Output ratios in a miniphantom for asymmetric fields shaped by a multileaf collimator

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Abstract
The integrated GE multileaf collimator (MLC) provides the ability to achieve `double' asymmetric fields: each of the 64 leaves allow an over-axis travel of 10 cm and the Y-jaws allow 20 cm. A formalism has recently been proposed by the authors to calculate the output ratio in a miniphantom for this type of MLC by the product of independent leaf and jaw correction factors. The original proposed formalism was restricted to regular or irregular fields including the collimator rotational axis. Introducing `reduced coordinates' for the correction factors in the present work this formalism is extended to asymmetric fields where central leaves or jaws overlap the collimator axis. The extended formalism is applied to asymmetric square, rectangular and irregular fields. For all fields checked at a given off-axis position, measured and calculated output ratios agree within 1% for 6, 18 and 25 MV photon beams. To relate output ratios normalized to off-axis points with output ratios on-axis, off-axis ratios are derived from film and miniphantom measurements. Both off-axis ratios agree to within 1% for 6 and 25 MV photon beams; a maximum deviation of 1.3% is observed at 18 MV. Calculated products of output ratios and off-axis ratios derived from films are compared with measurements for asymmetric square, rectangular and irregular fields, and agree mostly within 1% for all energies checked; maximum deviations of 1.3 and 1.6% are observed for 6 and 18 MV photon beams.