Radiation Biology Flashcards
Dose Units
for
Radiation Measurement
(4)
- Exposure Dose
- Absorbed Dose; RAD vs. Gray (G)
- Equivalent Dose; REM vs. Sievert (Sv)
- Effective Dose; REM vs Sievert
Terms: Exposure
Definition
Traditional
SI unit
Conversion
ionize air
R
air kerma
1 R= 2.58 x 10-4
Coul./kg
Terms: Absorbed dose
Definition
Traditional
SI unit
Conversion
energy absorbed by tissue
rad
Gy
1Gy = 100 rads
Terms: Equivalent dose
Definition
Traditional
SI unit
Conversion
modified by radiation weighting factor
rem
Sv
1 Sv= 100 rem
Terms: effective dose
Definition
Traditional
SI unit
Conversion
modified by tissue weight factor
rem
Sv
1 Sv= 100 rem
Exposure
* A measure of the
capacity of radiation to ionize air
skipped
Exposure
Traditional unit:
roentgen (R) = produce 2.08x109 ion
pairs in 1.0 cc of air at standard temperature and
pressure
Exposure
Metric equivalent unit (S.I.) :
air kerma (kinetic energy
released in matter) = sum of the kinetic energy of all
liberated charged particles/mass (Coulomb/kg)
Exposure
Conversion:
1 R= 2.58 x 10-4 Coulomb/kg
1 Coulomb/kg=3.88x103 R
Roentgen (R) -
unit of radiation exposure that
produces 2.08 x 109 ion pairs in 1.0 cc of air at
standard temperature and pressure
Metric equivalent (S.I.)-
Coulomb/kg
Conversion; 1 Coulomb/kg=3.88x103 R
»2.58 x 10-4 Coulomb/kg=1R
RAD - acronym for
Radiation Absorbed Dose
100 ergs or radiation energy in
1 gram of
absorbed material
Metric equivalent (S.I.) - Gy (gray) is Joule/Kg
Conversion;
1 Gy=100 RAD
0.0 1 Gy= 1 RAD
1R =
0.903 RAD
Equivalent Dose (I)
* To compare the
biological effects of different types of radiation
Radiation weighing factor (WR) depends on the
type and energy of the radiation involved
QF
❖X-ray =
❖High energy radiations=
1
>1
- high energy protons = 5
- alpha particles = 20
Quality Factor(Q.F)-
is a measure of the
biological effectiveness of a radiation to ionize
matter
the QF for x-radiation =
1
REM-
(2)
acronym for Roentgen Equivalent
in Man
* equivalent to RAD x Q.F.
- Since the QF for X-radiation = 1;
- RAD units for x-radiation are equivalent to
REM units
Equivalent Dose
S.I. unit =
Sv (sievert)
- S.I. unit = Sv (sievert)
- Conversion: 1 rem =
0.01 Sv
1 Sv = 100 rem
Diagnostic x-radiation is usually measured in
millirems (mRem)
Since 1 R ~= 0.903 RAD = 0.903 REM
therefore
1 mR ~ 1 mRad = 1 mRem
0.01mGy = 0.01 mSv
10 μGy = 10 μSv
E = Σ WT x HT
WT –
HT -
Tissue weight factor
Dose equivalence to tissue
Effective Dose
* This measure is used to specifically
calculate
risks of radiation to human
tissues on a common scale.
Effective Dose
* This measure is used to specifically
calculate risks of radiation to human
tissues on a common scale.
* The calculation is a product of the
sum
of dose equivalence to the specific
tissues or organs exposed and the
biological tissue weighting factor.
Effective Dose
* Use of the effective dose allows
comparisons of
different imaging
techniques to be made on a common
scale.
Effective Dose
* Use of the effective dose allows
comparisons of different imaging
techniques to be made on a common
scale.
* The value is an estimated measure of all
somatic and genetic radiation-induced risk
even if the entire body is not uniformly
exposed.
Effective Dose
Used to assess risk of non-uniform radiation to
localized part of body and degree to which this
would increase a person’s “whole body” risk of
(2)
- cancer induction and/or
- induction of genetic mutations
i.e., Stochastic effects
AREA EXPOSED is related to the
maximum size of the beam
Ionization:
Interactions of X-radiation
with Matter
RADIATION
INJURY
Mechanisms of Injury
from x-ray interaction with matter
………….(i.e., ionization)
Ionization form exposure sets of
a multiple direct and indirect
molecular reactions in
< 1 sec.
Enzymatic repair or further
deleterious molecular changes
occur in
minutes to hours
Determinsitic and stochastic
effects take place over time from
months, to decades, to
generations
Interactions of X-radiation
with Matter
1. No interaction ~
2. Photoelectric effect ~
3. Compton Scatter ~
4. Coherent (Thomson) Scatter ~
9%
27 - 30%
57 - 62%
7%
No Interaction
- X-ray photon enters object (eg. patient
or other biologic tissues) and exits with
no change in its energy
Photoelectric Interaction
- Accounts for 30% of all interactions
- X-ray photon collides with an orbital
electron and loses its energy - Ejected photoelectron loses it energy
- Results in an atom with an altered
electric state, i.e., “+” charge - (similar orbital electron reaction to characteristic
radiation production but no x-radiation is produced)
(5)
Photoelectric Interaction
* The ionized matter is unstable and seeks a more stable configuration.
* The new configuration may include new ionic bonds, different covalent bonding, etc…
* If the degree of photoelectric effect is significant,
this may affect, biologic structure, function or both.
* These effects are often deleterious biologic changes; e.g. altered metabolic function, malignancy, etc…
Compton Interaction/Scatter
(4)
- accounts for 62% of interactions
- X-ray photon collides with an outer
orbital electron losing some energy - X-ray photon continues in different
direction with less energy creating more
scatter until all the energy is lost - results in an atom with an altered
electric state, i.e., “+” charge
Compton Interaction/Scatter
* The ionized matter is unstable and seeks a more stable configuration.
* The new configuration may include new ionic bonds, different covalent bonding, etc…
* If the degree of photoelectric effect is significant,
this may affect, biologic structure, function or both.
* These effects are often deleterious biologic changes; e.g. altered metabolic function, malignancy, etc…
Coherent Interaction
(4)
- accounts for 8% of all interactions
- X-ray photon of low energy interacts
with an outer orbital electron and
changes direction - no photoelectron produced
- no ionization occurs
Mechanisms of Injury
from x-ray interaction with matter
………….(i.e., ionization)
(2)
- Direct
- Indirect
- Direct
- Indirect
both… (3)
– Both effects occur quickly
– Both effects take hours to
decades to become evident
– Both are a result of
ionization
Direct effect
(2)
➢ Directly ionizes biologic maromolecules
➢ Contributes to 1/3 of biologic effects
Indirect effect
(2)
➢ X-ray photons absorbed by H2O →
free radicals →biologic damages
➢ Contributes to 2/3 of biologic effects
Outcome of
Direct Effect of
UV Light
on Skin DNA
(3)
- Repair (healed)
- Inaccurate repair (mutation)
- No repair (death)
Indirect Effect
* Primary method of
cell damage from
radiolysis of water caused by x-radiation
Indirect effect
equaiton
Photon + H2O H2O* OH + H
Free radical:
a free atom or molecule
carrying an unpaired orbital electron
in the outer shell
Free Radical Formation
(2)
- Highly reactive and unstable
- Lifetime = 10-10 seconds
Toxins from Free Radicals
Free radicals seek a more stable
configuration which results in formation of
toxic substances
Dose-Response Curves
(2)
- Dose (amount) of radiation is correlated with the
response or damage - Curves are theoretical for diagnostic x-radiation
Threshold Non-Linear Curve
(3)
- Small exposures do a
substance do not
produce measurable
changes - A threshold must be
reached before
changes are observed - Most biologic effects
are non-linear
Linear Non-Threshold Curve
(2)
- Dose is proportional to the response
- No matter how small the dose, there is some
damage or risk
Nonlinear Nonthreshold Curve
(2)
- No threshold
- Minimal damage at first with increased rate
of damage with increased dose
Deterministic risk/effect
(2)
- Have a threshold
- severity is proportional to
the dose
Deterministic risk/effect
examples (7)
Erythema
●xerostomia
●cataract
●osteoradio-
necrosis
●fertility
●fetal devel-
opment
●alopecia
radiation Erythema
- Side-effect of head &
neck cancer treatment
Stochastic effects
(3)
- Have no dose threshold
- Probability of occurrence
is proportional to dose - Severity of effects does not
depend on dose
- To somatic cells -genetic mutations cause
malignancy
- To germ cells - genetic mutations cause
heritable
effects
Genetic Injury
(2)
– no affect on parent
– affects future
generation
Somatic Injury
(2)
– affects parent
– no affect on
future generation
Somatic effects/mutations
(3)
– Somatic cells –all those except reproductive cells
– Seen in the person irradiated
– NOT transmitted to future generations
* Induction of cancer, leukemia, cataracts
Genetic effects/mutations
(2)
– NOT seen in the person irradiated
– Passed on to future generations
Sequence of Radiation Injury
(3)
- Latent period
- Period of injury
- Recovery period
Latent Period
definition
* May be short or long depending on: (2)
* Shorter latent period if: (2)
* Genetic effects –
- Time that elapses between exposure and appearance of clinical signs
– Total dose
– Dose rate
– Increased amount of radiation
– Faster dose rate
may be generations before clinical effects are seen
Period of Injury
(6)
- Cell death
- Changes in cell function
- Breaking or clumping of chromosomes
- Giant cell formation
- Cessation of mitotic activity
- Abnormal mitotic activity
FACTORS MODIFYING EFFECTS
OF X-RADIATION
(6)
- Total dose
- Dose rate
- Oxygen
- Area exposed
- Cell type and function
- Age
- Total dose
increase Total dose → increase damage
- Dose rate
- Describe the frequency of dose delivery
- increase Dose rate = decrease cellular repairs = increase damages
- Oxygen
increase Oxygen content = increase radiosensitivity
= increase tissue damage
- Area exposed
- Cell type
Cell Type vs. Radiosensitivity
(CASARETT CLASSIFICATION)
Radiosensitive –
Radioresistant –
young, immature, rapidly growing
and dividing, least specialized
mature, specialized cells
Cell Type vs. Radiosensitivity
(3)
Species
Intrinsic resistance
Cells
Species
◦ Mammals more sensitive than reptiles, insects, bacteria
Cells
◦ Mitotic activity
increased frequency of cell division = ↑ sensitivity
◦ Mitotic activity
Immature cells/not highly specialized = ↑ sensitivity
◦ Cell metabolism
increased metabolism = ↑ sensitivity
High sensitivity to radiation
Least radioresistant
(6)
Blood cells
Small lymphocyte –most sensitive
Bone marrow
Reproductive cells
Intestinal mucosa
Mucous membrane
Intermediate sensitivity to radiation
(5)
Connective tissue
Breast (women)
Small blood vessels
Growing bone and cartilage
Salivary gland
Fairly low sensitivity to radiation
(2)
Thyroid gland
Skin
Low sensitivity to radiation
Most radioresistant
(3)
Muscle
Nerve
Mature bone
- Age
Pediatric Patients at risk
(2)
- Rate of cellular and organ growth puts tissues
at greatest level of radiosensitivity - Greater life expectancy puts children at 2-10
greater risk of being afflicted with a radiation
induced cancer
