Off-axis beam quality change in linear accelerator x-ray beams.

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Oncology

Background:

• Linear accelerators (LINACs) are crucial for radiation therapy.
• Flattening filters (FFs) in LINACs ensure a uniform dose distribution.
• FFs cause inherent beam hardening and spectral variation across the beam profile.

Purpose of the Study:

• To investigate the spatial variation of effective x-ray beam energy from LINACs.
• To quantify beam quality changes as a function of radial distance from the central axis.
• To assess the impact of flattening filter nonuniformity on beam characteristics.

Main Methods:

• Measurements of x-ray beam transmittance through a water column.
• Characterization of beam quality using Half-Value Layer (HVL) in water.
• Experiments conducted on multiple LINACs with energies ranging from 4 to 10 MV.
• Utilized good geometry setup for accurate transmittance measurements.

Main Results:

• The effective energy of the x-ray beam was found to vary with position.
• Beam energy consistently decreased with increasing distance from the central ray for all measured LINACs.
• HVL measurements confirmed spectral variations across the beam profile.

Conclusions:

• Nonuniform filtration by the flattening filter leads to spatial variation in LINAC x-ray beam energy.
• This energy gradient has implications for dose calculations and treatment planning in radiation therapy.
• Accurate characterization of beam quality across the entire beam profile is essential for precise radiotherapy delivery.