Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator.

A set of standard wedge filters has been modified for use with half-collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half-collimated field coverage (up to 20 x 30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off-axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.