Introduction: In radiation therapy, to calculate the dose absorbed by the patient, parameters such as Percentage Depth Dose (PDD), Tissue-Air Ratio (TAR)_, and Peak Scatter Factor (PSF)   etc. were used. For high energy photon beams, measurement of the TAR is subjected to some difficulties and because of the contamination of photon with electron it is inaccurate. The goal of this study was to find a formula for TAR from PDD and then calculate other dosimetry parameters as PSF and S_P.
 
Materials & methods:  PDD at different treatment depths for field size 5 × 5 to 40× 40cm ² was measured, then measuring the TAR for all treatment fields at the reference depth (10cm) and using the Tbl Curve Software, an analytical equation for TAR in 6MV and 18MV photon energies was obtained. Other treatment parameters S _C , were calculated from this equation.
 
Findings: The PSF value was greater than one; this value ranged from 1.03 to 1.18 for 6MV and ranged from 1.01 to 1.10 for 18MV. Both PSF and S_P increase with field, but didn’t have a significant and regular change with energy.
 
Discussion & Conclusion: There is a good consistency between obtained values and reported values. Determine of PDD in megavoltage treatment make calculate of TAR, PSF and S_P easy and possible. Using this formula prevents repetition of measurements and also it prevents the force on the accelerator.