ABSTRACT
The goal of radiotherapy treatment planning is to design a beam configuration which will
deliver a homogeneous dose to the specified planning target volume (PTV), ensuring that
normal tissue receives a reasonably low dose and that critical organs receive less than their
tolerance doses. ICRU 50 (ICRU 1993) suggests an acceptable dose variation ofC7%/K5%
of the reference dose, the reference dose point being the centre of the PTV, the isocentre, mid-
plane or some other suitable point (see Section 37.6). This is achieved by choice of treatment
modality (photons or electrons), beam energy, beam arrangement, use of wedges, compensa-
tors or blocks, and methods such as conformal planning (see Section 43.1) as appropriate. The
ICRU objectives are not always met and sometimes compromise may be necessary, depending
on the aims of radiotherapy. For example, if the treatment is palliative, long-term radiation
effects may not be a consideration and a simple beam arrangement will suffice. On the other
hand, if the treatment is radical, i.e. aiming at local control of the tumour, long-term effects
need to be taken into account and this may result in a more complex radiotherapy regime.
Centres may also be limited in the choice of photon beam energy or the availability of
electron therapy.
