Insensitivity of machine log files to MLC leaf backlash and effect of MLC backlash on clinical dynamic MLC motion: An experimental investigation.

PURPOSE: Multi-leaf-collimator (MLC) leaf position accuracy is important for accurate dynamic radiotherapy treatment plan delivery. Machine log files have become widely utilized for quality assurance (QA) of such dynamic treatments. The primary aim is to test the sensitivity of machine log files in comparison to electronic portal imaging device (EPID)-based measurements to MLC position errors caused by leaf backlash. The secondary aim is to investigate the effect of MLC leaf backlash on MLC leaf motion during clinical dynamic plan delivery. METHODS: The sensitivity of machine log files and two EPID-based measurements were assessed via a controlled experiment, whereby the length of the "T" section of a series of 12 MLC leaf T-nuts in a Varian Millennium MLC for a Trilogy C-series type linac was reduced by sandpapering the top of the "T" to introduce backlash. The built-in machine MLC leaf backlash test as well as measurements for two EPID-based dynamic MLC positional tests along with log files were recorded pre- and post-T-nut modification. All methods were investigated for sensitivity to the T-nut change by assessing the effect on measured MLC leaf positions. A reduced version of the experiment was repeated on a TrueBeam type linac with Millennium MLC. RESULTS: No significant differences before and after T-nut modification were detected in any of the log file data. Both EPID methods demonstrated sensitivity to the introduced change at approximately the expected magnitude with a strong dependence observed with gantry angle. EPID-based data showed MLC positional error in agreement with the micrometer measured T-nut length change to 0.07 ± 0.05 mm (1 SD) using the departmental routine QA test. Backlash results were consistent between linac types. CONCLUSION: Machine log files appear insensitive to MLC position errors caused by MLC leaf backlash introduced via the T-nut. The effect of backlash on clinical MLC motions is heavily gantry angle dependent.

Evaluation of the truebeam machine performance check (MPC): OBI X-ray tube alignment procedure.

Alignment of the On-Board Imager (OBI) X-ray tube is important for ensuring imaging to treatment isocenter coincidence, which in turn is important for accurate Image Guided Radiotherapy (IGRT). Varian introduced a new X-ray tube alignment procedure for the TrueBeam linac in software version 2.5 MR2 as part of the machine performance check (MPC) application. This study evaluated the new procedure against conventional methods and examined the clinical significance of X-ray tube misalignment. Long term stability and short term repeatability of MPC tube alignment was assessed as well as sensitivity of the method to setup error. Standard quality assurance tests expected to be sensitive to tube misalignment were performed before and after tube alignment. These tests included: IsoCal verification; MPC kV imager offset; Winston-Lutz: kV imaging to treatment/radiation isocenter coincidence; CBCT image QA using the Catphan phantom; and OBI image geometric accuracy and center pixel alignment. Tube alignment measurements were performed with MPC, the two-plate method, and wire-on-faceplate method. The X-ray tube was then realigned by approximately 1.01 mm in the tangential plane based upon MPC and the tube alignment and standard quality assurance measurements were repeated. The time taken for each tube alignment method was estimated. The MPC method of tube alignment was found to be repeatable, insignificantly sensitive to phantom setup error and quick and simple to perform. The standard QA tests were generally insensitive to the tube alignment change, possibly because of the IsoCal correction. However, reduction in the magnitude of IsoCal correction and MPC kV imager offset was recorded after tube alignment. There was also apparent improvement in CBCT image uniformity. The MPC procedure is recommended for X-ray tube alignment.