Elevated LET components in clinical proton beams.

Area of Science:

• Medical Physics
• Radiation Oncology
• Nuclear Physics

Background:

• Proton therapy utilizes proton beams for cancer treatment.
• Accurate dose and linear energy transfer (LET) calculations are vital for effective biological treatment planning.
• Secondary particles generated during proton interactions can influence dose distribution and biological effects.

Purpose of the Study:

• To quantify the contribution of secondary particles, particularly secondary protons and recoils, to the dose-averaged LET (LET(d)) in proton beams.
• To assess the impact of these secondary particles on biological treatment planning and radiobiological effectiveness.
• To investigate the influence of beam delivery techniques (passive scattering vs. active scanning) and phantom inhomogeneities on LET distributions.

Main Methods:

• Proton Monte Carlo simulations were performed in water phantoms and patient-specific models.
• Simulations accounted for primary protons, secondary particles (including secondary protons and nuclear recoils), and inelastic nuclear interactions.
• Dose-averaged LET (LET(d)) was calculated and compared with and without the inclusion of secondary particles.

Main Results:

• Secondary protons were found to have LET(d) values up to 10 times higher than primary protons at the same depth.
• The inclusion of secondary protons increased LET(d) by approximately 50% along the central axis and over 200% in the penumbra.
• The maximum LET(d) after the Bragg peak increased from 12 to 15 keV µm⁻¹ with secondary protons.
• Recoil nuclei (A > 3) contributed minimally (1.2%) to the dose in the entrance region for a prostate case, though their biological impact requires further study.

Conclusions:

• Secondary protons play a significant role in determining LET(d) in proton therapy, necessitating their inclusion in biological treatment planning.
• Accurate modeling of LET(d) requires consideration of secondary particle production, especially for analytical methods.
• Findings highlight the importance of secondary particles for LET-based radiobiological effectiveness calculations and analysis of radiobiological experiments.
• Inhomogeneities and beam delivery methods (passive vs. active scanning) subtly alter LET distributions, underscoring the complexity of proton beam modeling.