Electron beam versus photon beam radiation therapy for the treatment of orbital lymphoid tumors.

Shielding the lens of the eye while treating orbital lymphoid tumors and achieving a relatively homogeneous dose can be a dosimetric challenge. There are many clinical cases of tumors close to one or both eyes, where it is desirable to spare the lens as much as possible. We will show isodose curves comparing direct anterior fields using various electron beam energies to a single 6 mv photon beam to a multifield combination. The presence of an eye shield in a photon beam may impair the dose distribution at the back of the eye to an undesirable degree. The strong in-depth dose drop-off in electrons and the ease with which the beam can be intercepted suggests that an electron beam with an eye shield may offer advantages. We designed a suspended movable lens shield .8 cm wide with a height of 4.5 cm made of cerrobend surrounded by a brass cylinder mounted to a thin plexiglass plate to insert into a Varian electron 6 x 6 cone. A 1.5 cm gap between block and eye surface was used. Verification of dose distribution under the lens shield was obtained by film dosimetry.