Biomedical Engineering Reference
In-Depth Information
∗
At the time of writing, substantial changes are taking place in proton
beam therapy. For one thing, proton beam therapy has moved from
the physics research laboratory, where it was sequestered for several
decades, into the clinic and there has been a rapid growth in the
number of proton medical facilities worldwide. On the technical side,
practitioners are beginning to emphasize pencil beam scanning more,
and scattered beam delivery less. The reasons for this have already
been alluded to in Chapter 10. For uniform-beam planning, scanned
beams: (1) offer better sparing of tissues upstream of the target
volume because the depth of modulation can be varied throughout the
field on the basis of the extent in depth of the target volume; (2) can
in most cases be delivered without patient-specific hardware, thus
allowing treatments to be given more quickly, and to be adapted
to changing circumstances more easily; (3) can create non-uniform
beams and so be able to deliver intensity-modulated proton therapy
(IMPT); and (4) produce less neutron background radiation than
scattered beams. The downside is that one has additional issues to
deal with, related to patient and organ motion, as discussed in Chapter
10. While technically challenging, motion does not pose a funda-
mental limitation in all but a few instances - for which wobbled beam
delivery, described in Chapter 10, can be used.
I propose to begin with a discussion of a number of topics which
apply equally to all forms of beam delivery, although the technical
implementations may differ, depending on whether the beams are
delivered with scanned pencil beams or scattered or wobbled beams.
The first topic is the influence of inhomogeneities on a beam's dose
distribution.
I
NHOMOGENEITIES
A patient's tissues are highly inhomogeneous both in chemical
composition and density. Such inhomogeneities affect the dose dis-
tribution of proton beams which must therefore be designed to take
the patient's anatomy into account.
∗
Some of the material in this chapter is adapted, with permission, from the
article “Treating Cancer with Protons” which appeared in the September 2002
issue of Physics Today (pp. 45-50) by Goitein M., Lomax A.J. and Pedroni
E.S. A good source of information concerning proton beam therapy can be
found in ICRU78 (2007), from which portions of this chapter have been taken
with the permission of the Oxford University Press.
