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Energy parameters of three linear accelerators manufactured by different companies have been studied. All three machines produce 6 MV X-rays. The energy parameters specified by different manufacturers and different protocols were measured and evaluated. Profile measurements were carried out to find out the quality of the beam in the off-axis. These are affected by the beam production system which are also compared. Relative measurements were carried out using a radiation field analyser and the absolute measurements were carried out using 0.6 cc ion chamber and Baldwin Farmer dosemeter. Generally, the manufacturers of the linear accelerators specify % DD measurements at two different depths one at dmax and the other one at 10 cm depth or the depth at which 50% of the maximum dose occurs. The studies conducted on three different machines indicate that they are within the specifications but less than the quoted nominal energy of 6 MV. Atleast, % DD measurements should be carried out at two different depths one at a smaller depth and the other one at a larger depth. Profile measurements are within the specifications.
