LET/RBE/OER & Direct/Indirect Flashcards
RBE
Relative biological effectiveness
Biologic reaction compared to a standard
LET
Linear energy transfer
Energy deposited in tissue per unit of distance
Standard RBE
250 KV X-rays
OER
Oxygen enhancement ratio
Oxygen deprived versus oxygen rich environment
The amount of energy transferred to tissue per unit of distance travelled in tissue is
LET - linear energy transfer
High LET
Damage localized and severe (per unit of distance)
Low LET
Damage widespread and minimal (per unit of distance)
The higher the LET or energy deposited into tissue the greater…
Cellular damage will occur
As LET goes up, tissue damage…
Goes up
Whether LET is high or low depends upon:
Charge - neutral or ionic
Velocity - speed of light or slower
Mass - pure energy or matter
Charge of the particle will be
Attracted to opposites as they transverse matter.
A charged particle will do more damage than a neutral particle
A charged particle will do more damage than a
Neutral particle
Alpha particles cause the most damage because
It carries a +2 charge
X-rays and gamma rays cause the least damage due to
Being neutral (no charge)
Charged particles “pull” at tissue ironically as
They pass through
Velocity
The higher the velocity the more spread out the damage due to the large distance traveled in small time periods.
X-rays travel very fast so the damage caused is
Spread over a large area - small scale damage per unit of distance
Alpha particles travel relatively slowly causing damage to be
Limited to a small area - large scale damage per unit of distance
AMU
Atomic mass unit
Mass
The sum of an objects matter
Protons have a mass of
1 AMU
Electrons have an AMU of
5.486x10^-4 (.0005486 AMU)
Virtually no mass
X-rays have no mass making the
Highly penetrating
Alpha particles are
2 Protons and 2 neutrons
AMU = 4
Minimally penetrates matter - about 1mm
Summation of high LET
Particles are charged Move at slow velocity Have mass Greater possibility of cell death Low penetration of tissue Greatest concern when inhaled or ingested
High LET and DNA damage
Causes severe damage to DNA
- double strand breaks
- damage typically irreparable
High LET damage to DNA is usually
Lethal to the cell
Summation of low LET
No charge
Travel at speed of light
Have no mass
Highly penetrating
Low LET and DNA damage
Typically causes sub lethal damage that can be repaired by the cell.
Point mutations or single strand breaks are more common.
Low LET damage to DNA is usually
Repaired - the cell survives
Cells that exist in a higher aerobic state have a
Higher radio sensitivity than cells that exist in an anoxic state (no or little oxygen available)
As OER goes up, biologic damage…
Goes up
OER is expressed as
The dose required to cause damage in anoxic/aerobic conditions
Damage can be 2-3 times greater in oxygenated state than the damage to the same tissue in an anoxic state.
OER and LET
OER is enhanced with low LET radiation
OER is minimal with
High LET radiation
As LET goes up it will take less radiation to cause these e biological damage aaa
Low LET radiation. As LET goes up RBE goes up and vice versa
RBE is not practical for measured dose received in human sand is most commonly used for
Experiments I specific tissue types in labs
For radiation protection purposes we use the
Radiation Weighting Factor to calculate the equivalent dose and take into consideration different types of radiation
As LET increases biological damage
Will increase - the chance of tissue repair will decrease
As LET increases, RBE
Will also increase - damage due to mass and charge
OER increases with
Low LET radiation (indirect action)
Molecular effect of radiation
Direct and indirect Radio luzia of water Effects upon DNA Effects upon mitosis Target theory Cell death
Direct action of irradiation - physical damage
Biological damage due to ionization of atoms on master molecules.
Indirect action - chemical damage due to irradiation
Interraction of radiation with water molecules, production of ions and free radicals resulting in undesired chemical reactions - cell death due to indirect chemical reactions - toxic to cells
More likely with low LET radiation like X-ray.
Radiolysis of water
X-ray ionizes matter by removing an electron which causes disruption of normal water molecule consumption.
ionizing radiation and DNA
Single strand breakage aka point mutation.
Most common with low LET radiation.
Single strand breakage is aka
Point mutation
Double strand breakage ruptures both DNA side rails and is more common
With high LET radiation
Mutation
Alternation of DNA base sequence.
May not be reversible and may be passed along to daughter cells.
Ionizing radiation and effects upon chromosomes
Can cause breakage of chromosomes - both somatic and genetic cells.
Genetic fragments
Broken chromosomes can “stick” together.
Target theory
Master molecule within a cell, a unique or key molecule that cannot be replaced if damaged or destroyed. DNA is presumed to be the mater molecule.
Rapid exposure to 100,000 rads (1,000 gray) will cause
Instant death in a period of a few minutes or less
Reproduction death
Exposures of 100-1,000 rads.
Cells lose the ability to reproduce
Apoptosis
Programmed cell death
Non-mitotic
Cells die without attempting cell division - occurs in interphase
Mitotic death
Occurs when a cell dies after one or more divisions following exposure to ionizing radiation
Ionizing radiation can affect cell division
May retard mitotic process or permanently inhibit mitotic process.
Permanent inhibition leads to
Mitotic death
Mitotic delay can occur
With exposures as small as 1 rad.
Exposure just before cell division begins, delay in beginning mitosis. After the delay, the cell continues on as normal.
