Physical and biological factors determining the effective proton range

Area of Science:

• Radiation Oncology
• Medical Physics

Background:

• Proton radiotherapy is a growing cancer treatment.
• A constant relative biological effectiveness (RBE) of 1.1 is standard, despite data showing RBE increases with depth.
• This discrepancy may impact proton beam penetration and dose to surrounding tissues.

Purpose of the Study:

• To analyze the impact of a tissue and dose-dependent RBE on proton beam effective range.
• To compare this to the range calculated using a generic RBE of 1.1.

Main Methods:

• Treatment planning studies using Local Effect Model (LEM IV) and TRiP98 software.
• Systematic analysis of factors influencing biologically effective proton range.
• Comparison of passive and active range modulation techniques.

Main Results:

• Beam energy, tissue type, and dose significantly affected biological range extension.
• Up to 4 mm increased penetration depth observed compared to constant RBE.
• Range extension was more pronounced with passive modulation; maximum RBE higher with active modulation.

Conclusions:

• Physical characteristics of proton beams, like distal penumbra width, significantly influence RBE gradients.
• This impacts the biologically effective penetration depth of the proton beam.