SU-E-T-417: Tomotherapy Isocentricity: Is it Good Enough for Stereotactic Radiosurgery?

To evaluate the isocenter accuracy of radiation beam relative to the Tomotherapy linear accelerator's mechanical axis using a simplified Winston Lutz test, and to determine if Tomotherapy system is accurate for SRS applications.

BrainLab Winston Lutz pointer was setup on the Tomotherapy couch to the isocenter according to the green lasers. Four static plans were created to deliver radiation with 1cm jaw opening, all MLC closed except the central two leaves, and gantry at 0, 90, 180 and 270 degree, respectively. Gafchromic EBT films were taped on a solid water slab and placed on opposite side of gantry at a fixed distance for each plan. After the exposure, the films were scanned using Vidar scanner along with a 1 cm scale drawn (Figure 1-A, 2-A). The images were first processed by ImageJ, using Find Edges function to enhance the visibility of the boundaries of the circles, from a metal ball in the pointer, and the rectangles defined by two central leaves and the jaws (Figure 1-B, 2-B). Then two diagonal lines were drawn on each rectangle, and a cross on each circle, with the cross sections representing the centers of the rectangles and circles, respectively (Figure 1-C, 2-C).

The displacements of the centers of the circles from the centers of the rectangles for the first setup (films were 11cm from the isocenter) were 0.50, 0.707, 1.0 and 0.707 millimeters for each gantry angle, respectively, with an average of 0.73mm. The displacements of second setup (films were 25cm from the isocenter) were 0.54, 1.07, 0.87 and 0.54 millimeters, with an average of 0.75mm.

The preliminary data show that the radiation isocenter agrees with mechanical isocenter well within one millimeter tolerance required for SRS treatments, which indicated Tomotherapy system is suitable for such applications.