Biomedical Engineering Reference
In-Depth Information
Table 8.1. Comparison of principal advantages and disadvantages of beams
of various modalities used in external beam radiation therapy.
advantages
disadvantages
photons
widely available
good skin sparing
higher entrance dose than
tumor dose
high dose throughout
patient up to exit surface
electrons
finite penetration, thus
sparing tissues distal to
the target volume
very slight skin-sparing
broad penumbra due to
scattering
only suitable for quite
shallow target volumes
due to shallow fall-off of
the distal dose at higher
energies
protons
virtually no dose distal to
the target volume
somewhat reduced
entrance dose proximal
to the target volume
management of
inhomogeneities is non-
trivial
Penumbra becomes
substantial at large
depths (e.g.,
20 cm)
no skin sparing
very limited availability
≥
rectangular field. Here, the goals are largely clear. They are: (1) to
cover the entire CTV; (2 ) to do so with adequate margins to take
care of the various factors relating to patient and organ motion and
setup errors, and the beam penumbra as discussed in Chapter 7; and
(3) to avoid any OARs of particular concern
or, if one is not entirely
avoidable, then to minimize the volume of the OAR that is covered by
the beam.
A particularly useful approach is the design of the field shape in the
beam's-eye view (BEV). The beam's-eye view is a perspective view
of the patient's delineated anatomy as seen from the viewpoint of the
radiation source of one particular beam. As the planner changes the
beam direction the display changes, showing the new spatial
relationships between the target volume and the delineated anatomy
This allows the planner to choose a beam direction from which
particular OARs can either be avoided, or minimally included in the
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