Biomedical Engineering Reference
In-Depth Information
proximal surface of the target volume, nor can it modify the intensity
within the target volume.
Beam scanning can be used to deliver any physically possible dose
distribution. In particular, it can be used to deliver IMPT. Indeed,
scanning was developed with precisely this application in mind. With
beam scanning, it is possible to vary the intensity of the pencil beams
both across the field and in depth and, therefore, one can “turn off ”
pencil beams which terminate proximal to the target volume and
hence contribute no useful dose to the target. In this way the high
dose volume can be tailored to the target volume proximally as well
as distally.
Figure 11.13 shows three different dose delivery techniques, all using
scanned beams, for both a single beam and for a 3-field plan.
scanned
(uniform)
scanned
(uniform)
scanned
IMPT
scanned
IMPT
scattered
scattered
(a)
(a)
(a)
(a)
(b)
(b)
(b)
(b)
(c)
(c)
(c)
(c)
single
beam
single
beam
(e)
(e)
(e)
(e)
(f)
(f)
(f)
(f)
(d)
(d)
(d)
(d)
three
beams
three
beams
Figure 11.13. Dose distributions for single beam (
top
) and three-
beam (
bottom
) proton treatments.
Left column (a and d):
simulated
scattered beams;
middle column (b and e):
scanned beams, but
delivering uniform dose to the target volume; and
right column
(c and f):
IMPT. Figure adapted with permission from
Goitein
et al
. (2002)
The three techniques shown are: column 1: mimic of a scattered
beam - i.e., a beam which is uniform across the field and has a
fixed modulation in depth across the field (mimicking a SOBP);
column 2: beams as in (1) except that those pencil beams which
terminate proximal to the target volume are turned off, thus sha-
ping the high dose region proximal to the target volume; and
