2. Radiation Biology Flashcards
what are the 2 types of ionising radiaiton
electromagnetic and particulate
what is electromagnetic radation
oscillating electric fields which self propagate
wave form
what is particulate radiation
particles or photons rather than waves
low frequencies have long/short wavelengths
long
where does the split between ionising and non ionising radiation fall between
anything above UV light is ionising
what is an isobar
atom with the same mass of nucleons/same mass number
what is an isotope
atom with the same number of protons/same atomic number
what are the 2 forces holding and breaking the subatomic particles of an atom
what are these forces like in a stable atom
strong nuclear force holds atom together
electromagnetic force splits it apart
balanced forces in stable atom
what is radioactivity
activity is the number of disintergrations that occur per second
what is radioactivity measured in
becquerel (Bq)
what is alpha decay
2neutrons and 2 protons are emitted
what is beta decay
convert neutron to proton and ejects an e-
what is gamma decay
gamma particle like a photon is emitted
what is the radioactive decay law
number of atoms decaying per unit time is proportional to the total number of radioactive nuclei
what is the half life
time for activity to reduce to 1/2 its initial value
what is the equation for the radioactive decay law
N(t) = Noe^-(decay constant x t)
N = number of nuclei No = number of nuclei at time t = 0
what is the biological half life
time takes for body to excrete half of it (how quickly it clears it from the body)
what model does the radioactive decay follow
exponential
what is the physical 1/2 life
time for activity to reduce to 1/2 its initial value
what is the effective 1/2 life
time to reduce radioactivity level of internal organ/whole body to 1/2 its initial value due to both elimination and decay
effective half life is larger/smaller than the physical and biological 1/2 life for what reason
smaller
both act to reduce overall activity so 1/2 life overall should be smaller and quicker
what are the 2 types of photon interactions
compton scattering
photoelectric effect
what is compton scattering
is the atom ionised
incident photon impart some energy to outer shell e- of the atom
compton e- is ejected from atom
scattered photon loses energy and continues on a different path
atom is ionised so potential for biological harm
what is photoelectric effect
incident photon is completely absorbed by inner shell e-
photoelectron is ejected from atom
vacancy is filled by outer shell e-
secondary photon is produced releasing lower amounts of energy
what is exposure
any incident ionising radiation produces ion pairs
exposure is the amount of charge produced per mass of air
what is the unit for exposure
colombs per kg (C/kg)
what is the equation for exposure
X = Q/m
radiation particle interacts with nuclei to ionise them and produce what?
produces a charged pair
what is absorbed dose
as radiation is incident on tissue it imparts energy to that tissue
energy deposited in tissue per unit mass
what are the 2 units of absorbed radiation
joules per kilogram (J/Kg)
Gray (Gy)
what is the equation of absorbed dose
D = E/m
D = absorbed dose (Gy) E = energy (J) m = Mass (kg)
does absorbed dose indicate anything about biological harm
no, only how much energy is imparted on the tissues
what is kerma
kinetic energy released in matter
kinetic energy transferred to charged particles by indirectly ionising radiation per unit mass
what is air kerma
measure of tube output
what is the unit used for air kerma
mGy per 100mAs
what is entrance skin kerma
dose absorbed by skin at beam entrance
what is the unit of ESK
Gy
what are the 4 factors that effect the ESK
ESK are proportional to __
SSD
tube current
Length of exposure
square of kVp
does ESK say anything about how it interacts with tissue
no just at the surface, says nothing about biological harm
how do e- differ from alpha particles in the way that they interact and travel through tissue
e- = scatter through material and interact with an atom to produce ion pair
alpha = travels shorter distance but larger so deposits more energy in one place so can do alot of damage to the tissue they encounter
what is the path length
total distance particle travels
what is the range
penetration depth in the tissue
what is the specific ionisation
number of ion pairs produced per unit length
what is the linear energy transfer
energy transferred per unit length in tissue
product of average energy per ion pair and specific ionisation
(how much energy its depositing over the path it travels)
is the path length of e- or alpha particles greater
e-
is the range of the e- or alpha particles greater
about the same for both of them
do e- or alpha particles have a greater specific ionisation
e- travel trough longer path for same # of ion pairs but alpha produces lot more ion pairs in short distance so SI is higher than e-
for an e- is which is greater the path length or the range
path length
for an alpha particle is which is greater the path length or the range
both are equal
for an e- does it have high or low specific ionisation
low
for an alpha particle does it have high or low specific ionisation
high
for an e- does it have high or low average energy
low
for an alpha particle does it have high or low average energy
high
does an e- have high or low LET radiation
low
does an alpha particle have high or low LET radiation
high
what is a bragg peak
for heavy charged particles (Eg alpha, protons) specific ionisation increases as the particle loses energy
it carries charge but loses energy to neighbouring tissue so slows down resulting in an increased level to a certain point
when we talk about average energy of e- and alpha particle tissue interactions what do we mean that as
the average energy per ion pair
what characteristic of the x-ray beam and radiation determines what shielding it needs and how much energy it imparts
range
the range for alpha is small/large and e- and gamma can travel further/less distance
small
further
what is the equation for relative biological effectiveness
RBE = Dose of 250kV x-rays required to produce effect ‘X’/dose of test radiation required to produce effect ‘X’
could be at a different kV
what does RBE depend on
LET
what is biological effectiveness
how effective it is at producing biological harm for tissues
what is overkill
Even though high LET, low RBE
A single particle deposits much more energy than is required to kill a cell. Therefore, it kills less cells per absorbed dose
produce more harm than alpha particles so they’re damaging tissue but not going to penetrate the tissue anyways so lower bio effectiveness
what is the relative biological effectiveness
The ratio of the doses required by two radiations to cause the same level of effect. Thus, the RBE depends on the dose and the biological endpoint
what is direct interaction of ionising radiation with tissue
photons or e- directly ionise DNA molecules by removing atoms or breaking bonds
damages tissues as tissue cells replicate depending on DNA
what are the 2 ways that radiation interacts indirectly with tissue
incident photons and scatter e- produce free radicals
free radicals induce damage to DNA
what is the equation of H2O’s and radiation’s indirect interaction with tissue
what is its process of interaction
H2O + xray -> H2O+ +e-
this can then take 2 pathways
1/ e- + H2O -> H2O- and then H2O- -> OH- + H’ (hydrogen radical)
2/ H2O+ + H2O -> H3O+ +OH’ (hydroxyl radical)
photons interact with water molecule and ionises water to produce free radicals that then damage DNA
what is a product of the radicalisation of water by x-rays that can also cause tissue damage
H2O2
why are free radicals dangerous and harmful
They are highly reactive as they have an unpaired e- and can react with anything
As they react they tend to break down other bonds so will bind and damage DNA molecule
what is the equivalent dose
absorbed dose weighted for relative biological effectiveness
what is the unit of equivalent dose
Sievert (Sv)
what is the equation for equivalent dose
Ht = sum (WrDtr)
for xrays only:
Ht = Dt
Ht = equivalent dose (Sv) Wr = radiation weighting factor (Sv/Gy) Dtr = absorbed dose (Gy)
what is the equation for equivalent dose when only x-rays are concerned
Ht = Dt
Ht = equivalent dose (Sv) Dtr = absorbed dose (Gy)
how do you calculate the equivalent dose if you have different types of radiation coming in
If you have different radiation types coming in, take dose for each one and multiple by radiation factor and add them all up
equivalent dose and absorbed dose have the same value just different units true/false
true
what is the effective dose
equivalent dose weighted for biological sensitivity of tissues
what is the unit of effective dose
Sieverts (Sv)
what is the equation for effective dose
E = sum (WtHt)
E = effective dose (Sv) Wt = tissue weighting factor Ht = equivalent dose in tissue (Sv)
Why is the effective dose not always useful
why is it useful or what is it useful for
whys is it not useful or in what ways is it not useful
Effective doses are for a generic patient so while its useful to make comparisons, different people have different amounts of tissue types based on risk factors, gender, race, shape etc
Can’t use effective dose to describe individuals
While useful to say you get more dose here than there, you can’t use it to say anything about the risk/dose to an individual as there is too much variability
Useful for comparisons between examinations but not useful for assessing individual risk to a particular patient
