2. Photon Interactions

Question 1

Coherent scatter

Answer 1

• collision between photon and atom
• no energy transfer
• when collision takes place the photon changes direction
• low energy, increasing Z
• not important in RT

Question 2

Photoelectric effect

Answer 2

-photons collide with atom and electron in the orbital
-must have enough energy to over come binding
-photon gets absorbed and passes its energy to electron and atom
-empty space after electron is ejected
Net results:
- fast moving electron, initiates damage
-low energy and high atomic number and high density
-important with ortho and brachytherapy

Question 3

Compton scatter

Answer 3

• photon collides with atom
• portion of energy transferred to electron
  -overcomes binding energy
  -squires KE
  -free as stationary electron
  -photon energy changes
  -photon continues it’s way with different energy
  -high energy
  Net result
• fast moving electron, initiates damage
  -scatter can further interact with patient
  -important in RT

Question 4

Pair production

Answer 4

• energy of photon must be greater than 1.02MeV
  -higher energy
  -photon passing near nucleus subject to strong field
  -photon disappears and is replaced by electron/positron pair
  Net result
• fast moving positron and/or electron, initiate damage

Question 5

Attenuation

Answer 5

• number of photons will interact (absorbed or scattered)
• number of interactions, dn, is given by

dn =-u n dx

Where u=constant of proportionality (linear attenuation coefficient)

n= number of photons incident on thin slab

Question 6

HVL

Answer 6

Half Value Layer

- thickness that attenuates beam to 1/2 original intensity

Question 7

Indirectly ionizing

Answer 7

Net result photon/medium interaction in radiotherapy;

• fast moving electrons
• scatter, may interact further

Fast moving e initiate processes which result in damage

X-rays/ gamma rays are referred to as indirectly ionizing

Question 8

Indirect action of radiation

Answer 8

-fast moving electrons induce damage through indirect action

Interacts with other atoms/molecules-> creates a free radical-> free radical does the damage

Question 9

Free radical

Answer 9

• molecules with unpaired electron

- highly reactive

Question 10

Radiation induced cell death

Answer 10

Nonproliferating cells

• differentiated cells
• eg. Nerve, muscle
• death defined as loss of a specific function
• the s of thousands of cGy required to destroy cell function

Proliferating cells

• eg. Hematopoietic stem cells
• death defined by loss capacity for sustained proliferation
• mean lethal doses can be less than 200cGy