7.6 Nuclear Radiation in Medicine

A free GCSE Physics lesson for AQA (8463) from OBA School: tracers, gamma imaging and radiotherapy; choosing an isotope by its radiation and half-life; weighing risk against benefit. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how radioactive tracers and gamma cameras are used to look inside the body
• Describe how gamma beams, implants and absorbed isotopes are used to destroy cancer cells
• Choose a suitable isotope for a medical use from its type of radiation and half-life
• Evaluate the risks and benefits of using nuclear radiation in medicine

Key words

• Tracer: A small amount of a radioactive isotope put into the body so its path can be followed from outside with a detector.
• Gamma camera: A detector that builds up an image of where a gamma-emitting tracer has collected in the body.
• Radiotherapy: Using ionising radiation to kill cancer cells.
• Implant: A small sealed radioactive source placed inside or next to a tumour.
• Dose: A measure of the risk of harm from radiation absorbed by the body, measured in sieverts (Sv) or millisieverts (mSv).
• Risk–benefit: Weighing the possible harm of a procedure against the good it does.

Choosing the right isotope

For every medical use, doctors must pick an isotope with the right type of radiation and the right half-life. It should also be non-toxic and, ideally, decay into a stable product so the patient is not left with another radioactive substance.

• technetium-99m; Radiation γ; Half-life 6 hours; Typical use tracer for gamma camera images of bones, heart, lungs
• iodine-123; Radiation γ; Half-life 13 hours; Typical use imaging the thyroid gland
• iodine-131; Radiation β (and some γ); Half-life 8 days; Typical use destroying overactive or cancerous thyroid tissue
• iodine-125; Radiation low-energy γ; Half-life 60 days; Typical use permanent seed implants in tumours
• cobalt-60; Radiation γ; Half-life 5.3 years; Typical use source for external gamma beams

Key fact: Looking inside (diagnosis): use a gamma emitter, because gamma passes out of the body to the detector and ionises weakly. Killing cells (treatment) inside the body: use beta (or gamma), because it deposits its energy close to the source.

Tracers and gamma imaging

A tracer is injected, swallowed or breathed in. It travels with the blood or is taken up by one organ. A detector outside the body follows where it goes. A gamma camera uses a lead plate full of narrow holes, so only gamma rays travelling straight up reach the detector. A computer turns the signals into an image showing where the tracer has collected.

The half-life must be long enough for the tracer to reach the organ and for the scan to be done, but short enough that the activity in the patient soon falls to almost nothing. A few hours is ideal.

Worked example: how much is left?

A patient is given technetium-99m with an activity of 400 MBq. The half-life is 6 hours. What is the activity 24 hours later?
n = 24 ÷ 6 = 4 half-lives; 400 ÷ 2⁴ = 400 ÷ 16 = 25 MBq

Answer: 25 MBq, so the activity has fallen to 1/16 in a day

Treating cancer with radiation

• External gamma beams. Narrow beams of gamma rays (from cobalt-60 or from an X-ray machine) are aimed at the tumour from several directions. The beams overlap only at the tumour, so it gets a large dose while healthy tissue gets much less.
• Implants. Small sealed seeds or wires containing a beta or gamma emitter are placed in or next to the tumour. Permanent seeds use an isotope whose half-life is long enough to give the tumour a full dose over some weeks, but short enough that the activity then dies away.
• Absorbed isotopes. The thyroid gland takes up iodine. A patient drinks iodine-131, which collects in the thyroid, and its beta particles destroy thyroid cells only a few millimetres away.

Worked example: treatment time

Iodine-131 has a half-life of 8 days. How long until its activity in the thyroid falls to 1/8 of the starting value?
1/8 = 1/2³, so 3 half-lives; 3 × 8 = 24 days

Answer: 24 days

Weighing risk against benefit

All ionising radiation carries some risk of damaging healthy cells and causing cancer later. Doctors only use radiation when the benefit (finding or treating a serious illness) is greater than the risk. They keep the dose as low as possible: the smallest activity that does the job, the shortest suitable half-life, and shielding of other parts of the body.

Hospital staff protect themselves with lead screens and aprons, by keeping their distance and time near patients and sources short, and by wearing a radiation monitor badge. Patients with a large activity inside them may be asked to keep away from others for a short time.

Exam tip: A typical gamma scan gives a dose of a few millisieverts (mSv), similar to one or two years of natural background radiation. The risk is small, and a correct diagnosis can save a life.

Common mistakes

• Choosing an alpha emitter as a tracer. Alpha cannot get out of the body to the detector, and it is the most damaging inside the body.
• Saying the half-life should be “as short as possible”. Too short and the tracer decays before the scan is finished.
• Thinking the gamma camera sends out radiation. It only detects radiation coming from the tracer in the patient.
• Forgetting the other properties: the isotope should be non-toxic and ideally decay to a stable product.

Revision questions

Try these, then check your answers below each one.

Question 1. Which type of radiation should a tracer emit if a camera outside the body must detect it? (Choose from: gamma, alpha, beta, neutrons.)

Answer: gamma

Working: Gamma passes out of the body to the detector and is only weakly ionising.

Question 2. Which half-life is most suitable for a tracer used in a medical scan? (Choose from: a few hours, a few seconds, a few years, thousands of years.)

Answer: a few hours

Working: A few hours: long enough to complete the scan, short enough that the activity soon falls to almost nothing.

Question 3. A patient is injected with a tracer of activity 800 MBq. The half-life is 6 hours. What is the activity after 12 hours, in MBq?

Answer: 200 MBq

Working: 12 ÷ 6 = 2 half-lives; 800 → 400 → 200 MBq.

Related lessons

• Previous lesson: 7.5 Activity and Half-life
• Next lesson: 7.7 Nuclear Fission
• In this chapter: 7.1 Atoms and Radiation
• In this chapter: 7.2 The Discovery of the Nucleus
• In this chapter: 7.3 Changes in the Nucleus

Continue in the full course

This lesson is section 7.6 (AGCPH0706) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.