Dose measurements in the build-up region for the photon beams from clinac-1800 dual energy medical linear accelerator

Aim: Since the skill dose becomes the limiting factor while deciding the tu morcidal dose, the detailed analysis of dose distribution in the build-up l egion is necessary for high-energy photon beams. In this study the beam cha racteristics affecting the build-up and skin dose for 6- and 18-MV photons are analyzed. Materials and Methods: Measurements were made with 6- and 18-MV photons usi ng a PTW parallel-plate ionization chamber (B 23344-036) and a RDM-1F elect rometer. Build-up ionization measurements were made with the chamber fitted into a 25 x 25 x 25 cm polystyrene phantom with a fixed SSD of 100 cm. The entrance and build-up dose measurements were made with a polycarbonate aci d a mesh type metallic shielding tray and a 45 degrees wedge. Exit dose mea surements were carried out for the graphite patient supporting assembly tab le top, 1.0 cm thick piece of wood and the 1.0 cm thick patient supporting perspex base frame for head and neck treatments. Results: It was observed that the d(max) decreased slightly with field size as with other accelerators. For both photon energies the surface dose was observed to increase with increase in field size. It was also noticed that the dose in the build-up region increases slightly when the polycarbonate s econdary blocking tray is introduced with the increase in surface dose. The data show that the tray perturbation factor (TPF) at surface decreases ste adily with tray-surface distance for both photon beams for all field sizes. It was noted that the TPF was more when the polycarbonate tray was introdu ced at shorter tray-surface distances for both energies. At tray-surface di stances above 60 cm the TPF almost remained close to unity for 6-MV photons For all field sizes, whereas the continuous decrease in TPF could be noted for 18-MV photon beams even after the TPF reached unity. Conclusion: The increase in surface dose with field size for both photon en ergies is due to the electron scattering from the intervening materials. Th e use of wedge filters absorbs low-energy scattered electrons significantly and hence, the relative surface dose (RSD) is always less than unity. The increase in dose enhancement percentage with graphite compared to perspex s upporting assembly indicates that the electron backscatter is proportional to the atomic number of the medium.