Physical characteristics of 6 MV and 18 MV photon from a dual energy linear accelerator

Clinically pertinent data of the photon beams of nominal energy 6 MV and 18 MV from a dual energy linear accelerator is measured using radiation field analyser with semiconductor diodes and ionisation chambers. Percentage depth dose values are compared with available data from other linacs and BJR-17. Measurements made in the buildup region using parallel plate chamber show a marked increase in the surface dose and dsub(max) shifts by 3 mm and 14 mm for 6 MV and 18 MV photons respectively as the field size is increased from 4 x 4 sq. cm to the maximum. Variations of wedge angles with energy and field size are also determined upto the wedge widths of 15 cm for all the four available wedge angles. Isodose plots for both the energies are obtained using semiconductor diodes in a RFA-3 system. Output factors, wedge transmission factors and shielding tray factors were also measured in clear polystyrene phantom for both the photon energies at the depths of maximum ionisation. (author). 8 refs., 1 fig., 4 tabs