There is a lack of extensive data comprising energy spectra of high energy X-ray therapeutic beams used in teleradiotherapy, generated by medical linear accelerators. The energy spectra are not easy to determine because the experimental methods are very difficult to perform whereas the Monte Carlo calculations need a suitable computer power. Authors are usually presenting X-ray spectra that are determined in the air. However, the spectra in the air cannot be generally used to characterize accurately beam quality in another irradiated medium. In my investigations the energy spectra for the 6 MV X-ray beam were determined along the beam central-axis in the air and water - a medium recommended by the dosimetry protocols. The spectra were derived using the Monte Carlo method for open (3 cm x 3 cm, 10 cm x 10 cm and 40 cm x 40 cm) and wedged fields (3 cm x 3 cm and 10 cm x 10 cm) from Clinac Varian 2300. The simulation programs were verified by comparing the calculated depth-dose characteristics and profiles (for various primary energies) with already measured characteristics and profiles with use of the Markus ionization chamber and also by comparing calculated spectra and characteristics presented by different authors. This work showed that the shapes of spectra from different medical linacs depends strongly on the radiation field size and depth in water phantom. The second stage was reconstruction of energy spectra from measurements of transmission of X-ray beam. Measurements were performed by using Wood’s metal for Clinac Varian 2300 CD and CyberKnife for few radiation field sizes.
A detailed knowledge of the energy spectra of therapeutic beams from medical linacs is essential for calculating of the stopping power ratios or the beam quality correction factors and for dose calculation algorithms in advanced treatment planning systems, for investigations of treatment machine head design etc.