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SU‐E‐T‐195: Gantry Angle Dependency of MLC Leaf Position Error
Author(s) -
Ju S,
Hong C,
Kim M,
Chung K,
Kim J,
Han Y,
Ahn S,
Chung S,
Shin E,
Shin J,
Kim H,
Kim D,
Choi D
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4888525
Subject(s) - multileaf collimator , position (finance) , rotation (mathematics) , collimator , image guided radiation therapy , mathematics , quality assurance , linear particle accelerator , optics , nuclear medicine , physics , computer science , beam (structure) , artificial intelligence , medical imaging , geometry , engineering , medicine , operations management , external quality assessment , finance , economics
Purpose: The aim of this study was to investigate the gantry angle dependency of the multileaf collimator (MLC) leaf position error. Methods: An automatic MLC quality assurance system (AutoMLCQA) was developed to evaluate the gantry angle dependency of the MLC leaf position error using an electronic portal imaging device (EPID). To eliminate the EPID position error due to gantry rotation, we designed a reference maker (RM) that could be inserted into the wedge mount. After setting up the EPID, a reference image was taken of the RM using an open field. Next, an EPID‐based picket‐fence test (PFT) was performed without the RM. These procedures were repeated at every 45° intervals of the gantry angle. A total of eight reference images and PFT image sets were analyzed using in‐house software. The average MLC leaf position error was calculated at five pickets (‐10, ‐5, 0, 5, and 10 cm) in accordance with general PFT guidelines using in‐house software. This test was carried out for four linear accelerators. Results: The average MLC leaf position errors were within the set criterion of <1 mm (actual errors ranged from ‐0.7 to 0.8 mm) for all gantry angles, but significant gantry angle dependency was observed in all machines. The error was smaller at a gantry angle of 0° but increased toward the positive direction with gantry angle increments in the clockwise direction. The error reached a maximum value at a gantry angle of 90° and then gradually decreased until 180°. In the counter‐clockwise rotation of the gantry, the same pattern of error was observed but the error increased in the negative direction. Conclusion: The AutoMLCQA system was useful to evaluate the MLC leaf position error for various gantry angles without the EPID position error. The Gantry angle dependency should be considered during MLC leaf position error analysis.