Hall 1 - Physics & Chemistry of RT Absorption

Question 1

How does ionization differ from excitation?

Answer 1

Excitation = electron raised to higher energy level
Ionization = electron ejected

Question 2

How much energy is dissipated per ionizing event?

Answer 2

33 eV

Question 3

What is the minimum photon energy required to cause ionization?

Answer 3

10-25 eV
(not keV!)

Question 4

What is directly ionizing radiation?

Answer 4

Charged particles (electrons, protons, alpha)

Question 5

What is indirectly ionizing radiation?

Answer 5

EM radiation, uncharged particles (neutrons)

Question 6

What photon interaction dominates at therapeutic energies?

Answer 6

Compton

Question 7

What photon interaction dominates at diagnostic energies?

Answer 7

Photoelectric

Question 8

What is direct action?

Answer 8

Radiation directly damages DNA

DNA&raquo_space; DNA+ + e-&raquo_space; DNA*

High LET (neutrons, alpha)

No free radical intermediate&raquo_space; free radical scavengers less effective

Question 9

What is indirect action?

Answer 9

Damage DNA indirectly with hydroxyl free radicals from water lysis

H2O&raquo_space; H2O+ + e-&raquo_space; OH* + DNA&raquo_space; DNA* + H2O

Low LET (x-rays)

Hydroxyl radical is the most highly reactive and damaging species (75% of the damage)

Accounts for 2/3 of the photon effect

Question 10

Lifetime of a primary ion radical

Answer 10

10^-10 seconds

Creates a free radical

Question 11

Lifetime of a hydroxyl (free) radical

Answer 11

10^-9 seconds

Creates a DNA radical

Question 12

Lifetime of a DNA radical

Answer 12

10^-5 seconds

Breaks bonds/changes DNA

Question 13

How far can a free radical diffuse to damage DNA?

Answer 13

4nm
(2x diameter of DNA helix)