6.6.1 Diagnosis and treatment using radiation

Treating tumours with radiation

1. The radiation used is high-energy gamma rays or high-energy X-rays, because only very penetrating radiation can reach a tumour buried inside the body.
2. As the radiation passes through a cell it ionises molecules, and the damage that matters most is the damage done to the cell's DNA.
3. A cell with badly damaged DNA cannot copy it correctly, so the cell fails when it next tries to divide and it dies.
4. Tumour cells divide far more often than most healthy cells and are worse at repairing DNA damage, so the same dose kills a much greater share of them.
5. Healthy cells in the path of the radiation are damaged too, which is what causes the side effects of treatment, so every method of delivering the radiation is designed to give the largest possible dose to the tumour and the smallest possible dose everywhere else.
6. The total dose is usually split into many small fractions given on different days, which gives healthy tissue time to repair between sessions while the tumour is worn down.
7. The radiation can be delivered from a machine outside the body or from a source placed inside it.

External radiotherapy

1. A machine outside the patient produces a narrow beam of high-energy radiation and directs it at the tumour, so no radioactive material ever enters the body.
2. The beam is shaped by a collimator so that its cross-section matches the outline of the tumour as closely as possible.
3. The machine is then rotated around the patient, so the beam enters the body from many different directions during the session.
4. Every beam direction passes through the tumour, so the tumour receives the sum of all the doses and ends up with a very large total.
5. Any one region of healthy tissue lies in the path of only one or two of those directions, so it receives a much smaller total than the tumour does.
6. Advantages: nothing is implanted, so no surgery is needed; the patient is not radioactive at any point and can go home straight away; deep tumours can be reached; and the direction, shape and size of the dose can be adjusted between sessions.
7. Disadvantages: the radiation has to travel through healthy tissue on its way in and out, side effects such as tiredness, reddened skin and hair loss in the treated area are common, many sessions are needed over several weeks, and the patient must be positioned very accurately each time.

Internal radiotherapy

1. A radioactive source is placed inside the body, either in the tumour itself or immediately next to it.
2. The source may be a small sealed pellet or rod inserted during a short procedure, or a radioactive substance that the body itself concentrates in the target tissue.
3. An overactive thyroid or a thyroid tumour, for example, can be treated with radioactive iodine, because the thyroid gland absorbs iodine from the blood whether or not the nuclei are stable.
4. The radiation spreads out from the source in all directions, so its intensity falls rapidly with distance.
5. Tissue touching the source therefore receives an extremely large dose while tissue a few centimetres further away receives very little, which is exactly the pattern the treatment wants.
6. Advantages: the tumour gets a very high dose, healthy tissue further away is largely spared, treatment continues steadily while the source stays in place, and fewer hospital visits are needed.
7. Disadvantages: placing the source usually needs an invasive procedure, staff have to handle a radioactive source, the patient is radioactive while the source is inside them and so must limit close contact with visitors, and the method only suits tumours that can be reached or that take up a particular substance.

Tracers in diagnosis

1. A tracer is swallowed, injected or inhaled, depending on which organ is being investigated, and is carried around the body by the blood.
2. The radioactive atoms are attached to a chemical the target tissue takes up naturally, so the tracer collects where the doctor wants to look.
3. A gamma camera outside the patient detects the radiation coming out and builds a picture of where the tracer has gone.
4. A medical tracer must be a gamma emitter, because gamma radiation is the only kind penetrating enough to escape the body and reach the camera; alpha and beta radiation would be absorbed by the surrounding tissue and would only add to the patient's dose.
5. It must also have a short half-life, long enough for the scan to be completed but short enough that the activity inside the patient falls away within hours rather than years.
6. Technetium-99m is the isotope used most often, because it emits gamma radiation only and has a half-life of about $6hours$.
7. The doctor is looking for tissue that takes up more or less tracer than expected, since that pattern shows how the organ is working rather than just what shape it is.
8. Tracers are used to follow blood flow, to check how well a kidney or a thyroid gland is functioning, to find blockages in the circulation or the digestive system, and to locate secondary tumours in bone.

How a PET scanner works

1. The patient is injected with a tracer whose nuclei emit positrons rather than gamma rays, most often fluorine-18 attached to a molecule that behaves like glucose.
2. Tissue that is working hardest uses the most glucose, so the most active tissue, including many tumours, takes up the most tracer.
3. Each emitted positron travels only a very short distance through tissue before it meets an electron, of which there are enormous numbers in every atom around it.
4. A positron and an electron are a particle and its antiparticle, so when they meet they annihilate: both are destroyed and their mass is converted into energy.
5. That energy leaves as two gamma rays travelling in opposite directions, which is required because the pair had almost no momentum before they met.
6. A ring of detectors surrounds the patient, and the electronics look for two gamma rays arriving on opposite sides of the ring at almost the same instant.
7. Each such pair of detections means an annihilation happened somewhere along the straight line joining the two detectors.
8. A computer collects millions of these lines and finds the regions where they cross most often, and those regions are where the tracer is concentrated.
9. The result is a three-dimensional image showing how active each part of the body is, which is information that an X-ray image of structure alone cannot give.
10. PET scans are used to find tumours, to check whether a tumour is still active after treatment, and to study how the brain and the heart are working.

Making PET isotopes nearby

1. The isotopes that emit positrons have very short half-lives: about $110minutes$ for fluorine-18 and only about $20minutes$ for carbon-11.
2. A short half-life is chosen deliberately, because it means the activity inside the patient falls away within a few hours and the dose is kept small.
3. The same property applies before the scan as well, so the activity is falling steeply from the moment the isotope is made.
4. A long journey would use up several half-lives, and once too few nuclei are left the number of annihilations per second is too small to build a clear image.
5. Giving the patient a much larger amount to compensate is not an option, because that would raise the dose and undo the reason for using a short-lived isotope.
6. The isotopes are therefore made in a cyclotron on the hospital site or at a centre close to it, attached to the tracer molecule immediately, and injected within a few hours.
7. This is why PET scanning is expensive and why it is available at far fewer hospitals than ordinary X-ray imaging.