Forward scatter dose effect at metallic interfaces irradiated by x and gamma ray therapy beams

Aim: In this study forward scattering effects near different metallic inter faces are measured for Co-60 gamma and 6 and 18 MV photon beams. The studie d effects are the transport of secondary electrons across the metallic inte rface and the scattering of photons by the metallic inhomogeneity. Materials and Methods: All measurements were carded out with a PTW thin-win dow, parallel plate ionisation chamber (B23344-036) and an RDM-1F electrome ter with digital readout. Thin sheets of aluminium, mild steel, copper, cad mium and lead were used as in homogeneities. The inhomogeneities were place d between the polystyrene phantom and the front window of the chamber which was maintained at 100 cm SSD. Results: It was noticed that for a high energy photon beam (18 MV) the forw ard scatter dose factor (FSDF) increases rapidly as the thickness of the me tallic inhomogeneity increases. For low energy photons, there is a sharp in itial decrease of the FSDF until a minimum value is reached followed by a s low increase with increasing thickness of the inhomogeneity. It was also no ted that the FSDF variation at off-axis distances has slightly more slope c ompared with the ionization ratio (IR) curves for both 6 MV and 18 MV photo ns. However, the variation in slope is prominent for 1.8 MV compared with 6 MV photon beam. Conclusion: The sharp dose decrease observed downstream of a metallic inhom ogeneity at relatively Low photon energies (Co-60, 6 MV) is attributed to t he internal scattering of secondary electrons within the metal. The dose en hancement observed for high energy photon beams is attributed to the domina tion of the pair production process, increasing with atomic number. Since F SDF is dependent on the photon beam spectra, it can be used as a measure of beam quality across the beam.