PhDA model based on Monte Carlo techniques is developed to transport
ionising radiation through the radiation head of a 6MV linear
accelerator fitted with multileaf collimators Major emphasis is given
to the detailed geometrical descriptiqn of the multileaf collimator. The
model produces dose distributions in water from photon beams defined
by the jaws and the multileaf collimator. The model accounts for
contaminant electrons in the photon beam, off-axis x-ray radiation
originating at the collimator and the transmission and penumbra
effects of the side planes and front face of the leaves in the multileaf
collimator Dose distributions in water calculated by the model are
compared with experiment using lonisation chambers, diodes and film
and found to be within 1 5% The transmission and the penumbra of
the multileaf collimator leaves calculated by the Monte Carlo model
are compared with experiment and found to be in good agreemen
