Calculation of a pencil beam kernel from measured photon beam data

Usually, pencil beam kernels for photon beam calculations are obtained by M onte Carlo calculations. In this paper, we present a method to derive a pen cil beam kernel from measured beam data, i.e. central axis depth doses, pha ntom scatter factors and off-axis ratios. These data are usually available in a radiotherapy planning system. The differences from other similar works are: (a) the central part of the pencil beam is derived from the measured penumbra of large fields and (b) the dependence of the primary photon fluen ce on the depth caused by beam hardening in the phantom is taken into accou nt. The calculated pencil beam will evidently be influenced by the methods and instruments used for measurement of the basic data set. This is of part icular importance for an ac curate prediction of the absorbed dose delivere d by small fields. Comparisons with measurements show that the accuracy of the calculated dose distributions fits well in a 2% error interval in the o pen part of the field, and in a 2 mm isodose shift in the penumbra region.