Radiotherapy in Cancer Management Flashcards
Why is radiation a two edged sword?
Can treat and cause cancer
What can radiotherapy be used with?
- Surgery
- Chemotherapy
- Immunotherapy
When would it be used with surgery?
Local control of disease
When would it be used with chemotherapy/immunotherapy?
Palliation for improved QoL eg in alleviating painful bone metastasis
What are the 3 types of RT?
- External beam radiotherapy
- Brachytherapy (sealed source)
- Unsealed source
What is the aim of radiotherapy?
To maximise does to the tumour and minimise dose to the normal tissue
Is RT potent?
Yes
Is RT cost effective?
Yes
What % of cancer patients require RT at some point?
50%
How many of those treated with RT are treated with curative intent?
60%
What is the survival rate for those treated with RT?
> 70%
What do electromagnetic radiations interact with?
Electrons
What can absorption of radiation lead to?
- Excitation (raising e- to higher level)
- Ionisation (ejection of e-)
What does x-ray absorption depend on?
- The energy of the photon
- Chemical composition of the absorbing tissue
What process occurs with high energy photons?
The compton process - fast electrons produced and a deflected/scatterd photon with a lower energy.
What level of photon energy does diagnostic radiology use?
Lower energy
What is the process that occurs with lower energy photons?
Photoelectric process - fast electrons produced but the photon is entirely absorbed.
Why is the compton process used for radiation therapy?
It does not depend on atomic number of the absoring species so the problem of differential absorption by different tissues is avoided. Don’t want bone to be able to shield a tumour.
How can ionising radiation act at a molecular level?
- Directly
- Indirectly
What happens in directly acting ionisation?
The atoms of the target molecule aka DNA are ionised
What happens in indirectly acting ionisation?
The radiation interacts with other molecules to produce free radicals that migrate to the DNA
Can direct acting ionisation be modified?
Generally not
Can indirect acting ionisation be modified?
Yes, by sensitisers and protectors
What is an important quality of ionising radiation?
The energy is not uniformly released and is deposited unevenly.
What is the biological effect of ionising radiation determined by?
Photon energy size ie how concentrated is the beam? Very. This is why its so lethal.
What is the principle target for the effects of ionising radiation?
DNA
What kind of damage is ionising radiation very likely to cause?
Double stranded breaks
What other damage can ionising radiation cause?
- Base damage
- Sugar damage
- Single strand breaks
What are unrepaired DSBs thought to be?
Critical cell-killing lesions
How is the dose of radiation administered?
It is fractionated
What is the benefit of fractionation?
It spares normal tissue by allowing time for damage repair beteen doses (normal tissue recovers better than tumour tissue)
Why does normal tissue recover better than tumour tissue?
Normal tissue has a full compliment of DNA so full compliment fo DNA repair mechanisms. Tumour tissue has compromised DNA repair.
What is expressed by tumours when they become hypoxic?
Hypoxia inducable factor 1 (HIF-1) in region of hypoxia
What is the problem by hypoxic tumour cells?
They are radioresistant but still viable
How can we exploit this?
Dose fractionation - radiation kills some oxic cells. This allows reoxygenation of some of the hypoxic cells, which can then be targetted in the next dose of radiation.
What does multi-beam radiotherapy allow?
We can superimpose the dose over the tumour bearing region so the tumour gets a high does but the adjacent tissues are spared somewhat.
What can be used alongside superimposing the beams?
Multileaf Collimators
What do multileaf collimators do?
Shape the beam to the tumour volume
What is this combination called?
3D-Conformal Radiotherapy (3D-CRT)
What is the standard treatment in the UK?
3D-CRT
What is an Arc IMRT?
New method of radiation using a 360 degree beam
What is the downside of Arc IMRT?
No tissue is spared completely - all tissues get at least a small dose of radiation.
What is the Bragg peak?
Pronounced peak on the Bragg curve which plots energy loss of ionising radiation as it travels through matter
Where does the Bragg peak occur for protons?
Immediately before the proton comes to rest
What is the clinical relevance of the Bragg peak for protons?
It is lower than the exposure given from photons/x-ray beam so overall lower level of exposure to surrounding tissues.
What is the SOBP?
The spread out bragg peak - the sum of the indiviual Bragg peaks at staggered depths.
So why are protons better?
They can be targetted to stop at a certain point, where as x-rays go through the point and out the other side, causing more damage
Why is radiation a weak carcinogen and mutagen?
Its such a good cell killing agent - dead cells don’t cause cancer.
