Figure 3. Illustrating the principles of IMRT. A single (CT) slice of a patient is shown with the contours of the prostate (PTV, planning target volume), rectum (OR, organ at risk) and bladder (OR) outlined. The prostate has a concave outline and it is desired to achieve a high dose in the PTV sparing the ORs. Three intensity-modulated beams (IMBs) are shown. For simplicity they are shown as parallel beams and at arbitrary gantry orientations. It is not intended to suggest these three orientations are the most appropriate for the treatment. Each beam has a modulated fluence.