DOSIMETRIC EVALUATION OF LEAD AND TUNGSTEN EYE SHIELDS IN ELECTRON-BEAM TREATMENT

Purpose: The purpose of this study is to report that commercially avai lable eye shields (designed for orthovoltage x-rays) are inadequate to protect the ocular structures from penetrating electrons for electron beam energies equal to or greater than 6 MeV. Therefore, a prototype medium size tungsten eye shield was designed and fabricated. The advan tages of the tungsten eye shield over lead are discussed. Methods and Materials: Electron beams (6-9 MeV) are often used to irradiate eyelid tumors to curative doses. Eye shields can be placed under the eyelids to protect the globe. Film and thermoluminescent dosimeters (TLDs) we re used within a specially constructed polystyrene eye phantom to dete rmine the effectiveness of various commercially available internal eye shields (designed for orthovoltage x-rays). The same procedures were used to evaluate a prototype medium size tungsten eye shield (2.8 mm t hick), which was designed and fabricated for protection of the globe f rom penetrating electrons for electron beam energy equal to 9 MeV. A m ini-TLD was used to measure the dose enhancement due to electrons back scattered off the tungsten eye shield, both with or without a dental a crylic coating that is required to reduce discomfort, permit steriliza tion of the shield, and reduce the dose contribution from backscattere d electrons. Results: Transmission of a 6 MeV electron beam through a 1.7 mm thick lead eye shield was found to be 50% on the surface (corne a) of the phantom and 27% at a depth of 6 mm (lens). The thickness of lead required to stop 6-9 MeV electron beams is impractical. In place of lead, a prototype medium size tungsten eye shield was made. For 6 t o 9 MeV electrons, the doses measured on the surface (cornea) and at 6 mm (lens) and 21 mm (retina) depths were all less than 5% of the maxi mum dose of the open field (4 x 4 cm). Electrons backscattered off a t ungsten eye shield without acrylic coating increased the lid dose from 85 to 123% at 6 MeV and 87 to 119% at 9 MeV. For the tungsten eye shi eld coated with 2 similar to 3 mm of dental acrylic, the lid dose was increased from 85 to 98.5% at 6 MeV and 86 to 106% at 9 MeV. Conclusio n: Commercially available eye shields were evaluated and found to be c learly inadequate to protect the ocular structures for electron beam e nergies equal to or greater than 6 MeV. A tungsten eye shield has been found to provide adequate protection for electrons up to 9 MeV. The i ncrease in lid dose due to electrons backscattered off the tungsten ey e shield should be considered in the dose prescription. A minimum thic kness of 2 mm dental acrylic on the beam entrance surface of the tungs ten eye shield was found to reduce the backscattered electron effect t o acceptable levels.