Biomedical Engineering Reference
In-Depth Information
irradiate the target volume rather uniformly within a plane normal to
the beam direction and their dose will diminish near-exponentially
along the beam direction.
M
ULTIPLE
T
REATMENT
B
EAMS
A single photon beam would, because of the exponential attenuation
of X-rays in matter, lead to the delivery of a higher dose to the tissues
in front of the tumor than to the tumor itself, as shown in Figure 1.2.
In consequence, if one gives a
dose sufficient to control the
tumor with reasonably high pro-
bability, the dose to the upstream
tissues would be likely to lead to
to unacceptable morbidity. Such a
single beam would only be used
for very superficial tumors where
there is little upstream normal
tissue to damage and the skin-
sparing properties of X-rays are
useful.
Figure 1.2. Depth
−
dose curve of a
×
typical 10
10 cm 10 MeV X-ray
beam.
The solution is to use multiple cross-firing beams, all focused on and
encompassing the tumor, but coming from different directions so as to
traverse as far as possible different tissues outside the target volume.
This strategy markedly changes the distribution of dose, as is sche-
matically illustrated in Figure 1.3.
Figure 1.3. Schematic figure showing three different X-ray beam
arrangements (1, 2 and 4 beams). Each is designed to deliver 60 Gy at
the target center. The (very approximate) dose outside the target is
successively reduced as the number of beams increases. However, in a
complementary way, the volume outside the target volume that receives
an appreciable dose increases as the number of beams increases.
