Dependence of wedge transmission factor on co-60 teletherapy treatment depths and techniques

Abstract

Measuring the wedge factor (WF) for radiation field of 10 x 10 cm2 at a specified depth and Source to Surface Distance (SSD), and applying the value to all treatment depths and technique could introduce errors > ± 5 % of threshold stipulated for patient radiation dose delivery. Therefore, some Treatment Planning Systems (TPSs) provide for inputs of separate Percentage Depth Dose (PDD) and Tissue Phantom Ratio (TPR) data for wedged fields to account for WF dependence of treatment depths and techniques. Hence, relatively more measurements than usual are taken per wedge filter and photon energy to establish a TPS and obtain dosimetric data for estima-ting treatment time for wedged fields, which required sophisticated equipment and procedures. While many On-cology Centres rely on International PDD and Tissue Maximum Ratio (TMR) data, benchmark data for wedged beams are not readily available. To provide radiotherapy dosimetry of high accuracy and expediency, two emp-irical equations were developed for a GWGP 80 Cobalt-60 teletherapy machine at the Oncology Department, Korle Bu Teaching Hospital (Ghana). The equations were validated via linear interpolations by measuring WFs at various treatment depths using Source Axial distance (SAD) and SSD treatment techniques. The approach required only measurements of WF for a 10 x 10 cm2 field at depth of 5 cm employing SSD treatment technique per wedge filter. Using the empirical equations, WFs were determined to within ± 0.50 % of the measured valu-es over the entire treatment depth range of 1.5 to 15.5 cm for SAD and SSD treatment techniques respectively; and WFs could be obtained for any treatment depth and technique.