Lead-polystyrene transition zone dosimetry in high-energy photon beams

In order to study the dose enhancement under sheets of lead positioned directly on the skin of patients, parallel-plate ionization chamber measurements in high-energy photon beams (4-15 MV) were performed below a lead-polystyrene interface. The dose in the transition zone can be much higher or lower than in the situation with full buildup of polystyrene. The enhancement of ionization directly beneath the lead-polystyrene interface, compared to the ionization at a reference depth in polystyrene, increases with photon energy and field size. The field size dependence is due to an increase in relative contribution to the energy fluence of low-energy photons scattered in the phantom and for the 4 MV beam also to photons scattered in the head of the accelerator. By adding a thin (100 microns) plastic absorber against the lead, the low-energy and large-angle electrons, which give rise to the enhanced interface dose, can largely be removed. The data indicate that lead as bolus material should only be used with extreme caution.