Margins between clinical target volume and planning target volume for electron beam therapy

When growing a clinical target volume (CTV) to a planning target volume (PTV), it is necessary to determine suitable margins, based on the systematic and random uncertainties. For electron therapy, where treatments are usually given with single fields, the factors affecting the margin are very different in the direction of the incident beam from those in the perpendicular directions, since set-up errors do not affect the depth of the 90% isodose. For a typical case, the perpendicular margins are three times the margin in the direction of the incident beam. This gives rise to problems with volume growing algorithms if the beam axis is not aligned with a cardinal axis.