The measurement of linear accelerator isocenter motion using a three-micrometer device and an adjustable pointer

Abstract

Purpose : The small motions of the major axes of a linear accelerator observed during gantry and treatment table rotation were measured to improve beam-target alignment during stereotactic radiosurgery (SRS). Methods and Materials : Measurements of gantry isocenter motion and table rotational axis wobble were performed with an adjustable front pointer and a three-micrometer device. Nominal gantry and table isocenters were specified. The gantry motion path and table isocenter coordinates were then applied to offset simulated treatment target coordinates so as to compensate for gantry sag. Target simulation films were examined to document improvement of beam-target alignment. Results : The overall precision of the measurement of gantry and table isocenter coordinates was 0.2 mm. Over gantry rotation of 0 to 360°, the gantry isocenter was found to follow a pinched loop with a maximum point to point distance of 1 mm. Table axis motion was found to be negligible relative to the reproducibility of gantry isocenter motion. Thus, a table isocenter was defined that was invariant to table rotation. Conclusion : Results indicate that the three-micrometer device and adjustable front pointer are useful tools for three-dimensional (3D) mapping of gantry, collimator and table isocenters and their motions. It is suggested that such measurements may be useful in the quality assurance of linear accelerators, particularly to improve beam-target alignment during SRS and other high dose external beam therapy.