SU‐F‐T‐553: Sector Output Uniformity of the Perfexion Gamma Knife

Purpose: To study the uniformity of single-sector outputs from the 3 collimators of the Perfexion Gamma Knife (PGK) unit and investigate the potential impact on Gamma Knife treatment planning. Methods: An A16 ion chamber and a SFD diode detector were used to measure both the output factors and the single-sector outputs from a PGK unit. The detectors were placed in custom inserts in the center of a spherical solid water phantom and connected to a Max 4000 electrometer during the measurements. For single-sector outputs, a treatment plan with a prescription dose of 1 Gy at the focal point was generated for each sector of the 16 mm, 8 mm, and 4 mm collimators and was exported to the console computer for dose delivery. The procedure was repeated 3 times for each sector and an average value was taken as the sector output. Results: The differences between the output factors from the diode measurements and the clinical values for the 8 mm and 4 mm collimators were −0.4% and −2.1% respectively. The corresponding values for the A16 ion chamber were −3.5% and −17.1%. The maximum differences among the 8 single-sector outputs for the 16 mm, 8 mm, and 4 mm collimators were 4%, 3.7%, and 4.6% respectively. Variations of the sector outputs (defined as the standard deviation-to-mean ratio) among the 8 single-sector outputs for the 16 mm, 8 mm, and 4 mm collimators were measured at 1.16%, 1.32%, and 1.81%. Measurements using the ion chamber were similar to those acquired with the diode for the 16 mm and 8 mm collimators. Conclusion: Single-sector outputs for the 8 sectors of the 16 mm, 8 mm, and 4 mm collimators of the PGK differed by less than 5% and are within the specified range of planning accuracy.