Background: The lack of lateral electronic disequilibrium (LED) becomes a main problem in small field. This factor affects the dose in target volume cannot predict correctly. In addition, utilization of high-energy linear accelerator (10 MV) can emit some unwanted particles (electron contamination). Therefore, the aim of this study was to characterize head linear accelerator (linac) Varian Clinac iX 10 MV photon beam for square small field size (1×1, 2×2, 3×3, 4×4 and 5×5 cm²) using Monte Carlo (MC) simulation.
Materials and Methods: The commissioning process for this linac, has been conducted for field size 6×6, 10×10 and 20×20 cm² by comparing the measurement and MC simulation data. Head linac simulation was performed with BEAMnrc and dose calculation with DOSXYZnrc. The phase space (phsp) data from BEAMnrc was analyzed using BEAMDP to get the particles information in scoring plane. Results and Discussion: The scatter angle of particles depends on the field size. This factor affects the penumbra width in water phantom. On the other hand, PDD data show that the depth of maximum dose and penumbra width in small field shifted correspond with the number of scatter particle. The difference of relative output factor between measurement and MC results were found less than 2%. However, the 2% difference was still acceptable in photon beam dosimetry. Conclusion: From this simulation, the electron contamination give contribution in surface dose of water phantom about 13.0581% and less than 1% for field size 10×10 cm² and small field size, respectively.