Biomedical Engineering Reference
In-Depth Information
localization changes and uncertainties; beam scattering means that
perfect compensation is impossible; measurements of the location and
nature of the inhomogeneities are imperfect; and so forth. As a
consequence, one is obliged to make a relatively complex analysis of
the uncertainties and then design the beams so as to make the clinical
consequences of those uncertainties as acceptable as possible. Of
course, the same is true of photon beam therapy, but the proton
problems are more complex and require more complex solutions since
correction in depth as well as laterally is necessary.
Heterogeneous inhomogeneities
With one exception, I have
treated inhomogeneities as though they were internally homogeneous.
The exception is the complex situation in the base-of-skull, discussed
above. You will recall that the distal part of the proton beam was not
just shifted in range, but was smeared out. This was due, presumably,
to the many possible paths, each with a somewhat different water-
equivalent path length, that the scattering protons can follow. In
(Urie
et al
., 1986a) a dramatic degradation of the distal beam was
seen in the abdomen - probably mainly due to organ motion. And
similar smearing has been observed in the lung (R. Mohan, private
communication) and in granulated graphite (S. Vynckier, private
communication). The possibility of distal smearing by complex
inhomogeneities, which can be up to at least
±
2 cm, must be taken
into account when designing a proton beam.
Over-penetration in the lung
It is usual, in using
protons, to add a safety margin in depth (i.e., in energy), as well as
laterally, to account for the various uncertainties. This may be, say,
sufficient additional energy so as to provide in near-unit density
materials such as muscle or brain an extra 0.5 to 1.0 cm of
penetration beyond the target volume. However, lung has a low
-3
water-equivalent density (let us use 0.2 g
cm for the purpose of
illustration) which has the consequence that the same increase in
⋅
proton beam energy would lead to a factor of about 5 greater
penetration - that is, to a 2.5 to 5 cm overshoot in lung. Moreover,
organ motion can further increase this margin, due to the possibility
of tissues moving in and out of the beam path with the respiratory-
and cardiac-induced motion. Thus, after allowing for the inevitable
uncertainties, one may find that one has to treat an undesirably large
volume of lung, and/or that critical structures distal to the lung may
receive unwanted dose. One must strive particularly hard to reduce
the uncertainties when protons must traverse lung, so as to be able
to limit the safety margin to a small physical distance. Clearly
