An analytical calculation of output factors for 6 and 18 MV photon beams in varian 2100C/D linear accelerator

Output factor (O.F.) is one of the important and influential quantity in the calculation of treatment time in radiotherapy. The calculation of O.F. by a polynomial formula has been considered by researchers since 1985. All the previous studies employed empirical/semi empirical formulae. In the present research we propose a formula based upon a logic base and physical principles. The function could predict the output factors for radiation fields of Varian 2100C/D. One could change the variables in terms of and in the main function to obtain a new function g (u,v). Expanding Tailor's series for g (u,v) around point (10,10), and neglecting the second higher orders. According to this expansion and symmetry of the photon fields, the first and second orders of differentiation of the function are constant. One can measure the dose of at least three radiation fields for obtaining the constants O.F formula. The later formula could be used to calculate O.F for other radiation fields. These calculated results are compared with the experimental data. The calculated O.F data for radiation fields 5×5 to 40×40 cm2 have been compared with the experimental data for radiation energies 6 and 18 MV. The difference between the calculated and experimental data for the small radiation fields is more than that for the large fields because dimension of ionization chamber is large compare with the field size. This error is not important since the usual field sizes in radiotherapy are not small. Therefore this formula is suitable to be used in radiation therapy.