OBJECTIVEThis study was to discuss the influencing factors of Starshot analysis during the quality control of medical linacs.

METHODSDuring the quality control of a Siemens medical linac, Starshot analysis were carried out using FilmQA software and KODAK low sensitive films. The influence of setting to the results of Starshot analysis were explored, including Jaw width, shit of gantry angles, center shifts of Jaws, Beam energy and scan resolution.

RESULTThe offset of Jaws's center had effect on the analysis result and the center offset was linearly related to the result. The influence of Beam energy and scan resolution was much smaller, and the maximum deviation was less than 0.2mm. The deviation for gantry angle at 4 degree was a little larger than 2 degree which deviation was less than 0.1mm.The Jaw width had greater effort on the analysis result, and two trends were found in 2 tests.

CONCLUSIONIn clinic, if the result of Starshot analysis was out of tolerance, a careful check to multi factors should be made before the last determination.