LiF dosimetric studies of cavity displacement effect and reference depths for electron beams.

The total effects of the cavity displacement and the perturbation of the field by the cavity in thimble-chamber measurements of absorbed dose were investigated for electron beams of energy 15, 20, 30, and 39 MeV and for 60CO gamma radiation by LiF dosimeter rods of diameter 1 mm and length 6 mm. The measurements were carried out in a polystyrene phantom at recommended reference depths and at the respective electronic equilibrium depths, for cavities of varying sizes. Owing to the cavity effect, the ratio of absorbed dose of a 5-mm-diameter cavity to that of a point cavity, at the reference depth, was 1.028 for 60Co gamma radiation; this ratio varied from 1.022 at 15 MeV to 1.033 for 39-MeV electrons. Although the cavity effect is present over regions of the depth dose where dose is constant with depth, here it was less dependent on cavity size than when the cavity was positioned at ICRU depths. Both the displacement and perturbation of the radiation field effects are automatically corrected through the experimental arrangement in this work.