Exploring internal organs

1. Doctors can examine organs inside the body without surgery by using a radioactive tracer.
2. The tracer is a small amount of a radioactive isotope, often attached to a chemical that the organ being investigated naturally takes up.
3. Depending on the organ, the tracer is injected into the bloodstream, swallowed or breathed in.
4. The tracer is carried around the body and collects in the target organ, such as iodine-123 collecting in the thyroid gland.
5. As the tracer nuclei decay, they emit gamma rays that pass out through the body.
6. A gamma camera outside the patient detects the gamma rays and records where they come from.
7. A computer uses this information to build up an image showing where the tracer has collected.
8. Areas that take up too much tracer show up as bright hot spots, and areas that take up too little show up as cold spots.
9. Hot and cold spots can reveal a tumour, a blocked blood vessel or part of an organ that is not working properly.
10. The image shows how well an organ is working, not only its shape, because healthy tissue takes up the tracer at a normal rate.

Choosing a tracer

1. The tracer must emit gamma radiation, because gamma rays penetrate tissue well enough to escape the body and reach the camera.
2. Gamma rays are also weakly ionising, so they cause relatively little damage to cells as they pass out of the body.
3. An alpha emitter would be useless for imaging, because its particles would be absorbed inside the body and would strongly ionise the cells around it.
4. A beta emitter is also unsuitable, because most beta particles are absorbed within a few millimetres of tissue and add to the dose without reaching the camera.
5. The tracer must have a short half-life, usually a few hours, so that it lasts long enough for the scan but then decays quickly to keep the patient's dose low.
6. A half-life of a few seconds would be too short, because the tracer would decay before it reached the organ.
7. A half-life of several years would be too long, because the tracer would keep irradiating the patient long after the scan had finished.
8. Technetium-99m is the most widely used tracer, because it emits only gamma rays and has a half-life of about 6 hours.
9. The tracer must also be non-toxic, because it is put into the body.

Destroying unwanted tissue

1. Cancer cells divide out of control and form a lump of unwanted tissue called a tumour.
2. A high dose of ionising radiation damages the DNA of cells so badly that they die or can no longer divide.
3. Radiotherapy uses this effect to control the growth of a tumour or to destroy it completely.
4. Radiation damages healthy cells in exactly the same way, so the treatment must give a high dose to the tumour and as small a dose as possible to the tissue around it.
5. External radiotherapy
   1. Beams of high-energy gamma rays or X-rays are aimed at the tumour from outside the body.
   2. The beams are directed at the tumour from many different angles, either by rotating the source around the patient or by using several beams at once.
   3. The tumour sits where all the beams cross, so it receives a large total dose, while each region of healthy tissue is crossed by only one beam and receives a much smaller dose.
   4. The treatment is usually split into many small doses over several weeks, which gives healthy cells time to repair themselves between sessions.
   5. The patient is irradiated but not contaminated, so they are not radioactive when they leave each session.
6. Internal radiotherapy
   1. A sealed radioactive source, such as a tiny pellet or wire, is placed inside the body next to or inside the tumour.
   2. The source is chosen so that its radiation travels only a short distance through tissue, so most of the energy is absorbed by the tumour rather than by healthy organs.
   3. Some treatments use a radioactive substance that the tumour takes up, such as iodine-131, which collects in the thyroid gland and destroys thyroid cancer cells with beta particles.
   4. The patient now has radioactive material inside them, so they are contaminated for a time and must follow precautions, such as keeping away from pregnant women and young children for a few days.

Benefits and risks

1. Every medical use of radiation carries some risk, because the radiation that diagnoses or treats can also damage healthy cells and slightly raise the chance of cancer later in life.
2. Doctors use radiation only when the benefit to the patient is judged to be greater than the risk.
3. For a scan, the benefit is an accurate diagnosis without surgery, and the dose is kept low by using a short half-life gamma emitter.
4. For radiotherapy, the benefit of destroying a tumour usually far outweighs side effects such as tiredness, sore skin and hair loss in the treated area.
5. Hospital staff protect themselves by using shielding, keeping their distance from patients who contain radioactive material and wearing a dosimeter badge.