physics: Dosimetry

Question 1

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

Answer 1

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

Question 2

what is photon flux

Answer 2

the rate of photo fluence per second

Question 3

if all photons have the same energy then…

Answer 3

energy fluence =energy per photon x photon fluence

energy flux= energy per photon x photon flux

Question 4

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

Answer 4

sum the energy fluences for each energy

Question 5

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

Answer 5

• Kinetic Energy Released in Matter

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

Question 6

what is the units for kerma

what is kerma directly propotional to

Answer 6

gray

photon energy fluence

Question 7

KERMA can be split into two components (electrons)

Answer 7

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

Question 8

what is exposure

unit

Answer 8

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

Question 9

which paricles stopping power are we most interested in

Answer 9

electrons

Question 10

what is the mass stopping power

Answer 10

rate of energy lost per gram per cm^2

Question 11

what is stopping power

Answer 11

rate of energy lost per cm

Question 12

• what is the total stopping power
• equation
• sometimes called…

Answer 12

• is the sum of all energy losses
• Stot = Scol+Srad
• Selectronic and Snuclear

Question 13

stopping power is greater for…

why?

Answer 13

low atomic numbers

Question 14

what does fluence and flux tell us?

Answer 14

how much energy is in the beam

Question 15

what does exposure tell us

Answer 15

how much charge is produced

Question 16

what is work function

Answer 16

the amount of energy required to produce ionizing radiation

Question 17

what is centigray used for

Answer 17

routine radiotherapy absorbed dose usage

Question 18

what is milligray used for

Answer 18

diagnostic radiology absorbed dose usage

Question 19

what is millisieverts

Answer 19

radiation protection equivalent dose usage

Question 20

what produces dose

Answer 20

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

Question 21

refer to graph
in first region of graph…
why?

Answer 21

kerma and absorbed dose are not the same.

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

Question 22

At a certain depth (dmax) …

aka

Answer 22

kerma ends up the same as absorbed dose.

electron equilibrium or Ion pair equilibrium.

Question 23

further into the patient than dmax..

Answer 23

absorbed = kerma and just decrease following the usual processes

Question 24

in the build up region…

Answer 24

there is v little dose being absorbed

at skin surface this is know as skin sparing effect

Question 25

percentage depth dose

Answer 25

%Dn = (Dn/D0) x 100 ``` Dn= central axis depth dose (@ a particular point) D0= max dose along axis ```

Question 26

for small field size

Answer 26

the central axis dose is entirely un-interacted primary photons

Question 27

for larger field size

Answer 27

scatter photons in all directions but mostly foward directions

Question 28

primary photon is ____ by field size

Answer 28

unchanged BUT | scattered photon contribution increases when field size increases

Question 29

as photon energy increases...

Answer 29

foward scatter effects increases

Question 30

percentage depth dose tells us ..

Answer 30

characterisation of distribution of dose along central axis of beam

Question 31

4 different ways to characterise dose

Answer 31

tissues air ratio tissue phantom ratio TMR percentage depth dose

Question 32

what does normalizing mean

Answer 32

turning 100% into max value | eg 73% of whatever we found at dmax

Question 33

the higher the energy ...

Answer 33

the more dmax goes further into patient

Question 34

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

Answer 34

MV/ 4 = dmax in cm

Question 35

detector chamber gives ...

Answer 35

value of dmax at every depth in patient

Question 36

how to create the table

Answer 36

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.

Question 37

field size | if you have a small feild size

Answer 37

it will be mostly primary photons

Question 38

if you increase feild size ....

Answer 38

⇡ PDD and values of percentage | ⇡side scatter hitting central axis

Question 39

when photon energy increase

Answer 39

⇡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.

Question 40

high energy beam , amount of side scatter ....

Answer 40

is low to start w and doesnt change v much FOR LOW EN BEAM .. THIS HAS A BIGGER EFFECT

Question 41

BACKSCATTER | what is phantom

Answer 41

it scatters energy back we try make phantom bigger than a patient is ever likely to be.

Question 42

percentage back scatter

Answer 42

ratio of scattered to primary radiation

Question 43

back scatter - in diagnostic energy - high mV energies

Answer 43

- PE swamps CE therefore BS is low | - fowards scatter swamps backscatter

Question 44

UNDERLYING TISSUE | how is %DD usually calculated

Answer 44

w/enough tissue below target point to allow back scatter

Question 45

what is exit dose

Answer 45

absorbed dose delivered to surface where beam emerges

Question 46

equivalent square | sterlings rule

Answer 46

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

Question 47

How does the changing SSD affect the depth dose

Answer 47

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

Question 48

absolute dose rate...

Answer 48

decreases w inverse square law

Question 49

higher density medium...

Answer 49

will scatter more as there is more electrons to interact w

Question 50

the density of: lung bone

Answer 50

* 0.25g/cm^3 | * 1.8g/cm^3

Question 51

Tissue Inhomogeneities affect

Answer 51

primary beam | scatter

Question 52

the PDD depends on

Answer 52

``` field size medium energy ssd shape ```