tissue inhomogeneity in proton therapy and investigation of its effects on bragg peak by using mcnpx code

background: hadron therapy for malignant tumor is becoming increasingly popular. there are many factors which effect on implementation of a proper treatment planning. the purpose of this work is to investigate the inhomogeneity effects as affecting factor on proton range, full width at half maximum (fwhm) and 20% position of penumbra (p20) by mcnpx code. materials and methods: an inhomogeneous tissue (bone or adipose) with a variable thickness (0.1 cm, 0.5cm and 1.0 cm) was inserted into a phantom. then this phantom was irradiated by 108.8 mev proton beam located at 10 cm away from it. particle energy deposition (pedep) in mesh tally card was used for simulation of bragg curve and obtaining the proton range, fwhm and 20% position of penumbra (p20). finally, the mcnpx results were compared with geant4. results: the results of mcnpx for water phantom including 1cm adipose for range and p20 were 8.87 and 9.10 cm respectively, and for 1cm bone, range and p20 were 8.52 and 8.58 cm. by increasing the adipose thickness, range and p20 were reduced but it was vice versa for increasing the bone thickness. also fwhm does not show a regular variation in both bone and adipose tissues. conclusion: the results show that the location of the inhomogeneous tissues does not affect the variation of the whole shape of the bragg curve, while it radically affects the dose in the bragg curve. the comparison between mcnpx and geant4 results showed that the mcnpx results have the closer values than geant4 to references.