Assessment of variation of wedge factor with depth, field size and SSD for Neptun 10PC Linac in Mashhad Imam Reza Hospital

Background: In radiotherapy, wedge filters are used for optimizing the tumor dose distribution in patients. The attenuation in beam intensity due to the presence of wedge filter is compensated by means of a wedge factor measured at the central axis of the beam. The field size, depth and SSD dependence of wedge factor have been assessed for 9MV radiations of Neptun PC linear accelerator. Materials and Methods: Wedge factors (WF) at different SSD, field size (FS) and depth (d) in water were measured for 8 steel wedges with different sizes and angles of 15, 30, 45, and 60 degree. Experimental data were obtained using Neptun 10PC, Linac 9MV X-ray machine, a 3D water phantom, dosimeters and an electrometer. To study the effect of field size on WF, the wedge factor was measured for square field sizes from 5 ´ 5 to 20 ´ 20 cm, with 1 cm increment intervals for all wedges and, at the depth of 10 cm, SSD of 100 cm with monitor unit (MU) of 80. Effects of depth on WF were studied by measurement in various depths from 3-19cm for all wedge angles at SSD of 100cm, field size of 10 ´ 10cm and 80 MU irradiation. Effects of SSD on WF were investigated by a variation of SSD from 90-110cm with 5cm increment intervals while the dosimeter was set at depth of 10cm and field size of 10 ´ 10cm were irradiated for 80MU. Results: Linear dependence of WF with field size and depth of measurements were confirmed with 95% certainty. Shapiro-Wilk test, showed that the residual data of the regression tests have normal distributions (P>0.05). There was also found no linear relationship between WF and SSD (P>0.05). Conclusion: WF has linear dependence with field size and depth of measurements, but the rate of variations are less than 2.2% per 10cm variation in field size and less than 1.3% per 10 cm variation in depth of measurements, therefore, correction of WF for field size and depth of treatments in clinical trials is negligible. Iran . J. Radiat. Res., 2004 2 (2): 53-58