Radiotherapy Flashcards
1
Q
What are different form of radiotherapy?
A
- Brachytherapy
- Direct application (Pleisiotherapy, Eg strontium 90 wand)
- Implantation – Iridium wires (gamma rays)
- Systemic administration – Iodine 131 in feline
- Teletherapy
- External beam
– Orthovoltage source
– Linear accelerator
– Cobalt 60
2
Q
How is teletherapy used therapeutically?
A
- Radiation produced = Linear accelerators / Natural radioactive decay
- Electromagnetic radiation = X-rays, gamma rays, Electrons
- Particle beam therapy - heavy particle
3
Q
What is the compton effect?
A
- When x-ray Photons interact with electrons they deflect + move in a different direction and lose energy
- If needed to treat a superficial tumour (skin) need to use a bolus over skin to ensure maximum dose at surface
4
Q
How is DNA damaged by radiation? What is needed to inhibit repair?
A
- Critical target for therapeutic radiation is probably DNA
– DNA is very small, so chances of an incident photon directly damaging it are low - Damage is caused by ionisation of water molecules
– Water molecules around the DNA are ionised
– Free radicals are generated
– DNA is damaged by the free radicals - O2 is needed to inhibit repair of free radical induced damage
5
Q
What causes cell death?
A
– Induction of apoptosis
– Permanent cell cycle arrest
– Mitotic catastrophe
6
Q
How is the beam shaped in a linear accelerator?
A
- With jaws - like x-ray (rectangular field)
- Tumour shaped = multileaf collimator
7
Q
Is single beam or multiple beams better for radiotherapy?
A
- Multiple beam = can increase tumour dose while sparing surrounding tissue
8
Q
Why are electrons good for superficial tumours?
A
- Directly ionising
- Loses energy rapidly as passes through tissues = ionises superficially
9
Q
What are the 4 R’s of radiotherapy?
A
- Repair
- Repopulation
- Redistribution / reassortment
- Reoxygenation
10
Q
What is repair to radiotherapy?
A
- Repair of sublethal or potentially lethal damage to cells after exposure – Occurs rapidly
- Tumour cells and normal cells generally have similar repair capacities – Some tumours are really good at it e.g. malignant melanoma
- Total dose of radiation required to kill cells is less if a few large doses rather than lots of smaller doses are given – Fractionation
11
Q
What is repopulation after radiotherapy? What tumours are good at repopulating?
A
- Seen in rapidly dividing tissues
- Cells are recruited from G0 (resting phase)
- Protects rapidly dividing normal tissues
- Rapidly dividing tumours also repopulate effectively
12
Q
What is redistribution / reassortment after radiotherapy?
A
- Cells are more sensitive to radiation in some phases of the cell cycle than others (late G2 (pre-mitotic interval) and M (mitosis))
– Cells may become synchronised in the post treatment period
– Synchrony is soon lost
– Timescale over which redistribution occurs is variable and poorly defined
13
Q
What is reoxygenation after radiotherapy?
A
- Important factor
- Many solid tumours have poor blood supply – Sinusoidal vessels, poorly formed microvasculature
- Areas of hypoxia/necrosis
- Reoxygenation may occur after therapy
– Euoxic cell death
– Changes in tumour vascularity
14
Q
What is fractionation?
A
- Two doses of radiation given at separate times have less effect than the sum of the two doses given as a single treatment
– Between treatments cells can REPAIR sublethal damage
– Normal tissue and tumour tissue can REPOPULATE from cells that are in resting phases/cycle arrest - Fractionation is the practice of giving multiple small doses instead of one big one
15
Q
Why bother with fractionation?
A
- Reduce normal tissue toxicity =
– REPAIR
– REPOPULATION - Achieve better tumour cell kill =
– REOXYGENATION
– REDISTRIBUTION
