Mapping initial and general recombination in scanning proton pencil beams

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Particle Therapy

Background:

• Ion recombination in ionization chambers affects dose accuracy in proton therapy.
• Accurate measurement of dose requires understanding and correcting for ion recombination.
• Scanning proton beams introduce unique challenges for recombination correction.

Purpose of the Study:

• To investigate ion recombination in parallel-plate ionization chambers used in scanning proton beams.
• To evaluate the accuracy of different methods for recombination correction, including the two-voltage (TVM) and three-voltage (3VL) methods.
• To compare experimental results with theoretical models like Boutillon formalism and Jaffé theory.

Main Methods:

• Ion recombination was studied in parallel-plate ionization chambers at clinical proton beam facilities.
• The Boutillon formalism was employed to differentiate initial and general recombination.
• Recombination correction factor (k_s) was calculated using the two-voltage method (TVM) and the three-voltage method (3VL).

Main Results:

• The three-voltage method (3VL) showed good agreement with the Boutillon method across various energies and dose rates.
• The two-voltage method (TVM) showed limitations at lower dose rates, aligning more with Jaffé theory for initial recombination.
• The numerical code IonTracks accurately predicted both initial and general recombination under different dose rate conditions.

Conclusions:

• The three-voltage method (3VL) is a reliable approach for correcting ion recombination in scanning proton beams.
• Ion recombination parameters were quantified for specific clinical proton beam setups, providing valuable data for dosimetry.
• Numerical simulations like IonTracks are effective tools for understanding and predicting ion recombination phenomena.