Formulation of spectra-based attenuation coefficients in water as a function of depth and off-axis distance for 4, 10 and 15Â MV X-ray beams

On the basis of X-ray spectra information for 4, 10 and 15 MV X-rays from a linear accelerator, we propose a method for formulating the attenuation coefficient in water as a function of depth and off-axis distance. It was confirmed by transmission measurement that the formulation provides a good estimation of the attenuation coefficient in water. Using the formulation, we further assessed the effective attenuation coefficient in water as a function of off-axis distance.