Lead as surface bolus for high-energy photon and electron therapy

Dose distributions were evaluated under thin sheet lead used as surface bolus for 4- and 10-MV photons and 6- and 9-MeV electrons using a parallel-plate ion chamber and film. A narrow, significantly low dose region (-17%) was noted for 4-MV photons, whereas a 6% increase in dose was present for 10-MV photons. The dose was elevated 15%-22% near the surface of electron fields with lead bolus, but depth dose relationships were similar to soft-tissue-equivalent (STE) bolus. Investigation of partial-field bolus (2-cm-diam circle) documented reduced doses due to lack of lateral electron equilibrium for 10-MV photons, which was less evident using lead, and large edge effects (up to 30%) for electrons using either lead or STE bolus. Dose distributions on sloped surfaces with electron fields were similar for lead and STE; both require thickness adjustment to achieve a desired effective thickness normal to the surface. Lead bolus has been used successfully in clinical practice for photons.