Calculation of build-up factor of Co-60 by using of tissue air ratio concept

When a primary radiation interacts with a medium, the secondary radiations are produced, that the contribution of these secondary radiation should considered in the calculation of dose. Build-up factor is a dosimetric correction factor to assume effect of secondary radiation in calculation of dose. Calculation of this factor is very difficult and time consuming. Due to significance of this factor and difficulties in calculation of this factor in other method, the aim of this study is to calculate Build-up factor of cobalt-60 by using of Tissue Air Ratio concept in water Build-up factor equation for Co-60 obtained by using of general equation between Build-up factor and TAR. Build-up factors values calculated by using of this equation. Then, by using of TABLE CURVE software in several steps, general equation of Build-up factor obtained as a function of depth and field size. Build-up factor values and general equation of this factor were obtained as a function of depth and field size for Co-60 unit, to depth 30 Cm. graph of build-up factor plotted against depth for any field size, and physical formula of this factor was expressed as a function of depth for any field size with minimum regression (=.986) By increase of depth and field size due to increase of scattered radiation, build-up factors were increased. Build-up factors values in field size 35×35 have a good agreement with data in ANSI standard. (maximum differences=2.5%)