Dear Dariush,

I could not get your question. 

Roentgen is a unit which is only applicable in air for gamma or X-rays. It can not be measured in other materials. 

One Roentgen equals to 0.00877 grey. There is no conversion factor for other materials besides air.

Best wishes,

Ramin

Dear Dr Sadeghi

Actually the quantity f(medium) or simply the f factor is depends on the mass energy absorption coefficient of the medium relative to the air. 

f med = 0.876( (μ ̅/ρ)med) / ((μ ̅/ρ)air )

The physics of radiation therapy, Khan, table 8.1 list f factors for water, bone, and muscle under conditions of charged particle equilibrium for a range of photon energies. 

Best regards

Daryoush,

You are correct in stating that it is the ratio of the mass energy absorption coefficients.  You can look at a lot of these values at the NIST web site:

http://www.nist.gov/pml/data/xraycoef/

Look at Table 3 for elements and Table 4 for compounds and mixtures including some standard tissues.