Metallic interface dose perturbations with medical linear accelerator high energy x ray photons

Localised dose enhancement in the unit density phantom material for high energy photon beams, when they interact with high atomic number metal interfaces have been reported in literature. They arise from localised scatter and reflection of secondary electrons across the interface. The forward scattering effect has been measured for 6 and 18 MV photons from a medical accelerator with aluminium, mild steel, copper, cadmium and lead inhomogeneities. The measurements were carried out using thin window parallel plate ionisation chamber. For 6X photon beam the forward dose perturbation factors (FDPF) initially decrease up to a minimum value followed by a slow increase as the metal thicknesses are increased. For 18 MV beam, the FDPF increases rapidly and reaches saturation value for finite thickness of the inhomogeneity. These data will be relevant in clinical radiotherapy for determination of absorbed doses proximal to the metal interfaces, if the patient has metal prostheses. (author)