- Homogenous dry water phantom with precise spherical shape - 16 cm diameter
- Displacement of maximum dose from center of sphere shows degree of accuracy and stability of the system
- Checks the dose-calculation algorithm for a homogenous sphere
When the center of the sphere is used as the target for
testing the treatment unit, it must indicate the relationship
between the point of maximum dose and the target. These should
closely coincide for an accurate system.
A three-dimensional dosimetry cylinder at the center of the
sphere measures both the deviation of the maximum dose from
the target and the gradient of the dose along the three
orthogonal axes of the sphere.
TLD dosimetric cylinder is a stack of dry-water disks of 44 mm
in diameter. Seven of the disks have concentric rings of holes
for TLD rods, 1 mm in diameter and 3 mm long. Disk #4 is in
the central plane of the cylinder. A smaller target disk is at
the center of this disk. It has a Hounsfield number of about
130 for visibility in a CT scan.
The disks are assembled tightly by two nylon rods and round,
blind nuts, which locate the cylinder precisely, linearly and
angularly, in both sphere and head.
This is an optional cylinder, interchangeable with the TLD
cylinder. It is built up of 11 film disks and 12 dry-water
disks with the same nylon assembly rods and blind nuts.
A simple fixture is supplied so the user can punch the film
disks from film sheets.
is a dry-water cone, which fits precisely in a recess in the
sphere. It accepts the dosimetric cylinder (TLD or film). It
houses the dosimetric cylinder at the center of the sphere.
This is molded to locate the measuring volume of an ion
chamber at the center of the sphere. This cone is
custom-molded to suit the particular ion chamber specified by
- Total System Quality Assurance
- Checks the dose calculation algorithm with and without correction for tissue heterogeneities
- Measures the dose delivered to intracranial lesions
Provisions are made for installation of the dosimetric
cylinder at two locations in the cranium. One is on the CC
axis of the head close to the apex. The other is near one side
of the head at about ear level. Both cylinders are
perpendicular to the transverse planes of the head.
Since the dose at only one location is to be measured at a
time, the other location is filled with a dry-water cylinder
with no dosimetric provisions. The target can be located
uniquely by an initial CT scan.
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