Biomedical Engineering Reference
In-Depth Information
which the intensity modifying device is not present and within which
the photon intensity is relatively uniform throughout the field. There
are times, however, when one wants a non uniform-intensity beam.
These fall into two categories:
Standardized
linearly varying intensity distributions.
These are
used either when beams are combined (e.g., treatment using a
pair of beams at 90
to one another) or to roughly compensate
for a sloping patient surface. Such distributions are formed by
interposing appropriately angled wedge-shaped hunks of metal
°
into the beam (so-called “wedge filters”).
Patient-specific
intensity-modulated fields.
These can be used
either for providing varying degrees of beam attenuation
throughout the field to compensate for an irregularly shaped
patient surface and/or internal inhomogeneities, or for intensity-
modulated radiation therapy as discussed in Chapter 9. Intensity-
modulated fields can, as was done before the advent of multi-leaf
collimators, be made of metallic irregularly formed attenuators
(looking much like that schematically depicted in cross section
in Figure 4.11). However, especially for intensity-modulated
radiation therapy, they are most commonly created by dynamic-
ally modifying the position of each leaf of a multi-leaf collimator
and, thereby, the size and shape of the field, during the course of
the delivery of a beam.
How should the shape and intensity profiles of beams be designed?
Well, I am going to defer answering this question until Chapters 8
and 9. The final topic I want to briefly discuss in this chapter is:
D
OSE
C
ALCULATION
Herring and Compton (1971) presented an influential paper entitled
“The degree of precision required in the radiation dose delivered in
cancer radiotherapy”. In this paper, they discussed dose-response
data and clinical incidents in which wrong doses were delivered to
groups of patients due, for example, to errors in dosimetry. They
concluded that “the therapist needs a system which will permit him to
deliver the desired dose distribution […] to within
5% or possibly
even more accurately.” (I am sure you will share my frustration that
no indication was given of the confidence level with which this
accuracy was to have been obtained.) This paper, together with much
other consideration of the problem, focused attention on the need to
measure and calculate dose distributions accurately.
±
