0508 - Radiotherapy

Question 1

Define Ionising Radiation

Answer 1

Radiation with enough energy to remove electrons from the orbit of an atom, causing that atom to become charged or ionised.

Question 2

What are the types of ionising radiation used (particles)

Answer 2

Can use electrons, photons, or protons (less important)
Electrons dissipate just under the skin, good for breast.
Photons have a slow decay, so can hit the middle of the body from different angles.
Protons have a ‘Bragg Peak’ over a few mm in the center of the body.

Question 3

Describe the cellular target of radiation

Answer 3

Directly - Electrons are ejected from the atom, traverses DNA and causes breaks
Indirectly (more common) - Electrons interact with water, producing Hydroxyl free radicals that interact with DNA to cause breaks.

Question 4

What are the three possible outcomes for DNA that is broken by radiation?

Answer 4

Undergo repair
Cell death (apoptotic or mitotic - at next mitosis)
Cause mutations that can lead to carcinogenesis or heritable mutations.

Question 5

Develop an understanding of why Radiotherapy is fractionated (4 R’s)

Answer 5

Repair - Normal and malignant cells can repair, but normal cells are better at it.
Redistribution - Cells moving from a radio-resistant phase to radiosensetive phase of the cell cycle. Hit it another time (fraction) to get it when it’s sensitive.
Repopulation - Doubling time of a tumour decreases with treatment, so want to catch it before it doubles and when it’s undergoing mitosis.
Reoxygenation - Presence of oxygen prevents repair of the DNA breaks by binding to hydroxyl. By fractionating, you can get it when it’s oxygenated.
Fractionate to take advantage of these changes in the cell cycle.

Question 6

What is meant by therapeutic ratio?

Answer 6

Getting right balance between side effects and probability of cure/successful treatment. Not a question of avoiding side effects or definitely curing the cancer, but a conscious decision of trade-off.