On-axis and off-axis primary dose component in high energy photon beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiological Physics

Background:

• Accurate characterization of x-ray beams is essential for effective radiation therapy.
• Understanding primary dose components and off-axis variations is critical for treatment planning.

Purpose of the Study:

• To determine the depth dose of primary x-ray beam components, both on and off-axis.
• To investigate the impact of flattening filters on beam characteristics.
• To develop a method for correcting scatter and deriving primary photon fluence.

Main Methods:

• Transmission measurements in narrow beam geometry using a Perspex column.
• Iterative correction of experimental transmission curves using scatter coefficients (Sc) and attenuation coefficients (μ).
• Convolution of dose spread arrays and comparison with Monte Carlo simulations of bremsstrahlung spectra.

Main Results:

• Measured attenuation coefficients showed a linear increase with off-axis distance for all energies and linear accelerators studied.
• This off-axis attenuation behavior was observed both with and without flattening filters.
• The rate of increase in attenuation coefficients varied between different linear accelerators, even at the same nominal energy.

Conclusions:

• The observed off-axis beam hardening is consistent with angular variations in bremsstrahlung energy spectra.
• The developed methodology allows for accurate correction of experimental data to account for beam softening.
• These findings are vital for precise dose calculations in radiation therapy, particularly for off-axis regions.