In many experimental and simulation researches, water phantom is used instead of most body organs. Therefore, in this
study, we replaced the water phantom instead of some organs to calculate its effect on the proton stopping-power, and
range and the consequence of deposited energy and microdosimetric spectra in small sites. Some organs such as the
spleen, thyroid, pancreas, prostate, testis, and ovaries are considered. We calculated the proton stopping-power in these
organs using the SRIM code. Then using these results, we wrote a program in the programming language of Fortran
and computed the proton range and deposited energy in two sites of 1 and 100 micron. Also, using the Geant4-10-4
code, we simulated these sites and obtained microdosimetric spectra of protons at 1 and 5MeV energies. In order to
compare different states, the frequency-mean lineal energy, dose-mean lineal energy, these statistical uncertainties and
absorb dose in each case were calculated and reported. Also, we estimated the statistical uncertainty of quantities with
a new formula. We observed that using water instead of the organs causes a significant error in the calculations of the
range and the maximum relative difference percentage of 18% and 22% in deposited energy in 1 and 100 micron sites,
respectively. These differences depend on the energy of the incident proton, organ, and size site. Also, this replacement
changes microdosimetric spectra, the location, and intensity of the Bragg’s peak. The percent difference of location and
intensity of the Bragg’s peak for water instead of the spleen is -8.66 and 13.42%, respectively. Therefore, using
water instead of the body organs in microdosimetry calculations is not recommended