IMPLEMENTATION AND EVALUATION OF A THERMOPLASTIC IMMOBILIZATION DEVICE
FOR IRRADIATION OF PELVIC TUMORS
To explain how devices for pelvic immobilization were
fabricated and techniques developed. To compare the isocenter position
variations in patients with or without cast in the prone or supine position,
and to measure the skin dose increase due to cast in two, three,
four and six fields (4 oblique and 2 lateral) technique.
Methods and Materials
Aquaplast thermoplastic sheets measuring 18? x
24? and 2.4 mm thick were used for pelvic casts. Two acrylic frames were
made to handle the sheets in water at a temperature of about 80º
C. The water was heated in an electric heater and maintained
at constant temperature by two independent thermostats. When the Aquaplast
became transparent, it was taken out of the water, set on the body to shape
the pelvis, and fixed to a plastic baseplate. The frames can be adjusted
to the baseplate in
Ten pelvic casts were made on patients lying in
supine or prone position. The isocenter positions were checked once a week
by means of X-rays taken in a simulator Ximatron 5.
Longitudinal and lateral displacements were analyzed
during the simulation. The same procedure was applied in ten patients
without casts, whose skin was painted with a marker and silver nitrate.
The skin dose increase was assessed with thermoluminescent dosimeters
(rods TLD). The patients were irradiated with 10 MV photons.
The device was adjusted to every patient lying in both
treatment positions. Consolidation time for the plastic sheet was about
one hour. The mean deviation observed in patients without cast in every
axis was 4 mm. or higher, while the mean deviation in patients with cast
was 2 mm or lower, the longitudinal axis showing the greater displacement.
The skin dose value in patient with and without cast for 200 cGy are shown
in Table 1.
||115 cGy 166 cGy
||46 cGy 78 cGy
||70 65 cGy
||87 cGy 34
||77 cGy 106cGy
||38 47 cGy
||70 cGy 50
Table 1: Skin
dose values in patients with and without cast for 200 c Gy.
||57 cGy 36
The immobilization device contributes to radiation
therapy accuracy in pelvic tumors by improving the setup and reducing
involuntary motion. Besides, it is not necessary to use skin marks made
with markers or tattoos.
These results show that the skin dose was very high
when two fields were used in patients with casts.
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