Derivation of TMR from PDD in Absence of Scatter Factors

There are some complications for TMR(Tissue Maximum Ratio) calculation in radiotherapy treatment planning system. For example in TMR calculation from PDD(Percentage depth dose), BSF(Back Scatter Factor) or measurement is difficult and inaccurate. The goal of this study was to calculate TMR from PDD in absence of scatter factors and its conversion to a computer software for promotion of treatment planning system(TPS) in radiotherapy departments. Subjects and Measurements of PDD was performed using Varian and Siemens accelerators at Ahvaz Golestan Hospital radiotherapy department, by ionization chamber CC13 and scanditronix water phantom, for both 6 and 18 MV therapeutic energies. Then an analytical equation for TMR calculation from PDD was defined. By this equation TMR values for square fields in different depths for 2-50MV therapeutic energies were determined. Finally we employed MATLAB software to produce a computer program to be used in TPS software. There was a good agreement between TMR values calculated by presented analytical equation and TMR values in the BJR(British Journal of Radiology) report. In most energies for therapeutic fields, the difference between analytical and BJR values was less than 2% for depths up to 10 cm, and less than 4% for depths up to 20cm. The presented analytical equation can quick calculate and quantify TMR for all therapeutic fields and energies.