A hospital is considering using iodine-131, a beta and gamma emitter with a half-life of 8 days, to treat thyroid cancer. The thyroid gland absorbs iodine naturally. Explain why iodine-131 is suitable for use as a cancer treatment in this context, and explain one risk associated with its use.

OCR A-Level Physics B: Advancing Physics (H557) — 6.3 Nuclear and particle physics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Iodine-131 is a radioactive isotope that emits both beta particles and gamma rays. It has a half-life of 8 days. When a patient swallows iodine-131, the thyroid gland absorbs it because the thyroid naturally takes up iodine from the bloodstream.

Model answer (5 marks)

Iodine‑131 is suitable because it is taken up selectively by the thyroid, concentrating the radioactivity in the tumour. The beta particles emitted ionise the DNA of the cancer cells, destroying them, while the accompanying gamma rays penetrate the surrounding tissue and can reach any residual tumour cells that the beta particles miss. The 8‑day half‑life means the activity falls to negligible levels within a few weeks, limiting long‑term exposure.

A risk is that the gamma radiation can also irradiate healthy tissue around the thyroid, potentially damaging normal cells and causing side effects such as dry mouth, skin irritation or secondary cancers.

Examiner tips

  • Mention selective uptake by the thyroid, beta‑induced cell death, gamma penetration, and short half‑life for safety. Include a specific risk such as damage to surrounding healthy tissue.

Common mistakes

  • Failing to note that the thyroid concentrates the isotope; writing only about beta radiation; not mentioning the short half‑life; giving a vague or unrelated risk.

Mark scheme (5 marks)

  1. Gamma radiation is the most penetrating type of radiation and can pass through body tissue to reach the cancerous thyroid cells.
  2. The radiation (beta/gamma) ionises and kills the cancerous cells in the thyroid, controlling or destroying the tumour.
  3. The thyroid gland naturally absorbs iodine, so the iodine-131 concentrates at the target site (the thyroid), reducing exposure of other organs.
  4. The half-life of 8 days is short enough that the activity declines relatively quickly, reducing the long-term radiation risk to the patient.
  5. One risk: surrounding healthy tissue may also be irradiated by the gamma radiation, damaging or killing healthy cells and causing harmful side effects.

Key terms in this question

half-life

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