Dr AlHilali Consultant Clinical Oncologist

A closer look · External-beam radiotherapy

How radiotherapy
works.

Explore the relationship between a treatment target, nearby normal tissues and illustrative beam directions. Then see why geometry alone is not a treatment plan.

Explore how radiotherapy is planned

Illustrative cross-sectionNot to scale

A conceptual treatment target, surrounding tissue and three possible beam directionsA terracotta outlined target sits inside an abstract body contour. Two teal shapes mark illustrative normal-tissue regions. Three pairs of dashed amber lines cross the target from different directions. These lines show geometry only: they are not dose, beam intensity, a real person's anatomy or a recommended plan. Text below gives the full explanation. A B C Treatment targetNormal tissueNormal tissue
Target & illustrative margin Normal tissue Geometric beam routes only

No dose values or dose map. The colours do not represent radiation intensity. Dashed routes are not visible rays during treatment.

01 / Define the target

First, what needs to be treated?

The team uses planning images to outline the intended treatment target. Depending on the situation, it can include visible disease and areas that need treatment even though no tumour is visible there.

Margins may account for uncertainties such as position and movement. The dashed outline here is illustrative: it does not show a recommended margin.

A target is a clinical decision, not simply a shape drawn around a visible lump.

02 / Consider normal tissue

The tissue around it matters.

Nearby organs are outlined too. Their location and the amount of radiation they can safely receive influence how a plan is designed and assessed.

Normal tissue may be exposed along a beam's path and close to the target. It cannot always be completely avoided. Planning aims to balance target coverage with appropriate normal-tissue limits.

Precise targeting does not mean zero radiation outside the target.

03 / Explore directions

Different routes.
The same target.

Some external-beam techniques use several directions, or delivery during rotation, to shape where the dose is delivered. The paths here illustrate different routes through the body.

Toggling these lines does not calculate dose or show that more directions make a better plan.

04 / A plan is more than geometry

Where the lines meet
is only the beginning.

Clinically calculated beam contributions can be combined to deliver the prescribed target dose while limiting exposure elsewhere. Direction alone does not determine the result.

Beam shape, intensity, energy, the patient's anatomy, tissue properties and the treatment technique affect dose. Planning systems calculate the distribution; the team checks target coverage, organ limits and whether delivery is safe and achievable.

This is a concept explanation. It is not a planning system, a treatment simulation or a recommendation.

The explanation without interacting

  1. Define the target. Images and clinical judgement identify the area intended for treatment.
  2. Consider normal tissue. Nearby organs and their dose limits are part of planning.
  3. Consider delivery. Beam directions are one part of a much more detailed technique.
  4. Calculate and check. A clinical plan requires dose calculation, evaluation and delivery checks.

The drawing is an abstract two-dimensional teaching model, not a scan, actual anatomy or dose distribution. It omits depth-dose behaviour, scatter, beam shaping, modulation, motion, fractionation and treatment-specific constraints. It illustrates selected ideas about external-beam radiotherapy, not all radiotherapy techniques.

Read the existing approved radiotherapy article →

Sources & clinical review

The basis of this explanation

Clinically reviewed and approved by Dr AlHilali on 9 October 2026.

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