A generalized 2D pencil beam scaling algorithm for proton dose calculation in heterogeneous slab geometries

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Pencil beam algorithms are standard for proton therapy dose calculations.
• Existing 2D scaling models rely on specific scattering power assumptions, limiting their applicability in heterogeneous media.

Purpose of the Study:

• To develop a generalized 2D pencil beam scaling algorithm for proton therapy.
• The new model aims to be independent of the proton scattering power model and compatible with various radial kernel width expressions.

Main Methods:

• Derived a generalized expression for radial pencil beam width using Fermi-Eyges transport theory.
• Performed test calculations in homogeneous and heterogeneous phantoms using local and nonlocal scattering models.
• Evaluated dose calculations for narrow Gaussian proton beams.

Main Results:

• Original 2D scaling model showed significant under-prediction (up to 21%) and dose discrepancies (up to 32%) when using nonlocal scattering models.
• Errors increased with beam penetration in heterogeneous phantoms.
• The generalized 2D scaling model demonstrated excellent performance with a maximum dose error of 0.3% in a heterogeneous phantom.

Conclusions:

• A generalized 2D pencil beam scaling method has been successfully derived.
• The new model is independent of proton scattering power and robust to different radial kernel width functions.
• This generalized model offers accurate dose calculations in both homogeneous and heterogeneous environments for proton therapy.