ABSTRACT
The Planning Target Volume (PTV, see Section 29.2) must receive a dose large enough to
reach the therapeutic goal. Therefore, it must be covered by the beams that are used for treating
that volume. For skin tumours, the beam definition is straightforward; only one beam is used,
and the beam shape directly reflects the tumour lesion plus a margin. Treating tumours not
visible from the exterior of the patient requires the use of x-ray equipment or other medical
imaging devices to define the location of the PTV (see Section 30.3). Treatment simulators
(see Chapter 13) provide a diagnostic x-ray tube mounted in the same isocentric geometry as a
treatment machine. Using a simulator, the bony anatomy is easy to identify and beam
definition can be made relative to that. However, to obtain a precise match between the
field shape and the shape of the PTV, a projection of the PTV (together with an appropriate
margin) onto a plane perpendicular to the beam direction is required. This may be achieved in
one of three ways.
