Lecture 1 & 2 - Radiation Absorption and Ionizing Radiation-Induced Cell Damage

What is radiation biology?

a field of clinical and basic medical sciences that involves the study of action of ionizing radiation on living things

Radiation’s interaction with matter can cause two things, what are they?

1. excitation

2. ionization

Excitation

raising of electrons into higher orbitals/energy states

• BOND DOES NOT BREAK

Ionization

release of bound electron from the atom

• ion is created

Two types of ionizing radiation

1. electromagnetic radiation

2. particulate radiation

X-Rays vs. Gamma rays

Energies are similar, but source is different

• X-rays come from electron release

  • bremsstrahlung or characteristic

• Gamma comes from nucleus energy release

  • photon released from decay

what are the three types of ionizing electromagnetic waves?

1. X-Rays

2. Gamma

3. high energy UV rays (sometimes)

Photon speed, charge and mass

Speed = c

mass = none

charge = none

• pass undeflected through B field

Do photons interact with matter?

yes they can, but they can also pass through matter unimpeded and unchanged

How does the energy of electromagnetic waves relate to it’s frequency and wavelength

E=hv = E=h(c/wavelength)

• energy is proportional to frequency, and inversely proportional to wavelength

What does quantum mechanics suggest

some physical quantities exist and can change and interact, but only by discrete amounts (quanta) and behave probabilistically rather than deterministically

wave particle duality

a particle can behave either like a wave or a particle depending on how it is observed

heisenberg’s uncertainty principle

states that it is impossible to simultaneously and precisely measure both the position and momentum of a particle.

why is quantum physics important for radiobiology?

if photons are absorbed in living material, the energy deposited in the tissues and cells is deposited in discrete packets

what is the difference between nonionizing and ionizing radiation?

the size of the individual energy packets

• not total energy is involved

  • 1 Gy of the dose is stronger than 10 fractions of 10 cGy

What is the lethal dose of ionizing radiation

4 Gy (4 J/Kg)

• average man = 75 kg, so 300 J

  • 300 J is the same amount of heat energy transferred to bodt after drinking a 3.1 mL sip of coffee at 60 degrees celsius

What are electronic orbitals

regions of highest probability of where the electron will most likely be

types of particulate radiation

electrons, protons, alpha particles, neutrons, etc

electron mass, charge, and radioactive decay

mass: 9.11 × 10^-31 kg

charge: negative

radioactive decay: beta minus

How are electrons used in radiotherapy

due to their negative charge, electrons can be accelerated in an electromagnetic field to either be used to treat superficial tumors, or can be used to hit tungsten target to generate X-rays

Proton charge, mass, use in therapy

charge: positive

mass: 1836x mass of electron

therapy: proton therapy, bragg peak is more OAR sparing

Neutron charge, mass, radioactivity

charge: none

mass: slightly heavier than protons

radioactivity: can be used to collide with nuclear protons, ejecting them

• can be used to produce ionizations and excitations with these protons

• neutrons also used in fission reactions

where can humans be exposed to neutrons more frequently?

flying

alpha particles

nuclei of helium atom (2 P, 2 N), postively charged

• radium decay produces alpha particles

Examples of heavy charged particles that may be used in radiotherapy

carbon, neon, sillicon, argon

two types of ionizing radiation

1. directly ionizing

   • by charged particles with sufficient KE to break bonds

2. indirectly ionizing

   • by producing charged particles with high KE

What are the three electromagnetic interactions photons can undergo with subatomic structures?

1. photoelectric absorption

2. compton scattering

3. pair production

How is the probability of the interaction determined?

based on the energy of the photon and the atomic number of the target atom

photoelectric absorption

compton scattering

inelastic scattering of a photon by a charged particle

• energy of photon is decreased as part of energy is transferred to recoil e-

How does the compton effect vary with atomic number

compton cross section is independent of atomic number

how does the photoelectric effect vary with atomic number

cross section depends strongly on atomic number of absorbing material

• Z³

At what energies (low or high) does compton and P.E occur

Compton: prevails at higher energies

P.E: lower energies

Pair Production

for photons above the baseline of 1.022 MeV, they can interact with the electric field of the nucleus and form an electron-positron pair

• positron often annhilated with electron to release two more photons

photons energy is converted into mass of beta particles through mass-energy equivalence E=mc²

What photon energies would you use for X-ray radiation therapy, and what electromagnetic interactions would you see?

4-25 MeV

• PE, Compton, and PP all possible

What photon energies would you use for X-ray CT scans, and what electromagnetic interactions would you see?

20-150 keV

• mostly PE, potentially Compton, NO PP (E<1.022 MeV)

Direct Ionizing Radiation

dominant process for radiation with high linear energy transfer (LET)

Indirect ionizing radiation

radiation produces free radicals that interact with the target

free radical

a free radical is an atom, molecule, or ion that has unpaired valence electrons

hydroxyl radical, superoxide, and oxygen

what percent mass of a human cell does water make up

65%

what percent molecules of a human cell does water make up?

98.73%

When an indirect ionizing event occurs, what is the most likely target?

Water

When an indirect ionizing event occurs, what possible targets are there

all are theoretically possible

• DNA the least common

Radiolysis of Water Mechanism

H₂O + x-ray —> H₂O⁺+ e^-

e- pathway:

e^- + H₂O —> H₂O^-

H₂O^- —> H^. + OH^-

H₂O⁺ pathway

H₂O⁺—>H⁺ + OH^.

end product of radiolysis e^- pathway

H^. + OH^-

hydrogen free radical and hydroxide ion

end produc of radiolysis H₂O⁺ pathway

H⁺ + OH^.

Proton and hydroxyl radical

Describe the chain of event in X-Ray Absorption (indirect pathway)

Incident X-Ray beam —> recoil electron —> ion radical —> free radical —> chemical changes from bond breakage —> biological effects

Time for initial ionization to occur

10^-15 s

Ion radical life time

10^-10 s

Free radical lifetime

10^-5 s

• enough time to float around and cause bond breakage in cells

Time scale of radiation physics

10^-12 - 10^-15 s

• ionization, excitation, radical formation

time scale of radiation chemistry

10^-6 - 10^-9 s

• DNA damage formation

time scale of radiation biology

10³-10⁶ seconds

• repair replication, cell death, cancer, heritable mutations

Linear Energy Transfer (LET) definition

how much energy an ionizing particle transfers to material transversed per unit distance

What two factors does LET depend on

nature of radiation

nature of material

what travels deeper in tissue? high LET or low LET

Low LET

• high LET gives more energy into material, so may run out of energy before getting deeper into tissue

• low LET gives less energy to material, allowing it to go deeper into tissue

When could a high LET be used

to cause more severe damage to any microscopic structures near the particle tract

Gamma/X-Rays: high or low LET

Low LET, thus they need more shielding as they can penetrate deeper

alpha particles: high or low LET

high LET, less shielding needed