Head-scatter factors and effective x-ray source positions in a 25-MV linear accelerator

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Accurate characterization of linear accelerator (LINAC) beam properties is crucial for precise radiation dosimetry.
• Understanding the effective source position and head-scatter factors is essential for accurate dose calculations in radiotherapy.
• Variations in beam parameters can impact treatment planning and delivery accuracy.

Purpose of the Study:

• To investigate the behavior of the effective source position and head-scatter factors for 6 and 25 MV x-ray beams from a LINAC.
• To determine how these parameters change with field size, beam energy, and the use of wedges.
• To assess the distance dependence of head-scatter correction factors.

Main Methods:

• Measurements of primary photon fluence in air were conducted for various square field sizes (5x5 cm to 40x40 cm).
• Experiments were performed at different source-to-surface distances (80-140 cm) using both open and 60-degree wedged fields.
• Inverse-square analysis was employed to determine the effective source position, and head-scatter factors were calculated.

Main Results:

• For open fields, the effective source position was found to be consistent (~1 cm downstream) at both 6 and 25 MV across all field sizes.
• For wedged fields, the effective source position demonstrated dependence on field size, ranging from approximately 2 to 4 cm downstream.
• Head-scatter correction factors were observed to be largely independent of distance for both energies and field configurations.

Conclusions:

• The effective source position of a LINAC is energy-independent for open fields but exhibits field-size dependence for wedged fields.
• Head-scatter factors are robust with respect to distance variations, simplifying their application in clinical dosimetry.
• These findings contribute to improved accuracy in radiotherapy dose calculations and treatment planning.