Radiation Dose under Lead Block in Split Field Irradiation Using Cobalt - 60 Machine

Abstract

          Background:  Radiation dose which important organs under lead block received from split field irradiation using Cobalt-60 machine can be indirectly estimated by manual calculation taking longer time, or by computer planning, costing very high. Moreover, the data of radiation physics of each Cobalt-60 machine and the ability to protect radiation of each lead block probably differ. Objective: To study the radiation dose under lead block which has thickness of 5 cm and wideness of about 4.6 cm at a distance of 80 cm from the radiation source in split field irradiation with source-axis distance technique using Cobalt-60 machine (Theratron 780C). Materials & Methods:  By studying the % primary transmission of the lead block then the % transmitted dose under lead block. The study was performed in water phantom using ionization chamber NE 0.60 cc Robust Farmer 2581 (polystyrene cap) when the lead block the located on central axis and off-axis 1 to 4 cm. The field size and measurement depth studied were 10x10 to 25x25 cm² and 0.811 to 11.811 cm respectively. Results: The % primary transmission of the lead block was about 4%. The minimum % transmitted dose for all field size and measurement depth studied was 5.2113% at measurement depth of 0.811 cm with field size of 10x10 cm² and on off-axis 4 cm whereas the maximum was 21.7330% at measurement depth of 11.811 cm with field size of 25 cm² on central axis. Conclusion: The lead block can reduce radiation dose down to about 96% Radiation dose under lead block depends on field size, location of the shield and measurement depth.