Section 1 of 5Medical Tracers
A tracer is a small amount of a radioactive isotope that is swallowed or injected to follow the movement of a substance inside the body.
Gamma emitter
- Tracers emit gamma rays because these pass out of the body and can be detected by a camera outside.
Short half-life
- The half-life is a few hours, long enough to complete the scan but short enough that the activity soon fades.
Small quantity
- Only a tiny amount is used, so the patient receives as small a dose of radiation as possible.
Result
- The detector builds an image showing where the tracer collects, which helps doctors find problems in organs.
Section 2 of 5Treating Cancer
Treating Cancer
Radiotherapy
- Beams of gamma rays are aimed at a tumour to kill the cancer cells, which are damaged more easily than most normal cells.
Rotating the beam
- The source is moved round the patient so that healthy tissue receives only a small dose while the tumour gets a large one.
Internal sources
- Small radioactive pellets or implants can be placed in or near a tumour to irradiate it directly.
Side effects
- Healthy cells can still be damaged, which can cause illness, so doses are carefully planned.
Section 3 of 5Sterilising Equipment
Gamma radiation can sterilise medical instruments and some foods.
1
Instruments such as syringes and surgical tools are sealed in their final packaging.
2
The sealed packs are exposed to a strong source of gamma rays.
3
Gamma rays penetrate the packaging and kill bacteria and other microorganisms on the instruments.
4
The instruments do not become radioactive, because irradiation does not make objects radioactive.
5
The sterile items stay safe until the packaging is opened, without the need for heating that could damage plastics.
| Use | Type of radiation | Reason |
|---|---|---|
| Tracer | Gamma | Passes out of the body to be detected |
| Radiotherapy | Gamma | Penetrates to reach a tumour |
| Sterilising | Gamma | Penetrates sealed packaging |
Section 4 of 5Worked Example
Question
- A tracer has an activity of 240 MBq when it is injected and a half-life of 6 hours.
- Calculate its activity 18 hours later.
- Number of half-lives = 18 ÷ 6 = 3.
- After 1 half-life: 240 ÷ 2 = 120 MBq.
- After 2 half-lives: 60 MBq.
- After 3 half-lives: 30 MBq.
Answer 30 MBq
Section 5 of 5Exam Checklist & Tips
6.2.3 – Key Points for the Exam
- Tracer A radioactive isotope used to follow the path of a substance through the body.
- Radiotherapy The use of radiation to treat cancer by killing tumour cells.
- Sterilisation Killing microorganisms on an object so that it is safe to use.
- Gamma rays Highly penetrating electromagnetic radiation emitted by some unstable nuclei.
Common Mistakes
- Choosing an alpha emitter as a tracer; alpha cannot leave the body to be detected.
- Giving a very long half-life for a tracer, which would leave the patient radioactive for too long.
- Writing that radiation is harmless in medicine, when the benefits only outweigh the risks.
- Saying that sterilised instruments become radioactive after exposure.