  The present paper pertains to corrections which are due to the presence of
beam-limiting and beam-shaping devices in proton planning. Two types of
corrections are considered: those which are due to the nonzero thickness of
such devices (geometrical effects) and those relating to the scattering of beam
particles off their material. The application of these two types of corrections
is greatly facilitated by decomposing the physical effects (i.e., the
contribution to the fluence) of two-dimensional objects (i.e., of the apertures
of the devices) into one-dimensional, easily-calculable contributions. To
minimise the time requirements in the derivation of the scattering corrections,
a two-step process is introduced. The first step occurs at beam-configuration
phase and comprises the analysis of half-block fluence measurements and the
extraction of the one parameter of the model which is used in the description
of the beamline characteristics; subsequently, a number of Monte-Carlo runs
lead to the determination of the parameters of a convenient parameterisation of
the relevant fluence contributions. The second step involves (at planning time)
the reconstruction of the parameters (which are used in the description of the
scattering contributions) via simple interpolations, performed on the results
obtained during the beam-configuration phase. It is shown that the inclusion of
the scattering effects leads to substantial improvement in the reproduction of
the experimental data. The contributions from the block-thickness and
block-scattering effects have been presented separately in the case of a simple
water phantom. In this example, the maximal contribution of the block-relating
effects amounts to a few percent of the prescribed dose.
