Electron contamination and build-up doses in conformal radiotherapy fields

The dose in the build-up region depends upon the primary photon beam, backs cattered radiation from the patient and contamination radiation from outsid e the patient. In this paper, a model based on measured data is proposed wh ich allows the build-up dose for arbitrarily shaped treatment fields to be determined. The dose in the build-up region is assumed to comprise a primar y photon component and a contamination component that is a function of the field size and shape. This contamination component, for modelling purposes, is subdivided into contributions that correspond to elements of I cm by I cm cross-sectional area at the plane of the isocentre. The magnitude of the se components has been obtained by fitting measured data to an exponential function. The exponent was found to vary linearly with depth for energies b etween 4 MV and 20 MV. The coefficient decreased linearly with depth at 4, 6 and 8 MV, but exhibited a broad buildup region at 20 MV. The primary comp onent, in the build-up region, could be approximated by a 100 - (100 - PSD) e(-mu d) function, where PSD is the primary surface dose. The values obtai ned during the fitting procedure were used to calculate dose in the build-u p region for arbitrarily shaped fields. Good agreement was found in each ca se.