[An absorbed dose conversion factor due to the x-ray spectrum compare with the method by the half-value layer]

Area of Science:

• Medical Physics
• Radiological Dosimetry

Background:

• Absorbed dose conversion factors are crucial for accurate radiation dosimetry.
• Traditionally, effective energy (keV) derived from the half-value layer is used to determine these factors.
• However, this method may not precisely represent the X-ray beam quality.

Purpose of the Study:

• To investigate the necessity of X-ray spectrum measurement for precise absorbed dose conversion factor evaluation.
• To compare the accuracy of absorbed dose conversion factors derived from half-value layer versus spectrum measurements.
• To analyze the impact of X-ray tube voltage on these differences.

Main Methods:

• Comparison of absorbed dose conversion factors calculated using half-value layer versus detailed X-ray spectrum measurements.
• Evaluation across different phantom materials: water, soft tissue, and cortical bone.
• Analysis of the influence of varying X-ray tube voltages.

Main Results:

• Significant differences were observed in absorbed dose conversion factors between half-value layer and spectrum-based methods, particularly for cortical bone (up to 20.93% maximum difference).
• Differences were smaller in water and soft tissue (2.94% maximum difference).
• The discrepancy increased with rising X-ray tube voltage due to a higher proportion of high-energy photons.

Conclusions:

• X-ray spectrum measurement is essential for accurate absorbed dose conversion factor determination, especially for materials like cortical bone.
• Relying solely on the half-value layer can lead to substantial errors, particularly in heterogeneous media and at higher beam energies.
• Care must be taken when using absorbed dose conversion factors in dense materials like cortical bone due to potential amplification of errors.