging (“has a higher quality factor”)
Low LET: Photons, gamma rays, electrons and positrons
What is Relative Biologic Effectiveness?
The relative capability of radiation with differing LETs to produce a particular biologic reaction.
RBE = Dose of 250 kV x-rays / Dose in Gy of Test Radiation
How effective the test radiation is in producing the biologic reaction than the “standard” 250 kVp x-rays.
What is the “Kill Effect”?
As Linear Energy Transfer increases, Relative Biologic Effectivness will increase…to a certain point.
Above 100 keV/micrometer of tissue, RBE decreases with increasing LET - b/c the maximum potential damage has already been done - additional increase in LET is wasted dose.
What is Oxygen Enhancement Ratio (OER)?
The relative effectiveness of radiation to produce damage at different oxygen levels - biologic tissue is more sensitive to radiation in an oxygenated state.
Oxygen Enhancement ratio really only matters for low LET radiation.
With high LET radiation, the OER is often 1 (biologic damage without oxygen = biologic damage with oxygen).
When does oxygen enhancement ratio matter?
Oxygen Enhancement ratio really only matters for low LET radiation.
With high LET radiation, the OER is often 1 (biologic damage without oxygen = biologic damage with oxygen).
What is the majority of irradiation interaction in living cells?
Indirect - act on water - creates a free radical which jacks up the DNA - this process is promoted by the presence of oxygen.
Direct radiation most likely for high LET radiation (unusual in x-ray imaging).
Single vs Double strand DNA break?
Single - break one of the chemical bonds (point mutation) - more likely with low LET radiation. Repair enzymes can fix.
Double - break in multiple chemical bonds - more likely with high LET radiation. Harder to fix.
What is mutation?
Possible for radiation to cause a loss of or change in the nitrogenous base in the DNA chain. If cell doesn’t die from this, this incorrect information will be transferred as the cell divides.
Syndrome with the most sensitivity to x-rays?
Ataxia Telangectasia.
Common distractors:
Bloom Syndrome and Fanconi Anemia- both have genetic instability but no particular relationship with x-rays.
Xeroderma pigmentosa- more sensitive to UV radiation.
What is Chromosome Aberration?
Damage occurs in interphase (before DNA synthesis). Both chromatids are broken so each daughter will get a broken copy.
What is Chromatid Aberration?
Damage occurs later in interphase than chromosome aberration (after DNA synthesis). Only 1 chromatid is going to have a break (the other one will be fine).
Difference between Chromosome and Chromatid Aberration?
Chromosome Aberration - Damage occurs in interphase (before DNA synthesis). Both chromatids are broken so each daughter will get a broken copy.
Chromatid Aberration - Damage occurs later in interphase than chromosome aberration (after DNA synthesis). Only 1 chromatid is going to have a break (the other one will be fine).
What absorbed dose of x-ray or gamma ray will cause instant death of a large number of cells?
Dose of 1000 Gy in a period of seconds/minutes.
What is Mitotic Death?
When a cell dies after 1 or more divisions. Relatively small dose of radiation can cause this.
What is Mitotic Delay?
Very small dose (0.01 Gy) just before a cell starts to divide can cause a delay or failure in the timing of the normal dividing.
In what phase of the cell cycle are cells most sensitive to radiation?
M phase (mitosis)
In what phase of the cell cycle are cells least sensitive to radiation?
S phase
Order of sensitivity of cell phases to radiation?
M > G2 > G1 > S
Which phase of the cell cycle is most variable in length?
G1
What is surviving cell synchronization?
Blast group of cells with radiation, ones that survive wil have their cell cycles synchronized.
Reason is that the most resistant part of the cell cycle is late synthesis - so most surviving cells are in this phase.
What is the Law of Bergonie and Tribondeau?
Cell sensitivity is directly related to their reproductive activity and inversely related to their differentiation.
The more cells turn over (skin, blood, GI tract lining, reproductive cells) the more sensitive they are.
Less cell turn over and more differentiated they are (brain, nerves, muscles) the less sensitive they are
What is the most radiosensitive part of the GI tract?
Small bowel
What is the survival curve?
Graph of dose vs cell survival.
Has a “Quasi-threshold dose” at low dose which is the portion of the graph when repair mechanisms are trying to hold everything together. Measure of “sub-lethal” damage to the cell.
The linear slope of the curve is the radiosensitivity of the cell population. 1/Slope (D0). Higher the D0, the more radio-resistant the cell is.
What is the “Quasi-threshold dose” of the survival curve?
low dose which is the portion of the graph when repair mechanisms are trying to hold everything together. Measure of “sub-lethal” damage to the cell.
What is the slope of the survival curve?
The linear slope of the curve is the radiosensitivity of the cell population. 1/Slope (D0). Higher the D0, the more radio-resistant the cell is.
What does higher dose rate do to the survival curve?
Smaller shoulder and steeper drop in the curve.
What does a change in LET of radiation do to the survival curve?
Low LET will have a shoulder
High LET will NOT have a shoulder.
What will oxygen do to the survival curve?
Oxygen will make a steeper drop in the curve, more pronounced with low LET.
What are the most sensitive blood cells to radiation?
Lymphocytes
Dose of 0.25 Gy is enough to depress the amount of circulating in the blood.
Can stimulate the lymphocytes to divide and can score the number of chromosomal aberrations at first mitosis to reflect average total body dose.
What is Acute Radiation Syndrome?
Clinical response when the body is hit with a large amount of radiation.
3 subtypes (bone marrow, GI, and CNS) with 4 phases.
Feel bad (GI flu-like symptoms), then you feel better “latent phase,” then your syndrome subtype manifests - related to underlying organ system (bone marrow, GI, and CNS), then death/recovery.
Dose needed, latent period and outcome for Bone Marrow Acute Radiation Syndrome?
Dose needed: >2 Gy
Latent Period: 1-6 weeks
Outcome: Worse with higher doses, but possible to survive.
Dose needed, latent period and outcome for GI Acute Radiation Syndrome?
Dose needed: >8 Gy
Latent Period: 5-7 days
Death within 2 weeks
Dose needed, latent period and outcome for CNS Acute Radiation Syndrome?
Dose needed: >20-50 Gy
Latent Period: 5-7 days
Death within 3 days
Total body dose that will cause nausea about 30% of the time?
0.75 - 1.25 Gy
How do you triage patients with possible ARS?
Basic idea- earlier the symptoms, the worse they are going to do.
Early vomiting is a marker of severity/poor prognosis.
Surveillance for 5 weeks; Surveillance for 3 weeks, consider general hospital; Hospialize - burn center; Hospitalize - specialized radiation center.
What is Lethal Dose 50/30?
What is it in humans?
Dose which will kill 50% of people w/in 30 days.
3-4 Gy without treatment - can tolerate up to 8.5 Gy with medical treatment.
What is Lethal Dose 50/60?
What is it in humans?
What is it used for?
Lethal dose at 60 days for 50% of the population.
Used for bone marrow failure.
About 3-4 Gy.
What is lethal dose 50/4?
What is it in humans?
What is it used for?
Lethal dose at 4 days for 50% of the population.
Used for GI failure
About 10 Gy.
What is Genetically Significant Dose?
Term used for expressing genetic risk as an index of presumed impact on the entire population.
GSD is the dose if received by all members of the population that would result in the same hereditary damage as the actual doses received by the gonads of people who actually get radiation exposure. Depends on gonadal dose, and child bearing potential (age and sex of patient).
When is the effect of radiation on a fetus most severe?
Teratogenicity of radiation is dose dependent, and timing dependent.
Between 8-15 weeks is most vulnerable time. >15 weeks the brain is less sensitive to radiation.
Doses over 100-200 mGy are associated with reduced head diameter and mental retardation.
First two weeks (implantation) - 50-100 mGy may cause fetal loss. If the baby doesn’t die then likely to have no lasting effects (all or nothing).
It takes very low dose (just a few radiographs) to the fetus to increase the risk of childhood leukemia.
How much does IQ drop by per 1 Sv?
IQ is said to drop 30 points per 1 Sv, with the risk of retardation being 40% at 1 Sv.
When does the fetal thyroid begin to take up iodine?
Does not take up iodine prior to week 8.
If mom gets I-131 prior to week 8 the fetus will not by hypothyroid.
What is the risk of cancer per Sv?
Estimated by BEIR 5 and UNSCEAR - 8%/Sv
A reduction factor of 2 is used for low dose and low rate - so the working population has a risk more like 4%-5%/Sv
What is the calculation for risk for hereditary effect of radiation?
0.2/100 x Dose
Doses and onset for skin problems
Early transient erythema - 2Gy skin dose - hours
Severe “Robus” Erythema - 6 Gy skin dose - 1 week
Telangectasia - 10 Gy skin dose - 52 weeks
Dry desquamation - 13 Gy skin dose - 4 weeks
Moist Desquamation/Ulceration - 18 Gy skin dose - 4 weeks
Secondary Ulceration - 24 Gy skin dose - >6 weeks
Dose and onset for telangectasia
10 Gy skin dose - 52 weeks
Dose and onset of hair problems
Temporary epilation - 3 Gy - 21 days
Permanent Epilation - 7 Gy - 21 days
3 x 7 = 21
Difference between senile and radiation-induced cataracts?
Senile = anterior
Radiation-induced = posterior
20 years after exposure - latent period is inverse to the exposure amount.
Threshold for development from an acute exposure is around 2.5 Gy
Doses required for cataracts
Acute exposure threshold to cause cataract - 2.5 Gy
Annual Dose Rate Limit - 0.15 Gy/yr
What are the general rules for sterility/infertility with male and female?
Females: Threshold is age-dependent. Younger = more dose needed. Close to puberty ~10 Gy. Close to menopause ~2 Gy.
Male: Temporary sterility is going to occur somewhere between 0.15-2.5 Gy. More permanent sterility requires an acute dose around 5 Gy.
Doses for temporary and permanent male sterility/infertility?
Temporary: 0.15 - 2.5 Gy
Permanent: 5 Gy
Doses for sterility/infertility in female?
Age 12: 10 Gy
Age 45: 2 Gy
Female - no age given: around 6 Gy
Dose trigger for additional patient care/follow up?
15 Gy to a single exposure field
Most places will call this a “sentinel event”
Dose to cause nausea (30% of people)
0.75 - 1.25 Gy WB
Dose to depress circulating lymphocytes
0.25 Gy WB
LD 50/60 for marrow
3-4 Gy WB
LD 50/4 for GI
8-10 Gy WB
LD for CNS
> 20 Gy (20-100) WB
Dose for double the natural or spontaneous mutation rate
1 Gy
Effective dose from background radiation in the US
3 mSv/yr
Occupational exposure to lens
150 mSv/yr
Occupational exposure to radiation worker
50 mSv/yr
Occupational exposure to extremity
500 mSv/yr
Exposure limit for infrequent public exposure
5 mSv/yr
Exposure limit for continuous public exposure
1 mSv/yr
Exposure limit for embryo/fetus via Mother
5 mSv/yr (1 mSv to fetus directly)
Exposure to Embryo/fetus (post declared pregnancy)
0.5 mSv/month
Public exposure limit in controlled areas
50 mSv/yr
Public exposure limit in uncontrolled area
5 mSv/yr
Genetically significant dose to public
0.25 mSv
Effective dose from background radiation in the US
3 mSv/yr
What causes majority of energy received by biologic material from x-rays?
Transferred from electrons
Two thirds (around 60%) of x-ray damage to biologic material is mediated by free radicals
What can interactions of two separate chromosomal breaks lead to?
Aberrations such as dicentrics and rings
What are the most important lesions caused by x-rays?
Double stranded DNA breaks
The final number of double stranded DNA breaks is more important than the initial number of breaks - some will be repaired.
How can you score whole body radiation exposure?
Stimulating lymphocytes to divide
What is the risk of radiation induced cancer?
4-5% per Sv = adult
Up to 15% per Sv = child
1/10th for someone older than 50
When is transient skin erythema seen
Seen in hours
Main wave occuring after 10 days
Does radiation induced sterility in males affect hormone levels or libido?
Does not affect hormone levels or libido
There is a “latent” period between irradiation and sterility.
What does radiation induced sterility in females cause?
Causes symptoms similar to menopause
No hormonal effect in males or effect on libido.
Carcinogenesis by radiation is ______, based on what?
Stochastic (all or nothing)
Based on Beir 5 and UNSCEAR committes
What is a sentinel event dose?
15 Gy
What can radiation do to the thryoid?
Radiation-induced benign and malignant nodules
What can happen to Radon workers?
lung cancer
What contributes about 55% of the effective dose to the US population and is the largest single contributor to effective dose?
Inhaled Radon (alpha emitter)
What is the largest single contributor to effective dose in the US?
Inhaled Radon (alpha emitter) contributes about 55% of the effective dose to the US population.
What is the greatest source of exposure to ionizing radiation for the general population of the US due to human activity?
Medical imaging
For MRI, you must have a controlled access so that the fringe field outside the area does’t exceed __?
5 Gauss
For MRI, the SAR should not exceed ___?
4W/kg for whole body per 15 min
Specific absorption ratio - the RF power absorbed per unit mass
