Purpose: To develop and analyze an off-axis isocentricity test for the evaluation of a LINAC’s multiple metastasis single isocenter treatments and determine the machine’s off-axis limitations.
Methods: A QA plan was developed for use with an existing phantom in our department. The existing QA phantom has a series of 0.75 cm diameter BBs positioned throughout itself. The two BBs used for evaluation in this test were located at the beam’s isocenter and at a combined 3 cm lateral, 2 cm longitudinal, and 2cm vertical displacement. The radial displacement of this BB from the isocenter is 4.12 cm. Images were acquired with various couch, gantry, and collimator rotations using the EPID. An in-house script was developed to analyze these images and calculate the absolute 3D error of the machine’s isocenter off axis.
Results: The error in the test machine’s mechanical isocenter was observed to be 0.74 mm. At the off axis BB the absolute 3D error was observed to be 1.49 mm. A visual inspection was used to verify the accuracy of the script’s BB identification routine. The location of the BB as denoted by the script agreed with our visual inspection.
Conclusion: This proof of concept tool can successfully identify and localize two BBs in three dimensional space using a series of MV images. These tests confirmed that small mechanical errors are magnified away from the machine’s isocenter. This work paves the way for future simultaneous evaluation of several points off the beam axis. This future work will allow one to determine an acceptable cutoff for single isocenter multi-metastasis treatments.