Combining backscattered electrons and low energy photons to improve the dose distribution to an eyelid.

In electron beam therapy, backscattered electrons from metal shields result in a high dose to tissue in the vicinity of the shield. Data are now available which enable both the magnitude of the dose enhancement at the interface and the fall off in dose 'up stream' to be determined. With the aid of these data, a combined modality therapy (5 MeV electrons, 1.1 mm A1 HVL X rays) was given to an eyelid during which the lens was protected with a superficial X ray lead eye shield. The eyelid was treated to a mid lid dose of 45 Gy in 28 days, and the dose gradient on the beam central axis was more uniform (+/- 3%) than that obtainable with superficial X rays alone. The lens dose from the combined therapy was estimated to be less than 0.25 Gy.