Determining a Gamma Knife's isocenter using a starburst shot.

Area of Science:

• Medical Physics
• Radiosurgery Technology
• Quality Assurance

Background:

• The Gamma Knife is a crucial tool for stereotactic radiosurgery.
• Understanding the isocenter from individual Co60 source behavior is essential for precise treatment delivery.

Purpose of the Study:

• Introduce and validate the "starburst shot," a novel quality assurance (QA) technique.
• Analyze the convergence of individual collimator beam axes to better understand the Gamma Knife's isocenter.

Main Methods:

• Developed cylindrical phantoms for panoramic radiographic films across 4-, 8-, and 16-mm collimator sizes.
• Created an algorithm to map collimator central axes in 3D and used optimization to find the smallest sphere encompassing them, estimating the isocenter.
• Scored radial distances from the isocenter using the probability p0.5 of passing within 0.5 mm.

Main Results:

• The starburst shot method robustly determined the isocenter.
• p0.5 values were 1.00 for 4-mm, 0.81-0.93 for 8-mm, and 0.74-0.92 for 16-mm collimators.
• Isocenters for different collimator sizes were within 0.2 mm of each other; the technique accurately detected a 1-mm shift.

Conclusions:

• The starburst shot is an effective QA technique for detailed Gamma Knife isocenter analysis.
• It verifies isocenter position and size with submillimeter accuracy, offering a significant advancement over standard methods.
• Proposed tiered p0.5 values (0.90 for 4mm, 0.80 for 8mm, 0.70 for 16mm) as a clinical QA standard.