Biomedical Engineering Reference
In-Depth Information
date should perhaps be read as a confirmation that the theoretical
arguments for proton beam therapy have been upheld in the
limited number of situations in which they have been tested.
The physical rationale for proton beam therapy is unim-
peachable. Under virtually every scenario, protons deliver less
dose outside the target volume than do X-rays - typically
they deposit one-half or less integral dose to uninvolved normal
tissues than do X-rays (Lomax
et al
., 1999). This statement
holds no matter what the technical approach - it is the case, for
example, for intensity-modulated proton
vs.
photon therapy.
Glimelius
et al
. (2005) […] cite a remarkable 52 published
treatment planning comparisons which document this fact.
Faced with the possibility of receiving the dose distribution
possible through a proton treatment, it is hard indeed to imagine
anyone readily volunteering to receive an additional, say, 20 to
30 Gy to a large volume of tissue for which irradiation is not
medically indicated.
All this having been said, it is important to appreciate that the
application of protons is not without its difficulties and some
limitations. With regard to the former, we see it as essential
that anyone entering the field of heavy charged particle therapy
serve an apprenticeship at one of the existing heavy charged
particle centers. The physical/technical limitations include: the
management of the influence of internal tissue heterodensities;
the substantial problems posed by surgically implanted metallic
objects; the lack of superficial skin-sparing; the management
of moving target volumes; the unavoidably enlarged penumbra
at large depths; the distortion of the dose distribution under
conditions of tangential irradiation of structures with strong
differences in density (including the skin/air interface); neutron
backgrounds which are especially problematic when scattered
beams are used in pediatric treatments, and so forth. While
many of these limitations can be overcome, nevertheless protons
are not uncritically appropriate for all patients. One must always
keep in mind that the colorful and attractive pictures produced
by treatment planning programs may be misleading.
The commonly raised issue, ultimately, is that of economics.
[Most people are persuaded that, if it cost no more than X-ray
therapy, protons would in almost all cases be the preferred
modality.] Is the drawback of receiving the extra dose delivered
by X-rays worth the reduction in cost that they offer? In order to
