PET scanning and radiotherapy

PET: annihilation as a beacon

Positron emission tomography uses the same tracer principle with antimatter. The tracer is still a radionuclide attached to a molecule the body uses, and the image is still a map of where the biology took it; what changes is the radiation that is detected. The tracer, commonly fluorine-18 in a glucose analogue, is a beta-plus emitter, so what it sends out is positrons.

That looks like a breach of the rule that only photons get out of the patient, and it is not. Each positron travels only a millimetre or so before it meets an electron, so the positron itself never leaves the patient. The pair annihilates, both particles vanish, and their rest energy leaves as two gamma photons of 511 keV each, flying in opposite directions to conserve momentum. Those photons are what the scanner detects, and they escape as readily as any other gamma.

The scanner is a ring of detectors watching for coincidences, meaning two 511 keV photons arriving at opposite sides at effectively the same instant. Each coincidence pins the annihilation to the straight line between the two detectors, and hundreds of thousands of such lines intersect at the places the tracer gathered. The 511 keV is the electron's rest energy m₀c².

What the scan diagnoses depends on the molecule to which the fluorine-18 is attached. In a glucose analogue it goes wherever the body is burning sugar, so the image is a map of metabolic rate. Many tumours consume glucose faster than the tissue around them and show as bright spots, which is why PET, paired with CT, helps to detect certain cancers, to stage them by picking up secondary tumours anywhere in the body, and to check afterwards whether treatment has quietened them.

The uptake is not specific to cancer, though: infection and inflammation burn sugar too and also light up, and not every tumour takes up the tracer strongly, so a PET result is read alongside other evidence rather than as a verdict on its own.

The brain and the heart read the same map differently. Reduced uptake across particular regions of cortex is a marker of dementia, an abnormally active patch can locate where epileptic seizures begin, and heart muscle that still takes up the tracer after a heart attack is muscle worth restoring the blood supply to, while muscle taking up none has died.

Every one of those diagnoses rests on the same property. PET measures function, and it is poor at anatomy, so a bright spot on its own does not say which organ it is in. Scanners are therefore built as PET rings wrapped around a CT scanner, and the two images are laid on top of each other: one says what is happening and the other says where.

When the dose is the point

Diagnostic imaging treats the dose as a cost to be minimised. In treatment the same physics is used the other way, because ionising radiation damages cells: a tumour can be destroyed if it receives a much larger absorbed dose than the tissue around it. Radiotherapy rests on that comparison, and there are two ways of achieving it.

The first is to aim from outside. A beam of high-energy X-rays, megavolts rather than the kilovolts of diagnosis, is rotated around the patient so that it enters through a different path each time while always crossing the tumour. Each path through healthy tissue is used briefly and receives a small share of the dose, while the tumour lies on every path and receives the total. The energy is chosen high deliberately, because a hard beam is attenuated less on the way in and so deposits proportionately more where it is aimed; a megavoltage beam's dose also builds up over the first centimetre or two rather than peaking at the skin, and collimators shape each beam to the tumour's outline.

The second is to put the source inside. A radioactive implant is a sealed source, a seed or a small rod, placed in the tumour or against it, and the emitters this specification's model names are beta emitters, yttrium-90 or strontium-90 among them; clinical practice also uses other sources under the same geometry. Beta particles are absorbed within a few millimetres of tissue, so the dose is enormous where the seed sits and negligible a centimetre away, and the healthy tissue beyond that range is untouched however long the source is left.

That is the tracer rule applied in reverse, and it is worth saying so in an answer. A beta-minus emitter is unsuited to gamma-camera imaging precisely because its radiation cannot escape the body, and it is well suited to an implant for precisely the same reason. Unsealed sources exploit chemistry instead of geometry, iodine-131 being swallowed and concentrated by the thyroid itself, and there the tissue to be destroyed does the aiming.

Two numbers then shape the treatment. The activity of the source sets the scale of the dose rate, though the dose delivered also depends on the radiation's energy and on how the surrounding tissue absorbs it, and the half-life fixes how quickly that rate falls.

For a sealed implant the half-life that matters is the physical one, because the body cannot excrete a sealed source; the effective half-life belongs to unsealed tracers that can be cleared. So a permanent seed is chosen with a physical half-life short enough that it has spent itself once the planned dose has been delivered, while a stronger, longer-lived source is put in for a measured number of hours and taken out again.