Three-dimensional conformal radiotherapy plan of an oesophagus showing four beams entering from the front and back and one from left and
one from the right. The red outline (labelled PTV) is the volume to be treated and the coloured lines are the dose lines (isodoses) indicating what dose
the structures contained within that line receive. The ideal plan would have all the PTV contained within the 95% isodose with no areas of high dose
outside of this volume.