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Unusual Characteristics of the HD MLC On a TrueBeam STx

Annie Hsu

A Hsu*, E Mok, Stanford Cancer Center, Stanford, CA

PO-BPC-Exhibit Hall-15 Saturday  Room: Exhibit Hall

Purpose: During acceptance testing of the high definition MLC (HDMLC) on a Varian TrueBeam STx machine, unusual characteristics were noted in its mechanical capability. We present some of those findings and discuss their clinical impact.

Methods: Picket fence tests were run on both a Varian TrueBeam STx linac with an HD MLC and a Varian Clinac 21EX with a Millennium 120 MLC. Images were acquired with both EDR2 film and portal imaging (EPID).

Results: The picket fence result on a Millennium MLC shows the typical repeating pattern of gaps (dark due to exposure) and pickets (lighter due to the MLC blocking). For the HDMLC, the some of the gaps are much lighter than the pickets. This is was observed in both EDR2 film and EPID imaging. There also appears to be nonuniformity in the gaps over the width of the carriage and depending on which MLCs, the small 0.25cm leaves versus the larger 0.5cm leaves were being moved. The vendor was contacted and additional testing was done. One hypothesis is that this behavior maybe due to the intrinsic backlash of the MLC and is magnified by leaf curvature and the fact that the leaf gap for the TrueBeam is smaller than that for the Millennium MLC. It is also thought that leaf over travel is plays an effect. Discussions with the vendor yielded the additional information regarding software controls of the offset of the HDMLC.

Conclusion: The HDMLC on a TrueBeam is characteristically different than a Millennium MLC on a Clinac 21Ex. Reasons for this include hardware differences of the MLCs and software corrections that are handled by the TrueBeam. However, of primary clinical interest will be how these differences across the field profiles affect dosimetry and the modeling of the leaf gap in the planning system.


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