In this work a general source model has been developed to describe the
electron contamination in clinical linac heads when operating in photon mode. The
model includes two-sources that take into account the electrons generated in the linac
head and in the air gap between the linac head and the patient. Their geometrical
characteristics are determined by fitting the model of total fluence distribution to a
set of fluence data in air just before entering the water phantom and for various field
sizes up to 40 × 40 cm2
. The electron energy distributions are fitted to those obtained
from phase-space files calculated with PENELOPE at the patient surface and for the
largest field size. To verify the model, percentage depth doses and transverse profiles in
water obtained from a Monte Carlo simulation performed with a complete and detailed
linac geometry are compared to those found with the source model. Six linacs (three
operating at 6 MV, one at 15 MV and two at 18 MV) have been studied. A comparison
with the results obtained with a single source model is also carried out. The two-source
model proposed provides a reasonably good description of the dose absorbed in water
for all irradiation fields considered. The largest differences occur at the entrance of
the phantom, for the low energy configurations and the smaller fields, reaching ∼ 15%
at most. The two-source model describes much better than the single source one the
lateral profiles in depth.
