Calculation of The Relative Tray Transmission Factor For The Co-60 Teletherapy Unit

Relative tray transmission factor (RTF) have been determined for two types of trays as a function of field size and source surface distance (SSD) for Co-60 gamma ray beam with energy (1.25 MeV). The results show an increase of the relative tray transmission factor with increase field size up to (2.22) and (1.176) for the PMMA tray, and Al tray respectively when the field size is (25 cm x 25 cm) at (SSD = 60 cm(.
This result indicates that the relative tray transmission factor variation with field size is related to a change of the primary photon flounce. Our study also shows that the (RTF) decreases with increasing source surface distance (SSD) from (60 cm) to (80 cm) for the same type of trays due to a reduced contribution to the total dose from photons scattered in the trays.