Biomedical Engineering Reference
In-Depth Information
fact that the systematic variations were, at most anatomic sites, larger
than the random variations. These observations have been repeated in
many subsequent studies.
Figure 7.8 illustrates, in a simple example, the way in which random
and systematic variations differ so far as treatment outcome is
concerned. In this figure, a tumor is being irradiated by parallel-
opposed fields just large enough to cover the tumor if the fields were
correctly aligned. However, the fields are not correctly aligned, being
shifted half the time in one direction by, say, a distance of 20% of the
tumor diameter, and the other half of the time by the same distance in
the opposite direction.
In the case of random motion, all patients receive the dose illustrated
in Figure 7.8a, namely the prescription dose in the central 80% of the
tumor, and 50% of the prescription dose at the two sides. While this
dose is by no means ideal, it carries a finite chance of tumor control.
On the other hand, in the case of systematic motion, all patients will
have zero dose over 20% of their volume, as seen in Figure 7.8b, and
none will be controlled.
Figure 7.8 Schematic comparison of two types of motion.
(a) random motion, where half of the time during an irradiation the
beam is to the right and half the time it is to the left. The composite
dose distribution is shown in
red
(the dose profiles are slightly
vertically staggered for clarity).
(b) systematic motion, where half of the patients receive no dose on the
left side and the other half receive no dose on the right side.
While this example is highly simplistic, it illustrates the general point
that random positioning errors smear out a given patient's dose
distribution, whereas systematic positioning errors can leave each
