THE FIELD SIZE AND DEPTH DEPENDENCE OF WEDGE TRANSMISSION FACTOR FOR HIGH ENERGY PHOTON BEAMS

Abstract

The wedge transmission factors (WTF) usually were assumed to be used for clinical treatment planning system by independence on field size and depth of measure- ment. For this study, the field size and depth dependence of the in phantom WTF has been determined for Co-60 teletherapy unit and three Linear Accelerator energies of 6, 10 and 15 MV X-ray beam, containing 15°-60° lead, brass and alloy wedge filters. All measurements were made with a cylindrical ionization chamber in water or solid water phantom with a source-skin distance of 80 cm or 100 cm. Field sizes varied from 4x4 cm2 up to a maximum allowable size for each wedge filter. Several depths of measure- ment were selected: d ,5 cm (AAPM TG-21 calibration depth), 10 cm and 15 cm. The results show that use of single wedge WTF measured for 10x10 cm2 field introduces error less than 3% for field size not exceeding than 15x15 cm2 for all energies, but for a 22x22 cm2 field size, the error is up to 5%, 5.5%, 6% and 4.5% for Co-60, 6, 10 and 15 MV respectively. Moreover, for a 25x25 cm2 field size the error is up to 7.6% and 5.7% for 6 and 15 MV respectively. For the depth dependence study, we conclude that the WTF at depth for Co-60 differ not exceeding 3.5% from the determined values at TG- 21 calibration depth and for 6, 10 and 15 MV X-ray there are about 4.4%, 2% and 2.8% difference respectively. In this paper we have attempted to show that there is a definite dependence of WTF on field size and depth. Therefore a WTF measurement for a refer- ence field size and depth may not be valid for all field sizes and depths.