IMPLEMENTATION AND EVALUATION OF A THERMOPLASTIC IMMOBILIZATION DEVICE FOR IRRADIATION OF PELVIC TUMORS 

Purpose

    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
various positions.
    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.

Results

    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.
 
 
Fields % Anterior Uncasted
Oblique Uncasted
Anterior casted
Oblique casted
Difference (%) Lateral uncasted Lateral casted Difference
2 115 cGy       166 cGy 44     - -                - - - -
3 46 cGy         78 cGy 70     65 cGy 87 cGy     34 - -
4 77 cGy         106cGy 38     47 cGy 70 cGy     50 - - -
6 -                     - -        42cGy 57 cGy     36 32 cGy 50 cGy 56
Table 1: Skin dose values in patients with and without cast for 200 c Gy.

Conclusions

    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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