Photon Interactions

Question 1

What happens in photoelectric absorption?

Answer 1

The incoming photon must have enough energy to overcome the binding energy of the inner orbital electron
Photon is completely absorbed
Residual energy is given as kinetic energy to the electron, allowing it to escape
Atom is left with a vacancy, so an electron from a higher shell drops down to fill this space

Question 2

Does photoelectric absorption depend on atomic number?

Answer 2

Yes.

Absorption of xrays is proportional to Z^3

Question 3

Are more photons absorbed by photoelectric absorption in soft tissue or bone? Why?

Answer 3

Bone.

Because bone has a higher atomic number.

Question 4

What is the most dominant interaction process at radiotherapy energies?

Answer 4

Compton scatter

Question 5

What happens in compton scatter?

Answer 5

Incident photon transfers SOME of it’s energy to the electron
Electron is ejected from atom
Photon continues with less energy in a different direction

Question 6

Does compton scatter depend on atomic number?

Answer 6

No

Question 7

At what energies does pair production occur?

Answer 7

Above 1.022 MeV

Question 8

What happens in pair production?

Answer 8

Photon interacts with the NUCLEUS
Energy is transferred to the atom, annihilating the photon
An electron and a positron are created
Positrons are short lived and disappear when they meet an electron and are annihilated (forming 2 photons each of 0.51 MeV)
Electrons cause damage until captured by an atom

Question 9

Does pair production depend on atomic number?

Answer 9

Yes.

As is is dependent on the nucleus, a higher Z means an increased probability of pair production

Question 10

What is the concept of photons as ‘carrier waves’ in radiotherapy?

Answer 10

Photons generate electrons at depth within the patient

It is the electrons that cause damage and kill the tumour

Question 11

How do photons damage tumour cells?

Answer 11

They don’t
They generate electrons at depth within the patient
The electrons then cause damage

Question 12

What is ionisation?

Answer 12

The process by which a neutral atom gains a charge by:
Removal of an orbital electron (making the atom positively charged)
OR
In some cases an atom can acquire an electron (making the atom negatively charged)

Question 13

What is excitation?

Answer 13

The energy given to an orbital electron is insufficient to eject it from an atom, but raises it to a higher energy level
When this electron drops back to fill the vacancy, it will release the excess energy as a photon

Question 14

Why does a single photon interaction cause a chain of ionising events?

Answer 14

Electrons are small, light, and have charge, so have a higher probability of interacting
Each interaction can release another electron that can go on to interact, and so forth

Question 15

Why is most of an electron’s energy deposited locally (in the neighbourhood of the initial photon interaction)?

Answer 15

Because every time it interacts, it loses energy and slows down

Question 16

Is a photon beam monoenergetic or polyenergetic?

What does this mean?

Answer 16

Polyenergetic

It consists of a large number of photons with a spectrum of energies

Question 17

What is fluence?
What is the equation?
What are the units?

Answer 17

The number of photons passing through a sphere of cross sectional area ‘a’ in space
Φ = dN/da
unit = m^-2

Question 18

What is fluence rate?
What is the equation?
What are the units?

Answer 18

Fluence per unit time
dΦ/dt
unit = m^(-2)s^(-1)

Question 19

What is energy fluence?
What is the equation?
What are the units?

Answer 19

The sum of all the energies of all the photons passing through a sphere of cross sectional area ‘a’
It is the total energy carried by the photons
Ψ = dE/da
unit = Jm^-2

Question 20

What is the inverse square law?

Answer 20

The intensity of the radiation field is inversely proportional to the square of its distance from the source
I α 1/d^2

Question 21

What is attenuation?

Answer 21

The reduction in intensity of the beam due to interactions

Question 22

What are the 3 possible outcomes of attenuation (with regards to tumour kill)

Answer 22

Absorption (possible tumour kill)
Scatter (intensity reduced but no tumour kill)
Exit (beam intensity maintained)

Question 23

What are the 2 main factors affecting fluence at a point in tissue?

Answer 23

Inverse square law

Attenuation

Question 24

Does the chance of interactions increase or decrease as the photon beam is attenuated?
Why?

Answer 24

Decrease

Because there are less photons

Question 25

What is the linear attenuation coefficient?

What equation can we use to find it?

Answer 25

It describes the fraction of attenuated photons per unit thickness
I = I0e^(-μx)
Where I0 is the initial intensity, and μ is the linear attenuation coefficient

Question 26

What is the mass attenuation coefficient?

How can it be calculated?

Answer 26

The attenuation per unit mass rather than unit length

Calculated by dividing the linear attenuation coefficient by density

Question 27

What is the difference between primary radiation and scatter?

Answer 27

Primary radiation is the radiation reaching and depositing energy directly at our point of interest
Scatter arrives indirectly

Question 28

Does scattered radiation contribute dose to the patient?

Answer 28

Yes

Question 29

Is primary radiation dependent on field size?

Answer 29

No

Question 30

What are the sources of scatter?

Answer 30

Head scatter/leakage
Patient scatter (scatter in the patient due to interaction processes)

Question 31

Is head scatter the same for all linacs?

Answer 31

It varies between manufacturers

Question 32

Is scattered radiation dependent on field size?

Answer 32

Yes

The higher volume irradiated means more chance of scattered photons

Question 33

Can scatter be good?

Answer 33

Yes

It contributes dose to the tumour and gives is skin sparing