Once the LPO and RFO were established and leaf position was validated, a measurement of the gap width was performed. MLC files were created with narrow bands or gaps of defined widths between opposing pairs of leaves at specified intervals.
For these fixed width gaps moving at constant speed, the dose error is inversely proportional to the gap width.
2
SMLC files with gaps (or slits) of 0.5 cm, 1 cm, or 2 cm moving from X1 to X2 were imported into Eclipse and dose was calculated using the AAA algorithm version 8.9 for the TrueBeam STx with a HDMLC. The DLG parameter was
varied in Eclipse to different values of 0 mm, 0.2 mm, 0.5 mm,
1.0 mm, 1.5 mm, and 2.4 mm.
The expected value was 0.5 mm, which is roughly twice the RFO.
Using the procedure described in Sec.II.A, the current DLG parameter for
our Trilogy MX equipped with the Millennium 120 MLC had been set to 2.4 mm. DLG values were chosen between the extremes to characterize the response, and finer resolution was used to select the DLG as the error decreased.