Study of a spherical phantom for Gamma knife dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Accurate absorbed dose to water (Dw) determination is crucial for radiation therapy.
• Spherical water phantoms are commonly used for dosimetry, but shell materials can introduce perturbations.
• The Leksell Gamma Knife (LGK) requires precise dosimetry for effective treatment planning.

Purpose of the Study:

• To evaluate the dosimetric impact of different spherical phantom shell materials (PMMA, Plastic Water™, polystyrene) on absorbed dose to water (Dw) measurements.
• To determine if corrections are needed when using these phantoms with the Leksell Gamma Knife.
• To assess the potential of these phantoms for measuring LGK helmet output.

Main Methods:

• Utilized PENELOPE Monte Carlo code to simulate photon beams from the Leksell Gamma Knife (LGK).
• Modeled four 16 cm diameter spherical phantoms: homogenous water, and water phantoms with 1 cm thick shells of PMMA, Plastic Water™, and polystyrene.
• Simulated a 0.007 cm³ scoring volume to represent an Exradin A-16 ionization chamber at the phantom center.

Main Results:

• The PMMA shell required a small correction for Dw determination, within statistical uncertainty (+/- 0.71%).
• Plastic Water™ and polystyrene shells showed no significant dosimetric perturbation, allowing for measurements without correction.
• These findings suggest potential advantages for using these spherical water phantoms to measure LGK 4 mm helmet output.

Conclusions:

• Plastic Water™ and polystyrene spherical phantom shells are suitable for LGK dosimetry without correction.
• PMMA shells require minor corrections, but remain within acceptable statistical limits.
• Spherical water phantoms with these shell materials offer a practical approach for LGK output measurements.