14. Biomedical Applications

Question 1

What does radiation do to tissue as it penetrates it?

Answer 1

It loses energy within it and ionise molecules and atoms

Question 2

What are the two types of damage that radiation can cause when passing through the skin?

Answer 2

1. Direct damage - by altering cellular strucutre

2. Indirect damage - creates free radicals which cause cell damage

Question 3

What is a free radical?

Answer 3

An electrically neutral atom or molecule that has an unpaired electron
- Unpaired electron is highly reactive in tissue and can cause cell structure damage

Question 4

What is the oxygen effect?

Answer 4

Irradiation of oxygen molecules can alter the original strucutre as well as create a new radical
- Increases radiation efficiency

Question 5

What does the cell ability for self repair depend on?

Answer 5

How radiation is recieved over time

• Also big initial dose is worse than the same dose spread out over time
• Also depends on the organ and the area over which the radiation is recieved

Question 6

Define the absorbed dose, D

Answer 6

Amount of energy absorbed by a unit mass measured in Grays (Gy)
1Gy = 1J/kg

Question 7

What is the equivalent dose, H_T?

Answer 7

H_T = w_R * D_T,R
w_R - weighing factor
D_T,R - absorbed dose for given radiation
Unit in Sieverts = J/kg

Question 8

What is the weighting factor, w_R?

Answer 8

Tells us how the different types of radiation can have different severities on the cell for the equivalent dose
e.g. photon, electron = 1, alpha particle= 20

Question 9

What is the effective dose, E?

Answer 9

E = sum w_T H_T
w_T is the organ sensitivity weighting factor
e.g. gonads is 0.2, skin is 0.01
Units again in Sieverts

Question 10

Describe the difference between the abosrbed dose, the equivalent dose and the effective dose

Answer 10

Absorbed - Just a number on how much is absorbed
Equivalent - Weights the radiation in terms of the irradiating particle
Effective - Weights the radiation on the irradiating particle and the organ

Question 11

For an external source, what is the most important term in considering the dose recieved

Answer 11

Only the energy of the radiation

Question 12

For an internal source, what else must be considered?

Answer 12

Half life of source and the fraction of energy deposited

Question 13

What are the main sources of external radiation for humans?

Answer 13

Natural sources such as cosmic rays and common radioisotopes (85% of annual dosage)
- Can also get internal exposure from food

Question 14

What are the main sources of internal radiation for humans?

Answer 14

Breathing in radon gas

Question 15

Give some examples of ALARA in radiation protection

Answer 15

• Plan work in advance to reduce exposure
• Increase distance from source
• SHielding

Question 16

When might a worker need to wear a passive or active dosiometer

Answer 16

Passive if radiation recieved will be above 30% of natural sources over a period of time
Active if possibility of a signficant dose in a short period of time

Question 17

How are x rays used in medical applications?

Answer 17

Measure the attenuation as they penetrate the body by measuring the ratio of intensity in front and behind the patient e.g. calcium in bones absorbs better than tissue

Question 18

How does a CT scan work and what is a downside?

Answer 18

3D images containing depth information can be constructed from several 2D images taken at different angles
- Patient exposed to more radiation

Question 19

What is a PET scan?

Answer 19

Positron emission topography

• Inject a beta plus emitter
• positron will annihilate into two photons and define a line of sight as they travel in opposite directions
• By observing many annihilations, can observe where things happen

Question 20

What is the main concern when using radiation to irradiate tumours, and what is a solution to this?

Answer 20

Tumours are less oxygenated than the surrounding tissue

• Tissue is more sensitive than the tumour to radiation
• Want to irradiate the tumour with a highly powerful, focussed dose to kill it and cause minimal damage to tissue around it

Question 21

Describe using a gamma ray of photons to destory a tumour and the advantages and disadvantages

Answer 21

Generate a collimated beam of photons from a linear accelerator - Readily available to do

• However most of them deposit their energy very early in the material, meaning any tissue in front of the tumour would recieve a higher dose than the actual tumour
• Must irradiate the tumour from different sides to optimise tumour dose/healthy cells

Question 22

Why would electrons be used for a tumour on the surface rather than photons?

Answer 22

Photons can penetrate the body easier than electrons

- For a surface tumour, electrons can spare the tissue behind it from irratiation when compared to photons

Question 23

When would hadron therapy be beneficial?

Answer 23

They lose most of their energy in their narrow bragg peak at the end of their depth penetration
- Damage infront of and behind the tumour is less than in the tumour