Accuracy requirements of the primary x-ray spectrum in dose calculations using FFT convolution techniques

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Fast Fourier Transform (FFT) convolution is a key method for photon beam dose calculation.
• Accurate knowledge of the primary photon beam energy spectrum is essential for FFT convolution accuracy.
• Quantitative assessment of the required spectral accuracy has been lacking.

Purpose of the Study:

• To evaluate the sensitivity of dose calculation parameters to the incident photon spectrum.
• To determine the quantitative accuracy required for primary photon energy spectrum bins for FFT convolution.
• To establish criteria for spectral accuracy to achieve specific dose calculation precision.

Main Methods:

• Simulations were performed for 6- and 18-MV photon beams.
• Sensitivity analysis of central axis depth dose and dose under attenuators to spectral variations.
• Comparison of depth-dose data calculated with original and modified photon spectra.
• Systematic variation of energy bin weightings to assess impact on dose calculations.

Main Results:

• Maximum percent depth dose error on the central axis is proportional to changes in average incident energy.
• To maintain <1% depth-dose error, relative changes in average energy from single spectral bins must be <2%.
• Required accuracy for individual energy bins varies significantly based on their energy and weighting.

Conclusions:

• The study provides quantitative guidelines for the necessary accuracy of photon beam energy spectra in FFT convolution calculations.
• Specific energy bins require tailored accuracy levels, with lower energy bins often needing higher precision.
• Achieving precise dose calculations necessitates careful consideration of spectral accuracy, particularly for critical energy regions.