Physical chemical and biological basis of radiotherapy

Question 1

Direct vs Indirect ionising radiation

Answer 1

Direct: beams consist of charged particles
Electrons
Protons
Alpha particles - 2 neutrons and 2 protons, helium nucleus.
Heavy ions - charged particles larger than a helium nucleus.

Indirect: Beams of uncharged particles
Photons and neutrons.

Question 2

Particle vs electromagnetic radiation

Answer 2

Particles or photons.

Only high energy photons can cause ionisation.
Photon radiation is divided into X-ray radiation and gamma radiation.

Gamma radiation is generated in a nucleus

X-rays are generated outside the nucleus.

Question 3

What does one Gray mean

Answer 3

1 Gray means 1 joule of energy is absorbed per 1 kg of matter.

cGy means one hundredth of a gray.

Question 4

What type of radiation is usually used in RT

Answer 4

Photon radiation.

Question 5

What are the 3 results of photon radiation interacting with matter

Answer 5

Photoelectric effect

Compton effect

Pair production.

Question 6

Photoelectric effect

Answer 6

Photons are absorbed by matter if they have an energy equal to or slightly higher than the energy of an electron in that atom.
The photon is absorbed and a photoelectron is emitted from the atom.

This occurs mostly with low energy photons between 0.01 and 0.1 MeV.

As photon energy increases the photoelectric effect no longer occurs and compton effect dominates.
Higher atomic number nuclei absorb more. Which is how X-Ray images are created.

PEA is related to the atomic number of the attenuating medium (Z), the energy of the incident photon (E) and the physical density of the attenuating medium (p) by: Z³ p / E³.

Therefore, if Z doubles, PEA will increase by a factor of 8 (because 2³ is 8) and if E doubles, PEA will reduce by 8

Question 7

Compton effect

Answer 7

At energies mostly between 0.1 and 10 MeV.

A collision between the photon and the electron transfers energy to the electron, the higher energy Compton Electron is emitted, along with the photon, which has decreased energy and goes in a new direction , called Compton Scattering.

Question 8

Pair production

Answer 8

At very high energies, mostly above 10 MeV.

The photon must have a minimum energy of 1.022 MeV, the energy value for the matter that is created - an electron and a positron.

Positrons are shortly annihliated combining with electrons and releasing two photons with 0.51 meV in opposite directions.

Question 9

How do charged particles interact with matter

Answer 9

Interaction is determined by the Coulomb forces, interacts with virtually every atom it encounters, losing some of its energy.