Radiotherapy in Cancer Management

Question 1

Why is radiation a two edged sword?

Answer 1

Can treat and cause cancer

Question 2

What can radiotherapy be used with?

Answer 2

• Surgery
• Chemotherapy
• Immunotherapy

Question 3

When would it be used with surgery?

Answer 3

Local control of disease

Question 4

When would it be used with chemotherapy/immunotherapy?

Answer 4

Palliation for improved QoL eg in alleviating painful bone metastasis

Question 5

What are the 3 types of RT?

Answer 5

• External beam radiotherapy
• Brachytherapy (sealed source)
• Unsealed source

Question 6

What is the aim of radiotherapy?

Answer 6

To maximise does to the tumour and minimise dose to the normal tissue

Question 7

Is RT potent?

Answer 7

Yes

Question 8

Is RT cost effective?

Answer 8

Yes

Question 9

What % of cancer patients require RT at some point?

Answer 9

50%

Question 10

How many of those treated with RT are treated with curative intent?

Answer 10

60%

Question 11

What is the survival rate for those treated with RT?

Answer 11

> 70%

Question 12

What do electromagnetic radiations interact with?

Answer 12

Electrons

Question 13

What can absorption of radiation lead to?

Answer 13

• Excitation (raising e- to higher level)

- Ionisation (ejection of e-)

Question 14

What does x-ray absorption depend on?

Answer 14

• The energy of the photon

- Chemical composition of the absorbing tissue

Question 15

What process occurs with high energy photons?

Answer 15

The compton process - fast electrons produced and a deflected/scatterd photon with a lower energy.

Question 16

What level of photon energy does diagnostic radiology use?

Answer 16

Lower energy

Question 17

What is the process that occurs with lower energy photons?

Answer 17

Photoelectric process - fast electrons produced but the photon is entirely absorbed.

Question 18

Why is the compton process used for radiation therapy?

Answer 18

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.

Question 19

How can ionising radiation act at a molecular level?

Answer 19

• Directly

- Indirectly

Question 20

What happens in directly acting ionisation?

Answer 20

The atoms of the target molecule aka DNA are ionised

Question 21

What happens in indirectly acting ionisation?

Answer 21

The radiation interacts with other molecules to produce free radicals that migrate to the DNA

Question 22

Can direct acting ionisation be modified?

Answer 22

Generally not

Question 23

Can indirect acting ionisation be modified?

Answer 23

Yes, by sensitisers and protectors

Question 24

What is an important quality of ionising radiation?

Answer 24

The energy is not uniformly released and is deposited unevenly.

Question 25

What is the biological effect of ionising radiation determined by?

Answer 25

Photon energy size ie how concentrated is the beam? Very. This is why its so lethal.

Question 26

What is the principle target for the effects of ionising radiation?

Answer 26

DNA

Question 27

What kind of damage is ionising radiation very likely to cause?

Answer 27

Double stranded breaks

Question 28

What other damage can ionising radiation cause?

Answer 28

- Base damage - Sugar damage - Single strand breaks

Question 29

What are unrepaired DSBs thought to be?

Answer 29

Critical cell-killing lesions

Question 30

How is the dose of radiation administered?

Answer 30

It is fractionated

Question 31

What is the benefit of fractionation?

Answer 31

It spares normal tissue by allowing time for damage repair beteen doses (normal tissue recovers better than tumour tissue)

Question 32

Why does normal tissue recover better than tumour tissue?

Answer 32

Normal tissue has a full compliment of DNA so full compliment fo DNA repair mechanisms. Tumour tissue has compromised DNA repair.

Question 33

What is expressed by tumours when they become hypoxic?

Answer 33

Hypoxia inducable factor 1 (HIF-1) in region of hypoxia

Question 34

What is the problem by hypoxic tumour cells?

Answer 34

They are radioresistant but still viable

Question 35

How can we exploit this?

Answer 35

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.

Question 36

What does multi-beam radiotherapy allow?

Answer 36

We can superimpose the dose over the tumour bearing region so the tumour gets a high does but the adjacent tissues are spared somewhat.

Question 37

What can be used alongside superimposing the beams?

Answer 37

Multileaf Collimators

Question 38

What do multileaf collimators do?

Answer 38

Shape the beam to the tumour volume

Question 39

What is this combination called?

Answer 39

3D-Conformal Radiotherapy (3D-CRT)

Question 40

What is the standard treatment in the UK?

Answer 40

3D-CRT

Question 41

What is an Arc IMRT?

Answer 41

New method of radiation using a 360 degree beam

Question 42

What is the downside of Arc IMRT?

Answer 42

No tissue is spared completely - all tissues get at least a small dose of radiation.

Question 43

What is the Bragg peak?

Answer 43

Pronounced peak on the Bragg curve which plots energy loss of ionising radiation as it travels through matter

Question 44

Where does the Bragg peak occur for protons?

Answer 44

Immediately before the proton comes to rest

Question 45

What is the clinical relevance of the Bragg peak for protons?

Answer 45

It is lower than the exposure given from photons/x-ray beam so overall lower level of exposure to surrounding tissues.

Question 46

What is the SOBP?

Answer 46

The spread out bragg peak - the sum of the indiviual Bragg peaks at staggered depths.

Question 47

So why are protons better?

Answer 47

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

Question 48

Why is radiation a weak carcinogen and mutagen?

Answer 48

Its such a good cell killing agent - dead cells don't cause cancer.