Long-term geometric quality assurance of radiation focal point and cone-beam computed tomography for Gamma Knife radiosurgery system

To investigate the geometric accuracy of the radiation focal point (RFP) and cone-beam computed tomography (CBCT) over long-term periods for the ICON Leksell Gamma Knife radiosurgery system. This phantom study utilized the ICON quality assurance tool plus, and the phantom was manually set on the patient position system before the implementation of treatment for patients. The deviation of the RFP position from the unit center point (UCP) and the positions of the four ball bearings (BBs) in the CBCT from the reference position were automatically analyzed. During 544 days, a total of 269 analyses were performed on different days. The mean +/- standard deviation (SD) of the deviation between measured RFP and UCP was 0.01 +/- 0.03, 0.01 +/- 0.03, and -0.01 +/- 0.01 mm in the X, Y, and Z directions, respectively. The deviations with offset values after the cobalt-60 source replacement (0.00 +/- 0.03, -0.01 +/- 0.01, and -0.01 +/- 0.01 mm in the X, Y, and Z directions, respectively) were significantly (p = 0.001) smaller than those before the replacement (0.02 +/- 0.03, 0.02 +/- 0.01, and -0.02 +/- 0.01 mm in the X, Y, and Z directions, respectively). The overall mean +/- SD of four BBs was -0.03 +/- 0.03, -0.01 +/- 0.05, and 0.01 +/- 0.03 mm in the X, Y, and Z directions, respectively. Geometric positional accuracy was ensured to be within 0.1 mm on most days over a long-term period of more than 500 days.