Measurement of in-air output ratios using different miniphantom materials.

In-air output ratios, S(c), were measured using miniphantoms made of PMMA (thickness 2.4-24 g cm(-2)), graphite (1.8-26.5 g cm(-2)), copper (1.6-23.3 g cm(-2)) and lead (2.3-21.6 g cm(-2)), for collimator settings of 3 x 3 to 40 x 40 cm(2), and x-ray energies of 6 MV and 15 MV, respectively. The effects of the miniphantom on S(c) were quantified as correction factors as functions of collimator setting, material types and miniphantom thickness for each photon energy to correct the measured values. For miniphantoms with sufficient thickness to eliminate electron disequilibrium, the total correction factors can be expressed as multiplications of three factors: the attenuation correction factor, the mass energy absorption correction factor and the phantom scatter correction factor. This formalism implies that the collimator setting dependence of the correction factor is mainly caused by the energy spectrum shift. The narrow-beam attenuation coefficients in various phantom materials for different collimator settings were determined in narrow-beam geometries using a specially constructed collimator mounted on the tray holder of the accelerator. We have determined that the maximum total correction factor is approximately 1.01. For miniphantoms made of PMMA, graphite, copper and lead, at the miniphantom thickness of 10 g cm(-2), the maximum total correction factors are 1.002, 1.003, 1.005, 1.007, and 1.002, 1.003, 1.008 and 1.009 for 6 MV and 15 MV, respectively.