9) Radiotherapy

Question 1

How is radiotherapy currently used in cancer treatment?

Answer 1

Used in the local control of the disease
Can be used before or after surgery
Combined with chemo
For palliation

Question 2

What are the different types of radiotherapy?

Answer 2

External beam radiotherapy - potent
Brachytherapy (sealed source) - short range
Unsealed sources e.g. radioiodine in thyroid cancer

Question 3

What are the advantages of using radiotherapy?

Answer 3

Cost effective, curative, potent

Question 4

What are the effects of absorption of energy from radiation on atoms?

Answer 4

Excitation - raising electron to a higher energy level

Ionisation - ejection of the electron

Question 5

Explain the Compton process and which photons this happens in:

Answer 5

High energy photons (radiotherapy) - produces fast electrons that can go on to ionise other atoms and produce deflected/scattered photon of reduced energy

Question 6

Explain the photoelectric process and which photons this happens in:

Answer 6

Lower energy (diagnostic radiology) - produces fast electrons but photon entirely absorbed

Question 7

Why is the Compton process suited to radiation therapy?

Answer 7

Avoids the problem of differential absorption in tissues so can reach the tumour

Question 8

Why is the photoelectric process suited to diagnostic therapy?

Answer 8

Allows differential absorption of X rays to provide contrast between tissues on radiograph

Question 9

Explain the direct and indirect effects of ionising radiation:

Answer 9

Direct - atoms of target molecule are ionised

Indirect - production of free radicals that migrate to DNA - can be modified by protectors

Question 10

What are spurs and blobs?

Answer 10

Photon’s energy deposited in concentrated packets. Spurs are 3 ion pairs and blobs are 12 ion pairs

Question 11

Why is radiation dangerous?

Answer 11

Concentrated energy that can cause damage to DNA

Question 12

What are the effects of ionising radiation on DNA?

Answer 12

Base damage e.g. thymine glycols
Sugar damage - abasic sites
Strand breaks - single or double

Question 13

Why are double strand breaks particularly bad?

Answer 13

Critical cell killing lesions as the DSB repair is error prone

Question 14

Explain the process of fractionation:

Answer 14

Splitting of the total radiation dose into many single fractions

Question 15

What are the advantages of fractionation?

Answer 15

Better tumour control for a given level of normal tissue toxicity
Spares normal tissue by allowing repair between doses

Question 16

How can DNA damage in cancer be exploited in therapy?

Answer 16

Genomic instability in cancer cell so further genomic damage by therapy can kill cells as they are unable to repair the damage (damage to DNA repair genes)

Question 17

What are some examples of hypoxia markers?

Answer 17

HIF-1 and CA IX

Question 18

Why are hypoxic tumour cells a problem?

Answer 18

Radioresistant but still viable, also more aggressive

Question 19

How is the problem of tumour hypoxia overcome in therapy?

Answer 19

Dose fractionation allows reoxygenation of hypoxic cells so a great proportion of cells are radiosensitive

Question 20

What is multiple beam radiotherapy?

Answer 20

Allows radiologist to superimpose the X-ray dose over the tumour using multiple beams from different direction. High dose to tumour, sparing adjacent tissue e.g. liver, kidney, spinal cord

Question 21

What are the function of multileaf collimators?

Answer 21

Allows shaping of beam to tumour volume

Question 22

What is the Bragg peak and how can this be used in therapy?

Answer 22

Depth to which protons penetrate is determined by energy of beam so energy can be controlled so beam stops at target. Where protons stop is Bragg peak

Question 23

Why can proton therapy be advantageous to x-ray therapy?

Answer 23

Less damage to adjacent tissue by radiation as proton beam can stop at target tissue

Question 24

What is the risk of x-ray radiation?

Answer 24

Acts as a weak carcinogen and mutagen but small risk compared to other hazards in society