Biomedical Engineering Reference
In-Depth Information
Fig. 19.3
Absorbed depth dose (left) and lateral dose profiles (right) for an electron beam nominal
energy of 6 MeV and a field size of
cm
2
. Experimental measurements are shown
with symbols. Simulation results obtained with
PENELOPE
are shown with histogram. Standard
deviation bars are not shown being smaller than the symbol size
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of interest, and therefore needs dedicated treatment techniques. This is especially
challenging in the treatment of stage I Non-Hodgkin lymphomas of the conjunctiva
where external beam radiotherapy of the conjunctiva is the sole conservative and
curative approach [
22
]. The conjunctiva is a complex target volume. It covers the
anterior part of the eyeball excluding the cornea, reflects close to the equator to
form the fornix and covers the inner side of the lids. This topography results in a
form similar to a hemispherical shell that is extended from the surface up to a depth
of approximately 1.5 cm.
Before simulating the specific field used for the irradiation of the conjunctiva,
it is necessary to adjust the primary electron beam parameters so to reproduce
experimental absorbed dose data in water from a reference field. For that purpose,
a simulation of the Varian Clinac 2100 C/D at 6 MeV nominal energy with the
standard
cm
2
electron applicator is run. A drawing of the the simulated
geometry is shown in the upper right panel of Fig.
19.1
.
The simulated results shown in Fig.
19.3
are obtained from a monoenergetic
pencil-beam point-like primary electron source of 7.184 MeV. The agreement
between experimental and simulated data is better than 1% of the maximum dose,
with a distance-to-agreement of less than 1 mm. A PSF is tallied just upstream of
the third scraper of the electron applicator.
For the external beam radiotherapy treatment of the conjunctival lymphoma
a dedicated collimator inserted in the third scraper of the electron applicator is
used [
23
]. This collimator consists of a cerrobend block, 1.6 cm thick, with a central
cylindrical hole of diameter equal to 3.0 cm. A PMMA slab 0.2 cm thick is placed
on top of the insert. Aligned with the central beam axis, and attached from below
the slab, there is a hanging PMMA rod whose purpose is to shield the eye lens
of the patient. The length of the rod is 6.0 cm and its diameter equals to 1.0 cm
(see Fig.
19.4
). The distance from the upstream surface of the slab to the source is
93.2 cm.
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