Biomedical Engineering Reference
In-Depth Information
was directed at a complex but stationary object. It took me a little
while to appreciate that the patient is a living, breathing, moving
individual. That I was facing, in fact, a dynamic problem and that I
had better understand the time-varying processes if I were to make
any headway in my new profession.
When one speaks of motion, it should be with a broad understanding
of the term. As I use it, the term motion covers both short term and
long term changes over time of the patient's location, the location of
the patient's tumor and organs, the size and shape of the tumor and
normal tissues, including uncertainties in their delineation as
discussed in Chapter 3, the degree of filling of organs such as the
bladder, intestines, and rectum, and so forth. All of these phenomena
will affect the dose distribution due to a given beam, and all must
therefore be taken into account.
To deal with motion, as with any source of uncertainty, one needs to
proceed in three steps:
1.
understand qualitatively and quantitatively the nature and degree
of the problem(s);
2.
instigate measures to reduce the size of the problem(s) to the extent
that is feasible and practicable; and
3.
develop and implement strategies to take the inevitable residual
uncertainties into account in an optimal manner.
Tumor and organ motion can be classified into three categories,
namely: motion of the patient as a whole relative to some reference
object such as the couch top; intra-fraction motion of the tumor and
organs within the patient
(i.e., during delivery of
treatment preparations
treatment preparations
delivery of one fraction
delivery of one fraction
a single fraction); and
inter-fraction changes of
the position and/or size
and shape of the tumor
and organs within the
patient.
In managing a patient,
taking motion into
account, the sequence of
procedures shown in
Figure 7.1 is generally
followed.
immobilization
device made
immobilization
device made
localization
localization
simulation
simulation
verification
verification
simulation
simulation
simulation
simulation
verification
verification
treatment
planning
treatment
planning
treatment
treatment
when necessary
when necessary
Figure 7.1. Block diagram of the steps
involved
in
preparing
a
patient's
treatment. The colored steps are those
which are discussed in this chapter.
