Analysis of central-axis doses for high-energy x rays

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Dosimetry

Background:

• Accurate central-axis dose calculations are crucial for radiotherapy.
• Existing analytical models for high-energy X-ray beams require improvement, particularly for phantom-scattered photon contributions.
• Understanding scatter factors is key to precise dose delivery.

Purpose of the Study:

• To enhance analytical expressions for central-axis doses in high-energy X-ray beams.
• To specifically improve the modeling of the phantom-scattered photon component.
• To develop a more accurate and efficient method for estimating scatter factors.

Main Methods:

• Characterized X-ray beams using transmission, head-scatter, and phantom-scatter factors.
• Measured transmission in water (narrow beam) and head scatter (small phantom in air).
• Deduced phantom-scatter factors from central-axis doses and analyzed their dependence on depth (d), field size (s), and attenuation coefficient (μ).

Main Results:

• Developed a model where scatter factors are proportional to depth (d) for constant d/s ratio.
• Extracted two quality-dependent parameters: scatter probability (numerically close to μ) and scatter directional distribution (linear function of μ).
• Achieved high accuracy: 2.5% maximum error at 6 MV, 0.8% at 25 MV, and +/- 2% for external data (10-15 MV).

Conclusions:

• The proposed model provides a realistic and accurate method for estimating scatter factors.
• The method requires minimal measurements, making it efficient for clinical implementation.
• The improved analytical expressions enhance the precision of central-axis dose calculations in radiotherapy.