Clinical dosimetry of transmission factors for hard wedges and shielding trays used in T780 E telecobalt machines and its features

Abstract

In this study, three hospitals have been selected for this research project work, those hospitals are Badulla General Hospital, Kandy General Hospital and Tellippalai District Hospital. Brand of Theratron 780 E telecobalt machines manufactured by MDS Nodion, Canada were used in threc centers, as well as sets of wedges supplied by same vender. Wedge factors for different wedges were worked out and plotted against square field sizes. According to the measurements, wedge factors linearly increased with square field sizes for a particular wedge and shows nearly same wedge factor values for all centers. The important features of telecobalt machines are discussed including development history of telecobalt machines. Collimator scatter factors and shielding tray factors (beam shaping tray factors) were also measured for different field sizes. It was observed that collimator scatter factor linearly increased with field sizes from 5x5 cm² to 20x20 cm² at 80 cm SSD, due to the scatter radiation directly received by the sensitive volume of the ionization chamber. For large field sizes, scatter radiation due to the collimator and head assembly was not received by sensitive volume of the ionization chamber. Therefore the graph of scatter factor against field size becoming saturated hence formed a curve.