Dosimetric characteristics of wedged fields.

The beam characteristics of the wedged fields in the nonwedged planes (planes normal to the wedged planes) were studied for 6 MV and 15 MV x-ray beams. A method was proposed for determining the maximum field length of a wedged field that can be used in the nonwedged plane without introducing undesirable alterations in the dose distributions of these fields. The method requires very few measurements. The relative wedge factors of 6 MV and 15 MV X-rays were determined for wedge filters of nominal wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees as a function of depth and field size. For a 6 MV beam the relative wedge factors determined for a field size of 10 x 10 cm2 for 30 degrees, 45 degrees, and 60 degrees wedge filters can be used for various field sizes ranging from 4 cm2 to 20 cm2 (except for the 60 degrees wedge for which the maximum field size that can be used is 15 x 20 cm2) without introducing errors in the dosimetric calculations of more than 0.5% for depths up to 20 cm and 1% for depths up to 30 cm. For the 15 degrees wedge filter the relative wedge factor for a field size of 10 x 10 cm2 can be used over the same range of field sizes by introducing slightly higher error, 0.5% for depths up to 10 cm and 1% for depths up to 30 cm. For a 15 MV beam the maximum magnitude of the relative wedge factors for 45 degrees and 60 degrees lead wedges is of the order of 1%, and it is not important clinically to apply a correction of that magnitude. For a 15 MV beam the relative wedge factors determined for a field size of 6 x 6 cm2 for the 15 degrees and 30 degrees steel wedges can be used over a range of field sizes from 4 cm2 to 20 cm2 without causing dosimetric errors greater than 0.5% for depths up to 10 cm.