DP Chemistry: Nuclear medicine
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Nuclear medicine

D.8 Nuclear medicine (4 hours)

Pause for thought

Technetium-99m

Although much of this sub-topic deals with cancer treatment, technetium-99m, the most widely used radioisotope in medicine, is only used in diagnostic radiotherapy. It is ideal for this purpose as it is a gamma radiation emitter with a half-life of 6.0 hours. This means that within 24 hours 94% of it will have decayed to technetium-99. It also has a short biological half-life of one day, which means that it is readily excreted in urine from the body. Several other factors also make it an ideal diagnostic tool. The radiation emitted can easily be followed by conventional X-ray equipment and the breakdown product, technetium-99, has a much longer half-life of 2.11 x 105 years. Technetium-99 is a beta emitter, which breaks down to form stable ruthenium-99, this also contributes to keeping the patient’s total exposure to radiation low – although it is not totally risk free.

Technetium-99m is an example of a nuclear isomer. Technically a nuclear isomer is caused by the excitation of one or more of the nucleons (protons or neutrons) of an atomic nucleus to give a metastable state. It is metastable compared to the normal excited nuclear state as it has a longer half-life as it reverts back to the ground state. Even so the half-life of a nuclear isomer is normally very short (greater than 1.0 x 10-9 s) but some metastable isomers, including technetium-99m have longer half-lives measurable in minutes or hours.

Although technetium-99m’s half-life of 6 hours makes it ideal for diagnostic treatment it does pose a problem of storage, as the hospital needs a fresh supply every day that it is going to be used. In fact, hospitals are supplied with its parent nucleus, molybdenum-99, and they use technetium-99m generators to prepare the metastable isotope through the beta-decay of molybdenum-99.

The isotope is chemically extracted using column chromatography usually as the water-soluble sodium salt of the technetate(VII) ion, TcO4−. It is then normally reduced to the +4 or +3 oxidation state and converted in a complex ion using a ligand, such as exametazine, for diagnostic use.

Tc-99m exametazine complex

Nature of science

The benefits of the technique(s) being considered need to be balanced against the risks associated with the exposure to radiation.

Learning outcomes

After studying this topic students should be able to:

Understand

  • Many different types of emissions (e.g. alpha, beta, gamma, proton, neutron and positron) are used for medical investigations and treatment.
  • MRI (magnetic resonance imaging) is an application of NMR technology.
  • Radiotherapy can be used internally and/or externally.
  • TAT (targeted alpha therapy) and BNCT (boron neutron capture therapy) are two different methods used in the treatment of cancer.

Apply their knowledge to:

  • Discuss the common side effects of radiotherapy.
  • Explain (based on its half-life, emission type and chemistry) why technetium-99m is the most common radioisotope used in nuclear medicine.
  • Explain (based on the type of radiation emitted) why lutetium-177 and yttrium-90 are common isotopes used for radiotherapy .
  • Balance nuclear equations that involve alpha and beta particles.
  • Use the nuclear half-life equation to calculate both the percentage and the amount of radioactive material decayed and remaining after a certain period of time .
  • Explain TAT and how it can be used to treat diseases that have spread throughout the body.

Clarification notes

The discussion of common side effects should include loss of hair, nausea, fatigue and sterility.
The discussion should also include the damage to DNA and growing or regenerating tissue.

Students should know that the isotopes used in nuclear medicine include: Tc-99m, Lu-177, Y-90, I-131 and Pb-212.

International-mindedness

Culture, cost, availability and beliefs are some of the factors that can influence the use of nuclear technology in medicine and explain why its use is not consistent throughout the world.

Teaching tips

Although the use of radioisotopes in medicine is mentioned briefly under ‘utilization’ in sub-topic 2.1 : The nuclear atom, this is really the first time students will have encountered radioactivity. Explain to them the different types of radiation and make sure they know how to write the symbols for an electron (β- particle), proton, neutron, alpha particle and a positron (β+ particle) and then give them plenty of practice at writing nuclear equations, taking care to include all the isotopes listed in the guide, i.e. Tc-99m, Lu-177, Y-90, I-131 and Pb-212. They should also be familiar with the formation of Tc-99m (from Mo-99) and its subsequent breakdown to Tc-99 and Ru-99 and the reaction of boron-10 with a neutron.

Make sure they fully understand the concept of half-life and give them plenty of practice with half-life calculations. The data booklet is quite confusing here. There are three different equations listed in Section 1. Probably the easiest (where it does not involve a whole number of half-lives, which can be done by inspection) is to calculate λ by dividing 0.693 (ln 2) by t½ then use the equation N = Noe−λt.

The rest of the topic is mainly learning about why technetium-99m is used in diagnostic medicine, common side effects of radiotherapy, how TAT and BNCT work and the application and non-invasiveness of NMR (which changes its name to MRI in medicine so that patients are not alarmed by the use of the word ‘nuclear’). You could ask students to prepare short presentations on each of these areas.

Study Guide

Page 163

Questions

For ten 'quiz' questions (for quick testing of knowledge and understanding with the answers explained) see MC test: Nuclear medicine

For short-answer questions see Nuclear medicine questions together with the worked answers on a separate page Nuclear medicine answers.

Vocabulary list

positron
radiotherapy
radioisotope
half-life
MRI (magnetic resonance imaging)
TAT (targeted alpha therapy)
BNCT (boron neutron capture therapy)

Teaching slides

Teachers may wish to share these slides with students for learning or for reviewing key concepts.

  

Other resources

1. Students already know about 1H NMR from Topics 11 and 21. This video by, Howard R. Hart Jr., one of the pioneers of MRI, explains how an MRI scan works.

  How MRI works

2. A concise animated video by Richard Thornley on TAT, targeted alpha therapy.

  Targeted alpha therapy

3. A really neat video, also by Richard Thornley, which clearly explains BNCT, boron neutron capture therapy.

  Boron nuclear capture therapy

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