Weaver R D, Gerbi B J, Dusenbery K E
Department of Therapeutic Radiology-Radiation Oncology, Fairview-University Medical Center, University of Minnesota, Minneapolis 55455, USA.
Int J Radiat Oncol Biol Phys. 1998 Apr 1;41(1):233-7. doi: 10.1016/s0360-3016(97)00905-x.
To protect the lens and cornea of the eye when treating the eyelid with electrons, we designed a tungsten and aluminum eye shield that protected both the lens and cornea, and also limited the amount of backscatter to the overlying eyelid when using electron beam therapy.
Custom curved tungsten eye shields, 2 mm and 3 mm thick, were placed on Kodak XV film on 8 cm polystyrene and irradiated 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 measured by placing the micro-TLDs on the beam entrance side to check the 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 shields. The measurements were repeated with 2- and 3-mm flat pieces of lead to determine both the transmission and the backscatter dose for this material.
Tungsten proved to be superior to lead for shielding 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 dmax. At 6 MeV for a 3-mm lead plus 0.5-mm aluminum, .046 Gy was measured beneath the shield, a 9.5% decrease with the tungsten. Backscatter was also decreased 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 aluminum, at 3 mm depth the dose was .048 Gy compared to .079 Gy with lead and aluminum (39% decrease). Additionally, the backscatter dose was 3% less using tungsten. Simulating the lens dose 3 mm beyond the shield for the 2-mm and 3-mm custom curved tungsten eye shields plus 0.5 mm of aluminum was .030 and .024 Gy, respectively, using 6 MeV (20% decrease). Using 9-MeV electrons, the dose 3 mm beyond the shield was .048 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, respectively. 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 shield plus 0.5 mm aluminum. There was very little difference in backscatter 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.
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 electrons, tungsten provides the necessary protection for the lens and cornea of the eye and decreases the amount of backscatter to the eyelid above the shield.
在用电子束治疗眼睑时保护眼睛的晶状体和角膜，我们设计了一种钨铝眼盾，它既能保护晶状体和角膜，又能在使用电子束治疗时限制对上方眼睑的反向散射量。
将定制的2毫米和3毫米厚的弧形钨眼盾放置在8厘米聚苯乙烯上的柯达XV胶片上并进行照射，以评估通过眼盾的透射率。为模拟眼睑厚度并固定微型热释光剂量计（micro-TLDs），制作了一个水胶体模具以匹配眼盾的曲率。通过将微型热释光剂量计放置在束流入射侧来测量反向散射，以检查眼睑下侧的剂量。在钨眼盾上不添加铝、添加0.5毫米铝和添加1.0毫米铝的情况下进行测量。用2毫米和3毫米厚的铅板重复测量，以确定这种材料的透射率和反向散射剂量。
事实证明，钨在屏蔽下方结构和减少反向散射方面优于铅。在6兆电子伏特时，当向剂量最大值（dmax）输送1.00戈瑞剂量时，一块3毫米厚的平板钨加0.5毫米铝的眼盾下方剂量为0.042戈瑞。在6兆电子伏特时，对于一块3毫米厚的铅加0.5毫米铝的情况，在眼盾下方测得的剂量为0.046戈瑞，使用钨时减少了9.5%。当使用钨加0.5毫米铝与相同厚度的铅相比时，反向散射也从1.17戈瑞降至1.13戈瑞，减少了4%。使用9兆电子伏特以相同方式进行测量。对于3毫米钨和0.5毫米铝，在3毫米深度处剂量为0.048戈瑞，而使用铅和铝时为0.079戈瑞（减少了39%）。此外，使用钨时反向散射剂量少3%。对于2毫米和3毫米定制弧形钨眼盾加0.5毫米铝，模拟眼盾后方3毫米处的晶状体剂量，使用6兆电子伏特时分别为0.030和0.024戈瑞（减少了20%）。使用9兆电子伏特电子时，2毫米眼盾后方3毫米处的剂量为0.048戈瑞，3毫米眼盾后方为0.029戈瑞（减少了40%）。使用更厚的钨并不能进一步减少反向散射。对于6兆电子伏特束流，使用2毫米或3毫米定制钨眼盾加0.5毫米铝时，反向散射剂量分别为1.03和1.02戈瑞。对于9兆电子伏特，使用2毫米定制眼盾加0.5毫米铝时反向散射剂量为1.
