Radiation Therapy

Every technique covered so far in this module has one goal in common: image the body without damaging it. Radiation therapy flips that goal entirely. Here, radiation is deliberately aimed to damage and destroy targeted tissue — almost always a tumour — and the central engineering challenge shifts from image quality to a very different question: how do you deliver a destructive dose to one small region while sparing everything around it?

External high-energy X-ray treatment

In external-beam radiation therapy, a beam of high-energy X-rays is directed at a tumour from outside the body. The X-rays used here are typically at far higher energies than the diagnostic X-rays covered earlier in this module — where diagnostic imaging needs only enough penetration to pass through the body and reach a detector, therapeutic X-rays need to penetrate deeply into tissue and still deliver a genuinely damaging dose to a tumour that may be well below the skin surface. The aim is to damage the targeted cells' DNA sufficiently that they can no longer survive or reproduce, causing the tumour to shrink or die off.

Sparing healthy tissue: multiple beams, rotation, collimation

The central practical difficulty in external-beam therapy is that a beam entering the body damages whatever tissue it passes through, not just the tumour at its target depth — so a single beam delivering a damaging dose to a deep tumour would deliver a similarly damaging dose to all the healthy tissue along its entire path in and out. Three related methods address this problem.

Multiple beams

Rather than delivering the full dose along a single path, several separate beams are directed at the tumour from different external angles around the body. Each individual beam delivers only a modest, sub-damaging dose along its own entry and exit path through healthy tissue — but all the beams overlap at the tumour itself, and their doses add together there to reach a much higher, genuinely damaging combined dose specifically at the point where they intersect.

Rotation

Rotation extends the same underlying idea. Rather than using several fixed beams from different angles, the beam source rotates continuously around the patient during treatment, with the tumour kept at the centre of rotation throughout. Healthy tissue anywhere around that centre only ever receives dose from the beam briefly passing through it at each moment during the rotation, while the tumour at the centre receives dose continuously throughout the entire rotation — so, just as with multiple fixed beams, dose is spread thinly across surrounding healthy tissue but concentrated and accumulated at the central target.

Collimation

Collimation shapes the beam itself to more closely match the tumour's own shape, using absorbing material with an opening cut to the tumour's outline. Rather than irradiating a simple uniform cross-section that would unnecessarily expose healthy tissue around the tumour's edges, a collimated beam's cross-section is tailored closely to the target, reducing the volume of healthy tissue exposed to any dose at all.

Beta-emitting implants: an entirely different delivery method

Brachytherapy takes a completely different physical approach from external beams. Instead of directing radiation in from outside the body, a small radioactive source — specifically chosen as a beta emitter — is implanted directly inside or very close to the tumour itself.

Beta particles have a relatively short range in tissue compared with the more penetrating gamma radiation used for imaging elsewhere in this module. An implanted beta source exploits this directly: it delivers a high, damaging dose very locally to the tumour tissue immediately surrounding the implant, while healthy tissue only a short additional distance further away receives a much lower dose, simply because the beta particles' limited range means they can't reach that far in the first place.