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

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Abstract
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 state 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, large than those reported in the literature for other 6-MV linacs. Dosimetric results presented included central axis percentage depth dose (% DD) and tissue-maximum ratio (TMR), surface and buildup doses, output factors, and inverse square low applicability. The flatness and symmetry characteristics are within the manufacturer''s specifications for both large and small fields.