Background: Monte Carlo method (MC) has played an important role in design and optimization of medical linacs head and beam modeling. The purpose of this study was to compare photon beam features of two commercial linacs, Varian 21EX and Elekta SL-25 using MCNP4C MC code.

Materials and Methods: The 6MV photon beams of Varian 21EX and Elekta Sl-25 linacs were simulated based on manufacturers provided information. Photon energy spectra and absolute absorbed dose values were calculated for field sizes of 10×10 and 20×20 cm². Also, contamination electron spectra for field size of 20 ×20 cm² were scored for both linacs.

Results: Our results showed that the relative absorbed dose values and contamination electron spectrum were similar and comparable, but photon fluence and absolute absorbed dose values were 17% and 13% higher for Varian linac respectively for the field size of 10×10 cm².

Conclusion: Despite the differences in head components of two commercial linacs, their relative depth dose values were very close to each other. The absolute dose per incident electron showed some discrepancy, as well. Thus, this study suggests the use of absolute absorbed dose values as an invaluable factor when different linacs head are compared using Monte Carlo Method.