Biomedical Engineering Reference
In-Depth Information
use of this beam alone would
be a perfectly hopeless way
of treating the tumor. The
proximal dose is higher than
the already high dose
delivered to the tumor and
would lead to unaccept-
able complications in, for
example, the left temporal
lobe. And, the beam exits
through the patient's right
eye where, although the dose
is lower than the tumor dose,
it would still probably lead
to unacceptable visual com-
complications.
Figure 6.1. Dose distribution of a
Thus, for all but very super-
single posterior-oblique photon beam,
designed to cover the target volume
(white outline). Relative dose is coded
by color according to the color bar
below.
ficial tumors, one cannot
treat the tumor using only
a single photon beam. The
solution is simple: to use
multiple cross-firing beams that concentrate dose within the target,
but spread it around outside the target so that the dose to uninvolved
organs at risk (OARs) is more tolerable (see Figure 1.3 in Chapter 1).
The set of cross-firing beams, together with their
weights
1
comprise
what is called the
treatment plan
. This chapter is the first of several
devoted to discussing how a treatment plan is designed
an
introduction to which has already been presented in Chapter 1. I have
restricted my discussion to external beam therapy with photons. This
−
restriction is for simplicity and focus. The many other forms of
radiation therapy
e.g., external beam therapy with electrons,
intracavitary or interstitial implants, and intraoperative radiation
therapy - have, of course, additional planning issues.
−
1
The “weight” of a beam is a multiplicative parameter which determines the
dose it delivers. It may either determine the dose delivered to a point
within the patient, or the minimum, mean or some other level of dose
delivered to a volume within the patient.
