Biomedical Engineering Reference
In-Depth Information
given modality (protons in our case) may make radiation a good
choice, whereas, for other modalities, radiation may not be the
preferred choice. In deciding whether protons are likely to be helpful,
two important considerations need emphasis. These are:
Large targets
Protons have acquired a reputation for being particularly useful for
small targets. In my opinion this is an incorrect perception; protons
are probably more useful for the treatment of large targets than small
ones. This is because of dose-volume effects in the tissues outside
the target volume as discussed in Chapter 5. The larger the target, the
smaller the remaining volume of normal tissue and, therefore, the
greater the need to spare it. Hence, the larger the target, the greater
the clinical advantage of the dose-sparing properties of protons is
likely to be. Paradoxically, the excellent results in the treatment of
ocular melanomas with protons are a good example of this principle.
For, although the treated volume is physically small, it can be quite
a large fraction (up to at least one-third) of the relevant body
compartment - namely, the eye.
Complex geometry
Protons are good at solving patient-specific problems in which the
geometric relationships between the tumor and adjacent OARs pose
difficulties and normal tissue avoidance is important. The excellent
results in the treatment of base-of-skull sarcomas with protons are a
good example of this principle, for these tumors often wrap around, or
are very close to, sensitive normal tissues such as the brain stem and
optic chiasm.
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IFFERENCES IN
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LANNING
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ESIGN OF
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The effects of inhomogeneities
I have already devoted considerable space to a discussion of the
effects of inhomogeneities and I will not repeat the points already
made. However, in addition to the obvious fact that inhomogeneities
alter the penetration of protons in their shadow, and that one needs to
alter the beam by designing real or virtual compensation to take
account of this, there are a few additional points worth making:
Uncertainty
One cannot predict all the effects of inhomo-
geneities exactly. Mis-registration of the compensation scheme
relative to the patient can arise due to patient and organ localization
