Dynamic wedge factors for a comprehensive range of fields

Assessing the methods available to model wedge factors as applied to dynamic wedges over a comprehensive range of field elongations is the primary intention of this paper. Wedge factors for dynamically produced wedges were investigated by a series of measurements for square and rectangular fields. A Varian 600C linear accelerator was used for the experimental work. Dynamic wedge angles of 15, 30, 45 and 60 degrees were studied. (The wedge factor was defined as the ratio of the central axis dose reading at 10 cm deep of the wedged field to the reading at 10 cm depth for the same sized open field). The possibility of improving dynamic wedge factors for elongated fields by modelling the rectangular field with the equivalent square (Worthley) and also by the square field of equal area (Arthur) methods was evaluated. Dynamic wedge factors were found to depend on the field size in the wedged direction dominantly. The influence of field elongation in the non-wedged axis was found to be negligible. The experiments indicate that the derivation of a dynamic wedge factor for use with rectangular fields is best based on a square field with a side equal to that in the wedged axis. The application of two square field models (Worthley and Arthur) for elongated fields was found to produce significantly worsened wedge factors for the majority of the fields considered.