What is radiation quality?
A measure of the damage potential of a particular radiation type,this means alpha betas and gammas and positrons for example,has different abilities to cause damage to tissue,this damage potential is related to its size, charge and energy,we have a good example of this with amunitions, let's take a unit that you already know well,energy gained by electron being accelerated through a field of 1 V,now to begin the radiation damage to tissue, let's first see the linear energy transfer,it is a measure of energy absorbed by the material, keV/length...somewhat related to the track or path.let's consider a beta particle moving through a particle such as lead brick,it should be intuitive and it interacts, it transfers it's energy to the material.high LET=more energy transfer and shorter distance travelled.Because it loses energy quickly.ability to produce more damage,of course we are concerned with biological damage,before we begin to understand the damage of radiation, we must discuss radiation dose measurements in radiation symmetry.
radioactivity rate of decay= number of disintegration per unit time (usually sec)
exposure=the electric charge in air due to the presence of radiation, the total sum of kinetic energy of charged particles liberated by uncharged radiation.
absorbed dose=similar to exposure-consider tissue type
energy deposited in tissue.value of absorbed dose from same radiation will change with tissue type.
1Roentgen = 1Rad
100rad=1Gy
in human tissue, about 87% of 1R is absorbed 0.87R=1rad
dose equivalent=corrects absorbed dose for damage potential of radiation type
must provide a weight factor of radiation types.
QF or RBE,gives numerical value to the damage potential.
QF=dose from standard radiation to produce given biological effect/dose from test radiation to produce same biological effect
standard radiation -200 kVp x-rays
dose equivalent:
rem \ sievert
100 rem=1Sv,
Dose equivalent =absorbed dose x QF or RBE
ICRP defined values for QF/RBE
gamma x rays 1
electrons (beta/positron) 1
neutrons 10
alpha 20
QF INCREASES as LET increases
150mrad x 1 gamma - 150 rem
75 mrad x 20 alpha = 1500 rem
total dose equivalent = 150 + 1500 rem
for QF =1
general rule 1 R =1 rad = 1 rem
reality:
1 rad = 0.96 rem or 0.87 R = 1 rad
there is no universally accepted conversion from rad to rem.
exposure: air
absorbed dose: considers material
dose equivalent: considers radiation type
effective dose equivalent:
measures the biological risk on the whole body by considering different tissue types response differently to radiation ; also accounts for radiation to only part of the body vs the whole body.
to account for different tissue response, weight factors are used (yes, again), weight factors determined by ICRP.
2007 revision of ICRP recommendations:
14 designated tissues for organs + 1 for everything else.
to calculate: summation of- Dose Equivalent x Weight Factor
For all designated tissues/organs irradiated
H = summation Wt x Ht
How do we account for long term dose to an organ? inhaled, injected or swallowed
committed dose equivalent: Ht,50, the dose equivalent to an organ or tissue received during the 50 year period following intake of the radioactive material, we use 50 because there isnot so much damage after 50,as the receipent would likely to be perished by then.committed dose equivalent accounts for physical decay and body excretion.
biological half life: SAME principle as radioactive or physical half life,and it is how we account for body excretion of radioactive material,it is important to know that it is an approximation differs from person to person,sickness are different and medications are different,
Biological Half life:depends on the renal system and the bowels, the overall disappearance of radioactivity from the body depends on effective half life, consider both physical and biological decays,
1/effective halflife=1/biological halflife + 1/physical halflife
we have seen many ways to calculate dose now considering many variables.
it becomes very complex trying to calculate a specific dose to a specific organ.
source organ is lung, target organ is liver.after.....then same organ liver can be source and target.
to calculate dose to the liver precisely we need to answer few questions//
exact location in lungs, time spent in lungs, exact location lin liver, time spent in liver.
doses from each adjacent organ of liver must be considered, which becomes even more complicated.
another factor we must consider in calculating dose to tissue is the percentage of radioactive emissions that deposit energy in the tissue vs those that escape.this value is called the absorbed fraction.
depends on track, density and thickness, radiation type, energy and half life.
absorbed fraction will be expressed as a number b/w 0-1, where 1.0 equals 100% absorption.
with all of these factors, how can we ever calculate an accurate dose to an organ?
MIRD - Medical Internal Radiation Dose Committee:which is the permanent committe of nuclear medicine provided a widely accepted method to calculate internal dose,
the first method is to calculate cumulated activity;plot activity vs time, area under the curve is the cumulated activity;units = activity x time uCi x hr
it is important to understand that the cumulated activity is the first step in determining the dose to an organ that a patient receives from a nuclear medicine procedure.
S factor: consider most of the physical aspects of location of radioactive material,
total energy associated with the radiation typexabsorbed fraction in target of source/mass of the target organ.
complications of calculating S factor: absorbed fraction varies greatly with gamma rays, also all transitions, disintegrations and emissions must be included not just the primary decay, for 99mTc, there are altogether 14 radiations to include.this include conversion electrons, fluorescent x-rays and auger electrons,
lucky for us, MIRD published all the values of S-factor, so we don't need to calculate all these values.
DOSE equation of MiRD=cumulated activity x S-factor
Cumulated Activity = Biological Parameters
S-factor = Physical parameters
review:
LET and its affect on dose
all measurements of radioactive energy deposit
unit conversion b/w SI and traditional
how to incorporate QF in dodse calculations
how to calculate effective half life
how to calculate organ dose using MIRD scheme
Reference:
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