PROTON THERAPY IN WATER - Radiation Oncology: A Physicist's-Eye View - page 223

Nuclear Interactions: neutrons
Nuclear interactions create a halo of neutrons that largely escape the
patient without further interaction, but are responsible for a low dose
in and outside the beam that may, for example, have consequences for
secondary cancer production - especially in children or the fetuses of
pregnant women (Schneider, 2002; Hall, 2006).

Pencil beam broadening due to material upstream of the patient
Broadening of a pencil beam within the patient, as discussed above, is
unavoidable. Protons suffer the same type of interactions in material
upstream of the patient and the
amount and composition of this
material is, to some extent, under

one's control. Scattering in up-
stream material is exacerbated

by any drift path (i.e., gap)
between the material and the

patient. A drift path allows the

beam to expand after being
scattered, as shown schematically
in Figure 10.15. The extent of
upstream scattering depends on:
the method of beam formation
(i.e., scanning or scattering); the
method of energy control; the use
or not of double scatterers (see
below); the location of the aperture
if any; the path in air after the last
upstream material (Urie et al .,
1986b), and so forth.
One generally tries to minimize the amount and composition of any
upstream material (materials of low atomic number scatter less) so as
not to degrade the beam penumbra any more than necessary. In
addition, the location of any material is important. As a general rule
of thumb, one tries: (1) to locate the aperture close to the patient so as
to reduce the size of the penumbra - but not too close so as to avoid
superficial hot spots from protons scattered off the aperture edges (see

Figure 10.15. Lateral spreading
of a pencil beam: (a) without,
and (b) with upstream material
in the beam.

below); (2) to locate material upstream of the aperture as far from

the aperture as possible; and (3) to locate material downstream of
the aperture as far from the aperture as possible, i.e., as close to the