The range of applicator sizes supplied with a treatment machine is, necessarily, limited to the most common clinically used field sizes. However, in order to minimise irradiation of normal tissue, treatments can be customised to the shape and size of an individual patient’s lesion, using a lead cut-out. The thickness of the lead cut-out is dependent on the X-Ray energy; 1 mm is suitable for low energy treatments and 2 mm is suitable for medium energy treatments. The lead cut-out must be slightly larger than the applicator, with an aperture cut-out according to the lesion, plus a clinical margin.
The cut-out reduces the total dose received at the surface of the lesion, by the ratio of the backscatter factor of the area of the cut-out, to the backscatter factor of the area of the applicator.
The lead cut-out factor at beam quality E and aperture size A is given by:
(where F is the size of the applicator used)
Depth Dose Curves
If the dose at a depth, other than the reference depth, is required it is necessary to measure the relevant depth dose curve. Depth dose curves can be measured in water, using a waterproofed cylindrical ion chamber, or in solid water, using a low energy X-Ray parallel plate chamber. It is not recommended that parallel plate chambers designed for electrons are used.
Example depth dose curves can be found in BJR Supplement 25 .