Variations in the Dose Profiles of Physical and Virtual Wedge Filters of ONCOR Linear Accelerator

In this study, we measured the dose profiles of physical wedges (PWs) and virtual wedges (VW) for photon energies (6 MV and 15 MV), various field sizes (10 × 10, 15 × 15, and 20 × 20 cm²), depths (d_max, 10 cm, 20 cm), and wedge angles (15°, 30°, 45°, and 60°). This study was performed using a Siemens ONCOR IMRT Plus linear accelerator. The acquired dose profiles of PW and VW were compared and statistically analyzed.

The dose profiles were measured using IBA CC13 ion chamber in IBA Blue phantom (a three-dimensional water phantom). The source-to-surface distance was kept 100 cm and measurements were taken for 10 × 10, 15 × 15, and 20 × 20 cm² field sizes and for 15°, 30°, 45°, and 60° PWs. These measurements were taken for both 6 MV and 15 MV photon energies. VW profiles were obtained using LDA-99 linear detector array (IBA, Germany). The percent off-axis dose difference between PW and VW profiles were calculated, analyzed, and plotted. Statistics on the measured data was applied using SPSS version 13.

The percent dose difference between PW and VW beam profiles were calculated at different off-axis positions. These dose variations increased in the wedge direction (toe region), but were negligible in the nonwedge direction. The results of one-way analysis of variance show that the dose variation between PW and VW are significant with off-axis positions, wedge angles, and depth, but are statistically nonsignificant with energy and field sizes. These variations could produce abnormal doses, especially in large field sizes and wedge angles in clinical applications, which could be avoided by understanding the behavior of wedge profiles and comparing the calculated (from treatment planning system) and measured (actual) doses.