Shiu A S, Tung S S, Gastorf R J, Hogstrom K R, Morrison W H, Peters L J
Department of Radiation Physics, University of Texas M.D. Anderson Cancer Center, Houston, Texas 77030, USA.
Int J Radiat Oncol Biol Phys. 1996 Jun 1;35(3):599-604. doi: 10.1016/s0360-3016(96)80024-1.
The purpose of this study is to report that commercially available 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 advantages of the tungsten eye shield over lead are discussed.
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) were used within a specially constructed polystyrene eye phantom to determine 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 thick), which was designed and fabricated for protection of the globe from penetrating electrons for electron beam energy equal to 9 MeV. A mini-TLD was used to measure the dose enhancement due to electrons backscattered off the tungsten eye shield, both with or without a dental acrylic coating that is required to reduce discomfort, permit sterilization of the shield, and reduce the dose contribution from backscattered electrons.
Transmission of a 6 MeV electron beam through a 1.7 mm thick lead eye shield was found to be 50% on the surface (cornea) 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 to 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 maximum dose of the open field (4 x 4 cm). Electrons backscattered off a tungsten 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 shield coated with 2-3 mm of dental acrylic, the lid dose was increased from 85 to 98.5% at 6 MeV and 86 to 106% at 9 MeV.
Commercially available eye shields were evaluated and found to be clearly inadequate to protect the ocular structures for electron beam energies 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 increase in lid dose due to electrons backscattered off the tungsten eye shield should be considered in the dose prescription. A minimum thickness of 2 mm dental acrylic on the beam entrance surface of the tungsten eye shield was found to reduce the backscattered electron effect to acceptable levels.
本研究旨在报告，市售的眼盾（设计用于深部X射线）不足以保护眼部结构免受能量等于或大于6 MeV的穿透性电子的伤害。因此，设计并制造了一种中型钨制眼盾原型。讨论了钨制眼盾相对于铅制眼盾的优势。
电子束（6 - 9 MeV）常被用于将眼睑肿瘤照射至治愈剂量。眼盾可置于眼睑下方以保护眼球。在特制的聚苯乙烯眼模内使用胶片和热释光剂量仪（TLD）来确定各种市售的内部眼盾（设计用于深部X射线）的有效性。采用相同程序评估一种中型钨制眼盾原型（厚2.8 mm），其设计目的是保护眼球免受能量为9 MeV的穿透性电子的伤害。使用微型TLD测量因电子从钨制眼盾背散射而导致的剂量增加，测量时分别有无用于减轻不适、使眼盾可消毒以及减少背散射电子剂量贡献的牙科丙烯酸涂层。
发现6 MeV电子束透过1.7 mm厚的铅制眼盾在模体表面（角膜）的透射率为50%，在6 mm深度（晶状体）处为27%。阻止6 - 9 MeV电子束所需的铅厚度不切实际。取而代之的是制作了一种中型钨制眼盾原型。对于6至9 MeV的电子，在表面（角膜）、6 mm深度（晶状体）和21 mm深度（视网膜）处测量的剂量均小于开放野（4×4 cm）最大剂量的5%。未涂丙烯酸涂层的钨制眼盾背散射的电子使眼睑剂量在6 MeV时从85%增加到123%，在9 MeV时从87%增加到119%。对于涂有2 - 3 mm牙科丙烯酸的钨制眼盾，眼睑剂量在6 MeV时从85%增加到98.5%，在9 MeV时从86%增加到106%。
对市售眼盾进行了评估，发现其明显不足以保护眼部结构免受能量等于或大于6 MeV的电子束伤害。已发现一种钨制眼盾可为高达9 MeV的电子提供充分保护。在剂量处方中应考虑因电子从钨制眼盾背散射而导致的眼睑剂量增加。发现钨制眼盾束入射表面上至少2 mm厚 的牙科丙烯酸可将背散射电子效应降低至可接受水平。
