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PPCC > physics: Dosimetry > Flashcards

physics: Dosimetry Flashcards

1
Q

Radiation Intensity: what is photon fluence (N/A)

A

no. of photons (N) crossing a unit area (A)

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2
Q

what is photon flux

A

the rate of photo fluence per second

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3
Q

if all photons have the same energy then…

A

energy fluence =energy per photon x photon fluence

energy flux= energy per photon x photon flux

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4
Q

if there are different energies in photons then you need to…

A

sum the energy fluences for each energy

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5
Q

Radiation Dose and Kerma: what does KERMA stand for and what does it mean

A
  • Kinetic Energy Released in Matter

- sum of initial kinetic energies of all IP released (by uncharged particles photons) in a particular mass of medium

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6
Q

what is the units for kerma

what is kerma directly propotional to

A

gray

photon energy fluence

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7
Q

KERMA can be split into two components (electrons)

A

-inelastic collisions (ionisation and excitation of atomic electrons)
• radiative collisions with atomic nuclei (bremsstrahlung)

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8
Q

what is exposure

unit

A

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

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9
Q

which paricles stopping power are we most interested in

A

electrons

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10
Q

what is the mass stopping power

A

rate of energy lost per gram per cm^2

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11
Q

what is stopping power

A

rate of energy lost per cm

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12
Q
  • what is the total stopping power
  • equation
  • sometimes called…
A
  • is the sum of all energy losses
  • Stot = Scol+Srad
  • Selectronic and Snuclear
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13
Q

stopping power is greater for…

why?

A

low atomic numbers

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14
Q

what does fluence and flux tell us?

A

how much energy is in the beam

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15
Q

what does exposure tell us

A

how much charge is produced

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16
Q

what is work function

A

the amount of energy required to produce ionizing radiation

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17
Q

what is centigray used for

A

routine radiotherapy absorbed dose usage

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18
Q

what is milligray used for

A

diagnostic radiology absorbed dose usage

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19
Q

what is millisieverts

A

radiation protection equivalent dose usage

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20
Q

what produces dose

A

photons travel into patient and interact via comptom effect.
This produces short range els
photon continues w lower en

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21
Q

refer to graph
in first region of graph…
why?

A

kerma and absorbed dose are not the same.

some energy released (kerma) in being absorbed deeper in patient.

22
Q

At a certain depth (dmax) …

aka

A

kerma ends up the same as absorbed dose.

electron equilibrium or Ion pair equilibrium.

23
Q

further into the patient than dmax..

A

absorbed = kerma and just decrease following the usual processes

24
Q

in the build up region…

A

there is v little dose being absorbed

at skin surface this is know as skin sparing effect

25
Q

percentage depth dose

A

%Dn = (Dn/D0) x 100

Dn= central axis depth dose (@ a particular point)
D0= max dose along axis
26
Q

for small field size

A

the central axis dose is entirely un-interacted primary photons

27
Q

for larger field size

A

scatter photons in all directions but mostly foward directions

28
Q

primary photon is ____ by field size

A

unchanged BUT

scattered photon contribution increases when field size increases

29
Q

as photon energy increases…

A

foward scatter effects increases

30
Q

percentage depth dose tells us ..

A

characterisation of distribution of dose along central axis of beam

31
Q

4 different ways to characterise dose

A

tissues air ratio
tissue phantom ratio
TMR
percentage depth dose

32
Q

what does normalizing mean

A

turning 100% into max value

eg 73% of whatever we found at dmax

33
Q

the higher the energy …

A

the more dmax goes further into patient

34
Q

what gives you a rough estimate on where you would expect dmax to be

A

MV/ 4 = dmax in cm

35
Q

detector chamber gives …

A

value of dmax at every depth in patient

36
Q

how to create the table

A

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.

37
Q

field size

if you have a small feild size

A

it will be mostly primary photons

38
Q

if you increase feild size ….

A

⇡ PDD and values of percentage

⇡side scatter hitting central axis

39
Q

when photon energy increase

A

⇡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.

40
Q

high energy beam , amount of side scatter ….

A

is low to start w and doesnt change v much

FOR LOW EN BEAM .. THIS HAS A BIGGER EFFECT

41
Q

BACKSCATTER

what is phantom

A

it scatters energy back

we try make phantom bigger than a patient is ever likely to be.

42
Q

percentage back scatter

A

ratio of scattered to primary radiation

43
Q

back scatter

  • in diagnostic energy
  • high mV energies
A
  • PE swamps CE therefore BS is low

- fowards scatter swamps backscatter

44
Q

UNDERLYING TISSUE

how is %DD usually calculated

A

w/enough tissue below target point to allow back scatter

45
Q

what is exit dose

A

absorbed dose delivered to surface where beam emerges

46
Q

equivalent square

sterlings rule

A

a square field is equivalent to rectangular field if the ratio of area to perimeter is the same

47
Q

How does the changing SSD affect the depth dose

A

the field size defined is the same but when the SSD changes the projection of the field size changes

48
Q

absolute dose rate…

A

decreases w inverse square law

49
Q

higher density medium…

A

will scatter more as there is more electrons to interact w

50
Q

the density of:
lung
bone

A
  • 0.25g/cm^3

* 1.8g/cm^3

51
Q

Tissue Inhomogeneities affect

A

primary beam

scatter

52
Q

the PDD depends on

A 
field size
medium
energy
ssd
shape

PPCC (48 decks)

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