Characteristics of the 6-MV photon beam produced by a dual energy linear accelerator.

Clinical dosimetry data are presented for the lower-energy x-ray beam of a Varian Clinac 1800 linear accelerator. This beam has comparable characteristics to single energy linear accelerators with the same stated 6-MV x-ray energy. The nominal beam energy was found to be 5.3 +/- 0.3 MV on the central axis. Beam quality expressed in terms of half-value layer in water was found to vary by less than 10% over the entire field. The surface doses are only slightly, but consistently, larger than those reported in the literature for other 6-MV linacs. Dosimetric results presented include central axis percentage depth dose (% DD) and tissue-maximum ratio (TMR), surface and buildup doses, output factors, and inverse square law applicability. The flatness and symmetry characteristics are within the manufacturer's specifications for both large and small fields.