Build up for Vac Fix

Fieldsize:  10 x 10 cm     FSD 100cm

Ionization chamber:        NACP 05-05

At radiation of 10 MVR:

Skin dose:                                      28.5%
2 x .15mm (Plastic covering of Vac Fix Bag):    34.2%
Build up due to plastic covering:                5.7%

Skin dose:                                      28.5%
25mm Polystyrene spheres + 2 x .15mm (Plastic): 42.6%
Build up due to plastic covering and spheres:   14.1%


With Cobalt:

Skin dose:                                      69.6%
2 x .15mm (Plastic covering of Vac Fix Bag):    80.0%
Build up due to plastic covering:               10.4%

Skin dose:                                      69.6%
25mm Polystyrene spheres + 2 x .15mm (Plastic): 99.5%
Build up due to plastic covering and spheres:   29.9%

 

	At 10 MVR the build up effect of the 2 layers of 0.15mm plastic foil is only 5.7%, and two layers of the foil plus 25mm thickness of spheres (polystyrene fill) is only 14.1%. The spheres alone have a build up effect of 8.4%
	Using cobalt is very different. A big buildup effect is apparent. We strongly advise that two Vac Fix shells be made for the patients. One for the prone position having the back uncovered, and one for the supine position having the front uncovered. In this way, the build up effect is completely avoided.