US EPA

1755138 

Journal Article 

Dosimetric evaluation of lead and tungsten eye shields in electron beam treatment 

Shiu, AS; Tung, SS; Gastorf, RJ; Hogstrom, KR; Morrison, WH; Peters, LJ 

1996 

1 

International Journal of Radiation Oncology, Biology, Physics
ISSN: 0360-3016
EISSN: 1879-355X 

35 

3 

599-604 

8655385 

10.1016/S0360-3016(96)80024-1 

WOS:A1996UT72400023 

Purpose: 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. 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) 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. 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 (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
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. Conclusion: 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. 

eye shields; lead; tungsten; electron beams; backscattered electrons