A modified formula for defining tissue phantom ratio of photon beams

Tissue phantom ratio (TPR), for square fields of various dimensions has been determined at varying depths in water. The dose in water has been measured at a fixed source-to-surface distance (SSD) of 100 cm and reference depth of 5 cm for 6 MV photon beam of Siemens Linear Accelerator Primus II in German Cancer Research Center (DKFZ), Heidelberg, Germany. A modified formula has been developed to calculate the TPR value for isocentric treatment. The present article describes the conversion of the measured data values into a comprehensive and consistent data set by the modified formula, that gives the TPR from Percentage Depth Dose (PDD) with depth as a function of field sizes from 10 mm x 10 mm up to 300 mm x 300 mm) and depth (from 0 mm to 300 mm). DOI = 10.3329/bmrcb.v33i3.1140 Bangladesh Med Res Counc Bull 2007; 33: 92-97