Example of “Coning-Down”
5” diameter field A=pr2 = 3.14 x 2.5 x 2.5 =19.6 sq. in.
4” diameter field A=pr2 = 3.14 x 2 x 2 = 12.5 sq. in.
(5” to 4”) a 20 % diameter reduction yields a reduction of area irradiated by over 60 %
If the field size doubles, the tissue volume irradiated quadruples