The next part of the process is to create a combination of RT
beams that will deliver the dose required to the PTV (Figure 1).
At this stage, critical normal structures surrounding the target
volume are outlined, known as ‘organs at risk’ (OARs). These are
structures that limit the dose that can be delivered to the target
volume (e.g. the rectum is an OAR for prostate RT). Planning is
done by the physicists using a combination of three or more
beams entering from different directions. Physical avoidance of
normal structures is important to minimize toxicity. Once a plan
is created, it is approved by the clinician and the data is transferred
to the treatment machine.