Wedge factors for rectangular fields.

The variation of wedge factor with field size was measured for a range of square and rectangular fields for 45 degrees and 60 degrees wedges. Measurements were performed on accelerators with both externally mounted wedges, the Varian 600C of nominal energy 6MV and internally mounted wedges, the Philips SL75/5 of nominal energy 6MV and the Philips SL25 of nominal energy 25MV. Analysis of the results confirm previous investigations reported in the literature of the variation of wedge factor with field size and in particular the significantly greater variation for internally mounted wedges. A wedge factor for a rectangular field based on the wedge factor of the open field equivalent square gives a maximum error of 1.5%. A wedge factor for a rectangular field based on the wedge factor of the square field of equal area reduces the maximum error to 0.5% for all three accelerators. Analysis of results reported in the literature show a similar reduction.