Biomedical Engineering Reference
In-Depth Information
Interplay effects due to organ motion
The major problem with scanning is its sensitivity to organ and tumor
motion during the time a beam is being delivered, primarily due to
respiration. Organ motion can markedly affect the dose distribution
because of what are termed
interplay effects
(Bortfeld
et al
., 2002).
This term describes the possible interplay between the motion of the
scanned pencil beam, and the motion of, say, a cell (Goitein, 2005)
within the target volume. A given cell can either move so that it is
outside a pencil beam when it should be within it, resulting in a lower
than desired dose, or can
linger within a pencil beam
as it moves, resulting in a
higher than desired dose.
Figure 10.23 explains gra-
phically, although in an
exaggerated manner, how
this variation in dose, which
I term
dose mottle
, comes
about. On the positive side,
the average dose within the
target volume is essentially
unchanged as one is deliver-
ing the same total energy
no matter what movements
take place.
6
Figure 10.23. Schematic illustration of
how dose mottle arises. (a) and (b):
cell irradiated by the third pencil beam
moves right, into the way of the fourth
pencil beam, and so receives near-
double dose. (c) and (d): cell initially
outside the third pencil beam moves
left, avoiding being irradiated by
fourth pencil beam, and so receives
near-zero dose.
There are two ways to deal
with interplay effects, and
both are needed in situa-
tions where the amplitude
of motion is more than a
couple of millimeters or so.
The first approach is to gate
6
One can potentially have a problem any time the period of the beam
application is comparable to the period of organ motion. This is why, with
scattered beams, the range modulator is generally designed to rotate
rapidly (hundreds of Hz); its period is then much shorter than any
significant organ motion and, hence, interplay effects are averted. In
scanned beam delivery, because one scans in three dimensions, at least one
motion is pretty much guaranteed to have a period comparable to that of
the respiratory cycle, for example, and thus give rise to interplay effects.
