Source of Support: None, Conflict of Interest: None
Many investogators have reported that there is some reduction in the absorbed dose at the exit surface of the body when megavoltage radiations are used. In situations, where larger or multiple fields are used, a part of the beam at the exit portion meets the metallic or rigid surface of the table top. This introduces additional backscattered photons and electrons which enhances the exit surface dose. This increase in the exit dose because of the exit surface touching the metallic portion of the table top has been measured using a commercially available parallel plate chamber. The ionisation Ratio (IR) between the chamber reading with and without table top is a measure of the increase of dose. IR is reported for three different 60Co teletherapy machines and a Linear Accelerator (for 6 and 18 MV) for the field sizes 5 x 5,10 x 10,15 x 15 and for various thicknesses of the phantom (8 to 23 cm).