THE ANALYSIS OF DEPTH AND FIELD DEPENDENCE OF 'WEDGE' FACTORS FOR HIGH ENERGY PHOTON BEAMS

'Wedge' filters are used to compensate for missing tissue and to optimize isodose distribution to target volumes. The original photon energy spectrum changes due to 'wedge' filter because of scattering of primary photons. As such it becomes essential to account for the dependence of the 'wedge' factor on depth and field size in clinical dose calculation. The aim of this study is the determination of 'wedge' factors as a function of field size and phantom depth for Co-60 gamma-ray and X-ray beams in the range of 4, 6 and 15 MV containing 15°-60° brass and tungsten 'wedge' filters. For Co-60 and 4 MV X-ray beams standard 'wedge' filters with angles ranging between 15°-60°; for 6 MV and 15 MV X-ray beams only one 'wedge' filter with an angle of 60° have been used. Wedge factors have been found by the help of the proportion of wedge values to open field values under the same conditions. In order to search field and depth dependence relative wedge factors (RWF) have been examined. Dependence of the wedge factors on field size is up to 2.4 %, 4%, 2%, 2% for Co-60, 4-, 6-, and 15 MV energy respectively for 15° and 30° wedge filters. For Co-60 and 4 MV X-rays, the wedge factors vary up to 5% for large wedge angle. The change of wedge factor with depth is 7% and 3% respectively for 4 and 6MV photon energies for thick wedges. For Co-60 and 4 MV X-rays, the use of single wedge factor measured in reference conditions introduces error of up to 7% and 5% for 15x15 and larger fields for 45° and 60° wedge filters. It appears from these results that the best approach for treatment planning may be to use depth normalized RWFD and field size normalized RWFA with wedge angle greater than 30° in dose calculations for large depth. The wedge factor should be measured in reference depth instead of the maximum dose depth in the phantom because of the electron contamination.

Keywords: wedge factor, field dependence, depth dependence, photon beam