EVALUATION OF EYE SHIELDS MADE OF TUNGSTEN AND ALUMINUM IN HIGH-ENERGY ELECTRON-BEAMS

Purpose: To protect the lens and cornea of the eye when treating the e yelid with electrons, we designed a tungsten and aluminum eye shield t hat protected both the lens and cornea, and also limited the amount of backscatter to the overlying eyelid when using electron beam therapy. Methods and Materials: Custom curved tungsten eye shields, 2 mm and 3 mm thick, were placed on Kodak XV film on 8 cm polystyrene and irradi ated to evaluate the transmission through the shields. To simulate the thickness of the eyelid and to hold the micro-TLDs, an aquaplast mold was made to match the curvature of the eye shields. Backscatter was m easured by placing the micro-TLDs on the beam entrance side to check t he dose to the underside of the eyelid. Measurements were done with no aluminum, 0.5, and 1.0 mm of aluminum on top of the tungsten eye shie lds. The measurements were repeated with 2- and 3-mm flat pieces of le ad to determine both the transmission and the backscatter dose for thi s material. Results: Tungsten proved to be superior to lead for shield ing the underlying structures and for reducing backscatter. At 6 MeV, a 3-mm flat slab of tungsten plus 0.5 mm of aluminum, resulted in .042 Gy under the shield when 1.00 Gy is delivered to d(max). At 6 MeV for a 3-mm lead plus 0.5-mm aluminum, .046 Gy was measured beneath the sh ield, a 9.5% decrease with the tungsten. Backscatter was also decrease d from 1.17 to 1.13 Gy, a 4% decrease, when using tungsten plus 0.5 mm of aluminum vs. the same thickness of lead. Measurements using 9 MeV were performed in the same manner. With 3 mm tungsten and 0.5 mm of al uminum, at 3 mm depth the dose was .048 Gy compared to .079 Gy with le ad and aluminum (39% decrease). Additionally, the backscatter dose was 3% less using tungsten. Simulating the lens dose 3 mm beyond the shie ld for the 2-mm and 3-mm custom curved tungsten eye shields plus 0.5 m m of aluminum was .030 and .024 Gy, respectively, using 6 MeV (20% dec rease). Using 9-MeV electrons, the dose 3 mm beyond the shield was .04 8 Gy for the 2-mm shield and .029 Gy for the 3-mm shield (40% decrease ). Backscatter was not further decreased using thicker tungsten. With a 6-MeV beam, using the 2-mm or 3-mm custom tungsten eye shields plus 0.5 mm of aluminum, the backscattered doses were 1.03 and 1.02 Gy, res pectively. The backscatter dose with 9 MeV was 1.06 Gy using the 2-mm custom shield plus 0.5 mm aluminum and 1.05 Gy with a 3-mm custom shie ld plus 0.5 mm aluminum. There was very little difference in backscatt er dosage under the eyelid using 0.5 vs. 1.0 mm of aluminum. Therefore , for patient comfort, we recommend using 0.5 mm of aluminum. Conclusi ons: Tungsten is superior to lead as a material for eye shields due to its higher density and lower atomic number (Z). Using 6- and 9-MeV el ectrons, tungsten provides the necessary protection for the lens and c ornea of the eye and decreases the amount of backscatter to the eyelid above the shield. (C) 1998 Elsevier Science Inc.