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Objective
When stereotactic RT is applied, the mechanical isocenter stability (couch, gantry, collimator) has to be within a 1- (AAPM rep. 54) or 2-mm radius for higly conformal RT (SGSMP rec. 11). Due to its large weight and anisotropic gravity effect, the gantry needs special attention. Its rotation is characterized by a "isocenter path". The path extent has to be in congruence with the required RT conformity and therefore regularely checked.
Materials and Methods
A theoretical formalism and the iPhone application isoPath, based on an innovative optical measurement technique, are presented. IsoPath measures the extension of the isocenter path and the coincidence of the isocenter (focus of path) with the room lasers: A marker platelet is fixed to the front pointer next to the expected isocenter position. When the gantry is rotating, images of the markers are aquired at constant time intervals. Results are presented immediately after acquisition. Further evaluations of an export file are performed in Excel.
Results
In our two tested Elekta Synergy linear accelerators, the shape of the reproducible isocenter path in the plane of rotation is a double circle. Its diameter is 1.27 and 1.45 mm for the considered machines. The measurement accuracy of path and isocenter is better than 0.02 mm. Conclusions
The presented measurement method is accurate, time-saving and virtually without costs. The path information could be included in the TPS.