Introduction: In the last few decades, a lot of monte carlo codes have been introduced for medical applications. this is important because through simulation, we are fully independent of the spectral scattering factor that we are not able measure in real terms. The effect of the radiation energy, the dimension of the radiation field, the sensitive volume of the ion chamber on the scene of the phantom scattering factor.
Materials and Methods: In this study, the MCNPX calculation code was used to simulate the Siemens linear accelerators to determine the measure of phantom scatter factor.
The studied fields were chosen in this study with dimension 3×3, 10×10 and 20×20 cm2.
It was also simulated to examine dosimetry parameters including deep dose curves and dose distribution for 6 and 15 MeV, a water phantom at SSD=100 cm in order to measure the phantom scattering factor and a total scatter factor and a build-up cap in order to measure the collimator scatter factor at SAD=100 cm.
 
Results: For field sizes 3×3 cm2 with 6 MV photon beams, the phantom scatter factor 0.9398, collimator scatter factor 0.8907 and the total scatter factor 0.8366, and also for the field sizes 20×20 cm2 with 6 MV photon beams, the phantom scatter factor 1.023, collimator scatter factor 1.0259 and the total scatter factor 1.0492 were calculated from monte carlo simulation.
The Experimental values are equal to this 0.9398×0.8907=0.8370 for field sizes 3×3 cm2 and 1.023×1.0259=1.0494 for field sizes 20×20 cm2.
All of the above values are normalized to field sizes 10×10 cm2 in 6 MV photon beams.
 
Conclusion: Due to the results obtained, the sensitive volume of the ion chamber is affected by the smaller than 10×10 cm2.
It has also obtained phantom scattering factor independently and verified with the S(c,p) = Sc × Sp relation.