Evaluation of wedge factor measurements for high energy photon treatment field using physical wedge

ABSTRACT

To evaluate the parameters that affect the wedge factor for physical wedge using 6 and 18 MV photon beams. Measurements were performed for manual wedges 15°, 30°, 45°, and 60° using Varian Linac 6 and 18 MV photon beams at different depths dmax, 5, 10, and 20 cm for square fields 4x4 cm2 to 20x20 cm[2]. Also the measurements were performed for rectangular fields start from 2x4 cm[2] to 20x35 cm[2]. Measurements were taken at two different set up, first of all the source surface distance [SSD] was 100 cm and the second, source axis distance [SAD] was 100 cm. The wedge factors [WFs] were calculated to describe its dependence on the field size, detector type and depth, and wedge position for 6 and 18 MV photon beams. The dependence of wedge factors on field size were less than 1% at 6 MV and 1% at 18 MV x-ray beams for all wedges. For 6 MV using field size 20x20 cm[2] and 45° wedge the wedge factors increased by 2.6% while at 18 MV the dependence of wedge factors on field size reach to 3%. For 6 MV, as the depth increases the wedge factor increases reach to 1.5%, 3.1%, 5.7 and 8.9% for wedge angles 150, 300, 450, and 600 respectively. The detector type and setup were slightly affecting the wedge factor by less than 1.4% for different type of detector with SSD and SAD setup. The difference between wedge factor using In and Out position for wedge angle 60° was 3% and 3.5% for 6 and 18 MV respectively. This study has demonstrated the parameters that affect wedge factor measurements using photon beam. The results show variation in the wedge factor with field size, up to 2.6% and 3% for the 6 and 18 MV respectively. The wedge factor using 6 MV is highly affected by the depth reach to 8.9%. On the other hand the wedge factor for wedge angle 60 was dependent on depth for both energies 6 and 18 MV. The detector type and setup were slightly affecting the wedge factor by less than 1.4%. The difference between wedge factor using In and Out position for wedge angle 60 was 3% and 3.5% for 6 and 18 MV respectively. According to these measurements it seems that the measurement using fixed position of wedge angle and detector at field size 10x10 cm[2] at depth between 5 to 10 cm would represent a good choice for measuring the wedge factor