Determination of the attenuation coefficient for megavoltage photons in the water phantom

Background: Attenuation coefficient (μ) plays an important role in calculations of treatment planning systems, as well as determination of dose distributions in external beam therapy, dosimetry, protection, phantom materials and industry. So, its exact measurement or calculation is very important. The aim of this study was to evaluate the μ in different points in the water phantom analytically as a formula, in addition to derive and parameterize it with dosimetry measurements data results. Materials and Methods: To find the attenuation coefficients at each point along the central axis of the beam in the phantom for every size of the fields, the first mathematical approach was performed for derivation of μs from percentage depth dose (PDD) formula. Then by dosimetry for different fields in different depths of water phantom, one can parameterize the obtained formula for μ in any field and depth. Results: By comparing the mathematical and dosimetry results, the parameters of the μ-expression were derived in terms of the dimension of square field in different depths. From this formula one can find the μ for any field in different depths for two energies of the Varian 2100CD linear accelerator, 6, 18MV with the statistical coefficient of determination of R2>0.98. Conclusion: The measurement of the μ in each field size and depth has some technical problems, but one can easily measure the μ for every point of central axis of the beams in any field size. Iran. J. Radiat. Res., 2012 9(4): 251-255