Use of radioactive implants (A-level only) — AQA A-Level Physics

Radioactive implants are sealed sources of ionising radiation placed directly into or very close to a tumour. This technique, called brachytherapy, allows a high dose to be delivered to the cancerous tissue while sparing surrounding healthy organs. AQA A-Level Physics requires you to understand the properties of different types of radiation, the principles of absorbed and equivalent dose, and how half-life influences the choice of isotope for implantation.

This topic sits within the Nuclear Physics section of the AQA specification and links to medical physics applications. It combines ideas from radioactivity, decay equations, and energy transfer. Exam questions often ask you to compare implants with external beam radiotherapy, calculate dose quantities, or explain why a particular isotope is chosen for a given treatment. Mastering this topic strengthens your understanding of how physics is applied in real-world medicine.

• →Brachytherapy uses sealed radioactive sources (e.g. iodine-125, iridium-192, palladium-103) implanted into or near a tumour to deliver a localised dose of radiation.
• →The choice of isotope depends on the type and energy of radiation emitted, the half-life, and the required dose rate. Gamma emitters with short half-lives are common for temporary implants.
• →Absorbed dose D = E/m, measured in gray (Gy), where E is the energy absorbed by mass m. Equivalent dose H = D x w_R, measured in sievert (Sv), accounts for the biological effect of different radiation types.
• →The activity of an implant decreases exponentially with time: A = A_0 e^(-lambda t) or A = A_0 / 2^n, where n is the number of half-lives. This affects the dose delivered over the treatment period.
• →Advantages of implants over external beam radiotherapy include: precise localisation, higher dose to tumour, lower dose to healthy tissue, and shorter treatment times. Disadvantages include: invasive procedure, limited to accessible tumours, and radiation safety issues for staff.

• Beta particles are moderately penetrating but have a finite range in tissue, so the absorbed dose is concentrated near the implant and falls rapidly with distance.
• The implant is a sealed source, so the radioactive material is contained and does not enter the patient's general circulation.
• Choice of radionuclide depends on half-life, beta energy and the required treatment duration; a suitable half-life allows a useful dose rate without unnecessary long-term irradiation.
• The clinical aim is to deliver a high dose to the tumour while keeping dose to healthy tissue and staff as low as reasonably practicable.
• Dose depends on source activity, distance from the source and exposure time, so implant placement and treatment time are planned carefully.
• Staff and patient safety involve time, distance and shielding, plus monitoring and controlled handling of the source.

• 💡Name the emission type and explain how its range in tissue produces a localised dose.
• 💡Link half-life and activity to the practical treatment schedule rather than quoting them in isolation.
• 💡Use the principles of time, distance and shielding when discussing safety for staff and the patient.
• 💡Always state the units for each quantity: Bq for activity, Gy for absorbed dose, Sv for equivalent dose, and J for energy. Marks are often lost for missing or incorrect units.
• 💡When explaining the choice of isotope, refer to specific properties: type of radiation (penetrating ability), half-life (decay rate and safety), and energy (dose deposition). Use comparative language such as 'more penetrating than' or 'shorter half-life than'.
• 💡In calculations, show all steps clearly. Convert MBq to Bq, hours to seconds, and keV to joules before substituting into equations. This reduces the chance of power-of-ten errors.

• Treating beta particles as having unlimited range: correct this by stating that beta particles have a finite range in tissue and that dose falls steeply beyond it.
• Confusing beta-emitting implants with external beam radiotherapy: correct this by explaining that the source is placed inside or adjacent to the tumour.
• Assuming the implant irradiates the whole body uniformly: correct this by describing the steep dose gradient around the source and the need to protect healthy tissue.
• Students often think that a shorter half-life always means a safer implant. In reality, a very short half-life can deliver the dose too quickly and may require frequent replacement. The ideal half-life balances dose delivery with safety and practicality.
• Many students confuse absorbed dose (Gy) with equivalent dose (Sv). Absorbed dose is energy per unit mass, while equivalent dose includes a radiation weighting factor to reflect biological harm. For alpha particles, w_R = 20, so the equivalent dose is much higher than the absorbed dose.
• Some students believe that implants emit only gamma radiation. In fact, some implants emit beta particles or a combination of beta and gamma. The type of radiation determines the shielding required and the radiation weighting factor used in dose calculations.

1. 1Day 1-2: Review the properties of alpha, beta and gamma radiation, and the definitions of activity, half-life, and decay constant. Practice converting between units (MBq to Bq, keV to J).
2. 2Day 3-4: Learn the definitions and equations for absorbed dose and equivalent dose. Work through calculations for different radiation types and masses of tissue.
3. 3Day 5-6: Study the specific isotopes used in brachytherapy (e.g. iodine-125, iridium-192) and their half-lives, radiation types, and clinical applications. Compare implants with external beam radiotherapy.
4. 4Day 7-8: Attempt past paper questions on radioactive implants, focusing on 6-mark explanation questions and multi-step calculations. Mark your answers using the official mark scheme.
5. 5Day 9-10: Create a summary table of key isotopes, their properties, and uses. Test yourself with active recall prompts and flashcards. Review any weak areas.

• 📋Calculation of absorbed dose and equivalent dose from given activity, time, energy per decay, and mass of tissue. Advice: always convert units to SI before calculating, and state the radiation weighting factor.
• 📋Explanation of why a particular isotope is chosen for an implant, referring to half-life, type of radiation, and energy. Advice: link each property to a specific benefit or risk, and use comparative terms.
• 📋Comparison of radioactive implants with external beam radiotherapy, including advantages and disadvantages. Advice: structure your answer in terms of dose localisation, healthy tissue damage, and practical considerations.
• 📋Graph or data analysis question on decay curves, asking you to determine half-life or activity after a given time. Advice: read values from the graph carefully and show your working for the number of half-lives.