Biomedical Engineering Reference
In-Depth Information
T
REATMENT OF
O
CULAR
M
ELANOMAS
If only because the treatment of ocular melanomas has been one of
the largest clinical experiences with proton beam therapy and has
been very successful, I will say just a few words about these
specialized treatments (Gragoudas
et al
., 2002; Egger
et al
., 2003;
Goitein and Miller, 1983).
Figure 11.17 shows several steps in the treatment process: Panel (a)
shows a wide-angle fundus view of an ocular melanoma; often ocular
melanomas are first discovered upon fundus examination. Panel (b)
shows a pathological specimen and, schematically, how at operation
when the posterior of the eye is exposed, the eye can be trans-
illuminated. Panel (c) shows radio-opaque clips, some 2 mm in
diameter, which are sutured to the sclera around the periphery of the
tumor. The location of the clips relative to the tumor base can be seen
during trans-illumination.
A computer model of the eye is built up as in panel (d); the normal
structures are taken from a library of structures of interest, scaled to
the dimensions of the eye as measured on A-mode ultrasound. The
tumor base is drawn on the surface of the retina as shown in panel (e),
based on drawings of the tumor-to-clip relationships made at the time
of operation, and on the tumor shape as seen in the fundus picture.
The body of the tumor is then added as seen in panel (f), based
primarily on ultrasound to measure the tumor's height and shape and
on visual examination of the eye.
A direction of gaze is chosen by having the computer interactively
move a virtual light source around with the eye following the light
until a desirable interrelationship of the target volume and the normal
structures, as seen in the beam's-eye view, has been achieved. This
process is based on the planner's experience. An aperture is then
drawn (panel (g)) with a margin of the order of 2 mm to allow for
sub-clinical extension of the tumor, alignment uncertainties, and the
90-50% penumbral width, which is typically about 1 mm. The dose
distributions can then be examined in sections through the eye (panel
(h)) and as isodose lines drawn on the curved retinal surface, as seen
in panel (i) which can be compared with panel (a). Dose-volume
histograms (panel (j)) can also be inspected.
