Picket fence test of Agility MLCs using linac log data

Abstract

The multileaf collimator (MLC) performance is critical during intensity modulated radiotherapy (IMRT) delivery. Picket fence test is typically performed by acquiring images of MLC gaps using film or electronic portal imaging device (EPID) to assess the MLC positional accuracy as part of a comprehensive weekly quality assurance (QA) programme. This study aims to investigate the accuracy of MLC position of Agility MLCs (Elekta, Stockholm, Sweden) using linac log data. The positions of MLCs were tracked during picket fence test using Elekta’s propriety optical tracking system on the linac head. The data were recorded at four cardinal gantry angles over six weeks. The fluence from the picket fence was constructed using the tracked MLC positions and compared with the fluence obtained from EPID. The fluence of the picket fence pattern constructed from the tracking data of MLC position showed that the measured range of gap size error within -0.3 mm to 0.8 mm. The average MLC positional error tracked was 0.07 mm ± 0.14 mm (mean ± 1 SD). Gap size measured from tracking data was within the range of the results obtained from EPID. Tracking the MLC position using the optical tracking system provide a more accurate test method for routine QA of the MLC performance. Over the period of investigation, it was found that the MLC performance was within the tolerance of ± 1 mm recommended by AAPM TG142.