Biomedical Engineering Reference
In-Depth Information
unchanged in the region not shadowed by the inhomogeneity.
However, in the shadow of the interface region, the dose is
additionally perturbed due to the difference in the strengths of
multiple scattering in the two adjacent materials. Namely, there is a
dose enhancement (hot spot) on the low density side, and a dose
reduction (cold spot) on the high density side (Goitein, 1978;
Goitein
et al
., 1978). The geometry is illustrated in the left side of
Figure 11.3 for the extreme case of a parallel beam of protons
impinging on an air/plastic interface situated in air.
Figure 11.3: Influence of semi-infinite slab intersecting a broad
proton beam, traveling in air: (a) schematic drawing (see text); and
(b) measured (
red
) and Monte Carlo simulation (
black histogram
) of
the dose in air 25 cm below a 2.5 cm thick half-slab of plastic. The
blue curve
shows measured data when a 1.25 cm thick infinite slab
of plastic is placed just above the semi-infinite slab.
Protons on side 1 miss the inhomogeneity and travel on, unperturbed.
On the other hand, protons on side 2 will be scattered by the
inhomogeneity. Protons reaching the region marked A in Figure
11.3a come entirely from side 1 and will deliver a dose equal to that
which would be delivered if the slab were not present. The same
fluence of protons as reaches A will reach regions such as that marked
D which are well away from the shadow of the edge of the
inhomogeneity since, although protons have been scattered by the
inhomogeneity, the net flux of particles does not change.
However, near the shadow of the edge of the inhomogeneity things
are different. A region such as B will be traversed both by protons on
