Back-scattering from metals in superficial therapy with high energy electrons

In the treatment of ear, eyelid, nose and lip lesions with 3–8 MeV electrons, lead shields are often used to protect underlying tissue. The characteristics of electron backscatter from lead and metals of different atomic number, Z, were investigated for electron beams in the above energy range using monocrystalline layers of LiF powder to measure dose.

The dose from electron back-scatter increased approximately in proportion to Z^½ but decreased slightly with increasing primary electron energy. The energy of the back-scattered electrons was sufficient to penetrate several millimetres of polystyrene and increased with Z and primary energy.

The overdosage produced by back-scatter from eye-shields in the total skin treatment of patients with 3·1 MeV electrons may be reduced from 80 per cent to 35 per cent by using 2·2 mm of stainless steel instead of lead. In the treatment of localized lesions at 6·4 MeV, the overdosage may be reduced from 73 per cent to 15 per cent by covering the lead with 2 mm of aluminium.