Determining a Gamma Knife's isocenter using a starburst shot

The Gamma Knife is an important treatment unit for stereotactic radiosurgery. The isocenter is well understood from the collective behavior of all its 192 Co60 sources but is not known from the behavior of individual sources.

To introduce and validate the "starburst shot," a new quality assurance (QA) technique to better understand the Gamma Knife's isocenter by analyzing the convergence of all individual collimator beam axes.

We built cylindrical phantoms to acquire panoramic radiographic films for 4-, 8-, and 16-mm collimator sizes. We developed an algorithm to locate the centers of each entrance and corresponding exit exposure on the panoramic film, map their centers back to the cylinder, and connect the centers with a line segment in 3D. These lines represent the collimators' central axes, and we analyzed the resulting cluster of 192 lines using an optimization algorithm to locate the smallest sphere that simultaneously touches all lines. This sphere's center is an estimate of the isocenter, and the radial distances from the isocenter were scored by the probability $p_{0.5}$ of passing within 0.5 mm of the isocenter.

This optimization method provided a robust determination of the isocenter. For the 4-mm collimators, $p_{0.5}$ was 1.00. For the 8-mm collimators, $p_{0.5}$ ranged from 0.81 to 0.93, and for the 16-mm collimators, it ranged from 0.74 to 0.92. The isocenters determined for the 4-, 8-, and 16-mm collimators were found to be within 0.2 mm of each other. In a double-exposure experiment, the starburst shot accurately found a 1-mm shift introduced between exposures.

The starburst shot is a new, effective technique for detailed Gamma Knife isocenter QA, providing a comprehensive analysis of all 192 beams. It successfully verifies the isocenter's position and size with submillimeter accuracy. The starburst shot differs significantly from standard techniques, such as the pinprick tool test, and we propose a tiered system of acceptable $p_{0.5}$ values of 0.90 for 4 mm, 0.80 for 8 mm, and 0.70 for 16 mm, as a QA standard for clinical use.