Direct Table Rotational Isocenter Determination Using Radiation Isocenter 📝

Author: Christopher Ackerman, Chang Chang, Yansong Liu, Stephen Kenneth Northway, Bailey Vallejo 👨‍🔬

Affiliation: University of California San Diego / California Protons, California Protons Cancer Therapy Center, California Proton Cancer Therapy Center 🌍

Abstract:

Purpose: To determine mechanical isocenter of a 6-degree-of-freedom (6DOF) couch using radiation isocenter. Coincidence between the isocenters is quantified using MyQA Phoenix amorphous silicon detector (IBA Dosimetry)
Methods: The detector was positioned using kV imaging on the couch horizontally with the gantry at 0°. The couch is rotated between +90° to -90° in 15° increments. A spot proton beam at 160 MeV is delivered at every couch angle. The detector recorded beam spot at each angle. The beam center was determined by locating the pixel of maximum intensity, and the detector's geometric center was obtained from DICOM metadata. A half pixel shift was applied to ensure subpixel accuracy in both coordinates. Since the location of beam spot remained constant regardless of couch rotation, the measured offsets reflect the effect of couch rotation on detector position. These offsets were fitted to a circle representing the couch's rotational isocenter. The distance between this fitted center and the spot location is determined to be the misalignment between the couch's and the beam's isocenters. The radius of the fitted circle represents the misalignment of the detector's center relative to the couch's rotation isocenter.
Results: The circle fit showed that as the couch rotates, the detector traces a nearly perfect circle of less than 1 mm in radius around the beam isocenter. A small (<1 mm) offset between the fitted circle center and the beam isocenter was observed, demonstrating stable and precise mechanical alignment of the 6DOF couch.
Conclusion: By employing the MyQA Phoenix detector and a simple beam center mapping approach, we achieved a direct, quantitative assessment of the 6DOF couch's rotational isocenter relative to the proton beam isocenter. The submillimeter accuracy confirms robust system performance and supports this method as a practical QA tool for routine verification of isocenter congruence in proton therapy.

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