Verification of source and collimator configuration for Gamma Knife® Perfexion™ using panoramic imaging

Purpose: The new model of stereotactic radiosurgery system, Gamma Knife® Perfexion™, allows automatic selection of built-in collimation, eliminating the need for the time consuming manual collimator installation required with previous models. However, the configuration of sources and collimators inside the system does not permit easy access for the verification of the selected collimation. While the conventional method of exposing a film at the isocenter is useful for obtaining composite dose information, it is difficult to interpret the data in terms of the integrity of each individual source and corresponding collimation. The primary aim of this study was to develop a method of verifying the geometric configuration of the sources and collimator modules of the Gamma Knife®Perfexion™. In addition, the method was extended to make dose measurements and verify the accuracy of dose distributions calculated by the mathematical formalism used in the treatment planning system, Leksell Gamma Plan. Methods: A panoramic view of all of 192 cobalt sources was simultaneously acquired by exposing a radiochromic film wrapped around the surface of a cylindrical phantom. The center of the phantom was mounted at the isocenter with its axis aligned along the longitudinal axis of the couch. The sizes and shapes of the source images projected on the phantom surface were compared to those calculated based on the manufacturer's design specifications. The measured dose at various points on the film was also compared to calculations using the algorithm of the planning system. Results: The panoramic images allowed clear identification of each of the 192 sources, verifying source integrity and selected collimator sizes. Dose on the film surface is due to the primary beam as well as phantom scatter and leakage contributions. Therefore, the dose at a point away from the isocenter cannot be determined simply based on the proportionality of collimator output factors; the use of a dose computation algorithm is required. Scatter and leakage dose contributions from neighboring sources were calculated and found to be 6.3% (ranging from 4.5% to 7.4%), 16.7% (12.5%-19.3%), and 66.6% (38%-78%) for the 4, 8, and 16 mm collimators, respectively, at the centers of the source images. The measured average dose on films with 16 mm collimators agrees with the dose model of the treatment planning system to within 1.0%. The average doses on the film were 24.0, 60.8, and 186.2 cGy for 4, 8, and 16 mm diameter collimators, respectively, when the machine was set to deliver a reference dose of 100 Gy to the center of an 80 mm radius spherical dosimetry phantom. Conclusions: A method of simultaneously capturing and analyzing the panoramic images of 192 cobalt sources has been developed to verify the source and collimator configuration of GK systems. The method was extended to verify the dose calculation model of the treatment planning system by comparing the measured doses on the panoramic film images and the corresponding calculated doses. The method presented can play a significant role in comprehensive commissioning and routine quality assurance testing of the Gamma Knife systems.