The regions within the penumbra of the 40 x 40 cm2 diagonal profiles for all linacs and energies were matched to < 0.5 %. The TPR20,10 measurements were within 0.5 % for all energies. The global average gamma evaluation index (2 %, 2 mm) of all matching verification measurements was 100 %. The DLG for 6X was matched to 1.40±0.03 mm resulting in a DLG of 1.56±0.04 mm (15X), 1.25±0.03 mm (6XFFF) and 1.45±0.04 mm (10XFFF). The jaws were calibrated in each linac with a precision of 0.3 mm, and the jaw junction at isocenter was calibrated with a precision of 0.2 mm, significantly improving the calibration method provided by the vendor (Table 1).
Varying the Spot Size
resulted in a negligible effect on output. The Spot Size
was therefore optimized based on penumbra widths of small MLC-defined lateral dose profiles and resulted in 0.5 mm and 0 mm in X- and Y-axis respectively for all energies (vendor recommendation is X=1 mm and Y=0 mm). The global average gamma evaluation index (2 %, 2 mm) of all AAA
verification measurements was 100 % (Figure 1). DLG
was optimized for 6X (6XFFF and 10XFFF) to 1.8 mm (1.6 mm, 1.8 mm) to match a 1.4 mm measured DLG
(1.25 mm and 1.45 mm). No DLG
optimization was done for 15X because no dynamic treatments are used clinically for this energy.