The aim of this work is to improve photon dosimetry in composite fields such as are used in delivering IMRT. In a typical IMRT treatment, the total dose at a point in the PTV arises from an appropriately weighted combination of in-field, out-of-field and penumbral contributions.

Measurement with an ion chamber needs to take account of its varying sensitivity to these contributions and the possible lack of electron equilibrium. Effective electronic equilibrium may only be achieved in the combined treatment.