Monte Carlo simulation of photon beams of radiotherapy from Clinac 2100

In this work we use the M.C.N.P.4.B. Monte Carlo code to simulate the photon fluence in the beam of a radiotherapy machine. This study is of great importance when there is no completed information about the target material and the flattering filter used in the accelerator which is wanted to simulate with the code. From the obtained technical data about the set-up of the machine head of the Clinac 2100 equipment of nominal potentials of 6 MV and 15 MV, given by Varian maker and available in the literature, it was calculated the percentage depth dose for a field of 4 x 4 cm2, allowing thus, to obtain the photon spectrum of the equipment that work under 6 MV and 15 MV potential. The validation of the method was done comparing the obtained spectrum in the simulation with the spectrum obtained in the literature, and also comparing the P.D.D. measured and calculated. The spectrums used for comparison are results from simulations of Monte Carlo code. The average energy for each obtained spectrum agrees with the one of the literature about ± 5%. The P.D.D. calculated by M.C. were obtained with an error of 1% and with this error they agree with the P.D.D. measured for small fields. With the obtained results we could conclude that the approach used in this work do not induce significant errors to the results and at the same time they considerably reduce the time for the equipment that it is usually necessary in this kind of simulation. (authors)