physics: Dosimetry Flashcards
Radiation Intensity: what is photon fluence (N/A)
no. of photons (N) crossing a unit area (A)
what is photon flux
the rate of photo fluence per second
if all photons have the same energy then…
energy fluence =energy per photon x photon fluence
energy flux= energy per photon x photon flux
if there are different energies in photons then you need to…
sum the energy fluences for each energy
Radiation Dose and Kerma: what does KERMA stand for and what does it mean
- Kinetic Energy Released in Matter
- sum of initial kinetic energies of all IP released (by uncharged particles photons) in a particular mass of medium
what is the units for kerma
what is kerma directly propotional to
gray
photon energy fluence
KERMA can be split into two components (electrons)
-inelastic collisions (ionisation and excitation of atomic electrons)
• radiative collisions with atomic nuclei (bremsstrahlung)
what is exposure
unit
total charge of the ions (of one sign)
produced when all the ion pairs (electrons and positrons) released by the photons in dry air are completely stopped
coulomb/kg
which paricles stopping power are we most interested in
electrons
what is the mass stopping power
rate of energy lost per gram per cm^2
what is stopping power
rate of energy lost per cm
- what is the total stopping power
- equation
- sometimes called…
- is the sum of all energy losses
- Stot = Scol+Srad
- Selectronic and Snuclear
stopping power is greater for…
why?
low atomic numbers
what does fluence and flux tell us?
how much energy is in the beam
what does exposure tell us
how much charge is produced
what is work function
the amount of energy required to produce ionizing radiation
what is centigray used for
routine radiotherapy absorbed dose usage
what is milligray used for
diagnostic radiology absorbed dose usage
what is millisieverts
radiation protection equivalent dose usage
what produces dose
photons travel into patient and interact via comptom effect.
This produces short range els
photon continues w lower en
refer to graph
in first region of graph…
why?
kerma and absorbed dose are not the same.
some energy released (kerma) in being absorbed deeper in patient.
At a certain depth (dmax) …
aka
kerma ends up the same as absorbed dose.
electron equilibrium or Ion pair equilibrium.
further into the patient than dmax..
absorbed = kerma and just decrease following the usual processes
in the build up region…
there is v little dose being absorbed
at skin surface this is know as skin sparing effect
percentage depth dose
%Dn = (Dn/D0) x 100
Dn= central axis depth dose (@ a particular point) D0= max dose along axis
for small field size
the central axis dose is entirely un-interacted primary photons
for larger field size
scatter photons in all directions but mostly foward directions
primary photon is ____ by field size
unchanged BUT
scattered photon contribution increases when field size increases
as photon energy increases…
foward scatter effects increases
percentage depth dose tells us ..
characterisation of distribution of dose along central axis of beam
4 different ways to characterise dose
tissues air ratio
tissue phantom ratio
TMR
percentage depth dose
what does normalizing mean
turning 100% into max value
eg 73% of whatever we found at dmax
the higher the energy …
the more dmax goes further into patient
what gives you a rough estimate on where you would expect dmax to be
MV/ 4 = dmax in cm
detector chamber gives …
value of dmax at every depth in patient
how to create the table
take reference measurement ( place detector chamber at dmax)
take subsequent measurements from the detector moving up and down
we ÷ each of those measurements by value of reference measurements.
field size
if you have a small feild size
it will be mostly primary photons
if you increase feild size ….
⇡ PDD and values of percentage
⇡side scatter hitting central axis
when photon energy increase
⇡forward scatter
this means field effect gets smaller for higher energy beams
eg for 4 MV beam a small change in feild size has a bigger effect.
high energy beam , amount of side scatter ….
is low to start w and doesnt change v much
FOR LOW EN BEAM .. THIS HAS A BIGGER EFFECT
BACKSCATTER
what is phantom
it scatters energy back
we try make phantom bigger than a patient is ever likely to be.
percentage back scatter
ratio of scattered to primary radiation
back scatter
- in diagnostic energy
- high mV energies
- PE swamps CE therefore BS is low
- fowards scatter swamps backscatter
UNDERLYING TISSUE
how is %DD usually calculated
w/enough tissue below target point to allow back scatter
what is exit dose
absorbed dose delivered to surface where beam emerges
equivalent square
sterlings rule
a square field is equivalent to rectangular field if the ratio of area to perimeter is the same
How does the changing SSD affect the depth dose
the field size defined is the same but when the SSD changes the projection of the field size changes
absolute dose rate…
decreases w inverse square law
higher density medium…
will scatter more as there is more electrons to interact w
the density of:
lung
bone
- 0.25g/cm^3
* 1.8g/cm^3
Tissue Inhomogeneities affect
primary beam
scatter
the PDD depends on
field size medium energy ssd shape
