Determination of Correction Coefficient of the Sterlling's Formula

For high energy x-ray beams used in radiation therapy the head-scatter factor (Sc) account for the variation in scattered radiation with collimator setting of the incident beam that reaches the point of measurement on the central axis. The scattered radiation from the jaws that determine dimensions of treatmet field, is due to high thickness of jaws that are not located at the same level of the patient's body. Therefore scattered of them on the treatment area is not the same. This makes the replace of treatment field Beacuase of are not symmetric. the Sterlling's formula for the equivalent fields must be corrected.The correction of each system takes place by applying the coefficient in Sterlling's formula that iscalled the Vadash̒s factor which is important in the treatment planing of patients. Subjects and By measuring the amount of Sc for square fields,the variation of Sc in terms of square field size was plotted, the relationship between Sc and field size was determined by Exel software.By this relation, the equivalent square fields for rectangular fields attained and average Vadash's factor) was calculated for them.A selection of different values around this average, for every energy (6MV and 18MV) the value of A were determined, finally A that total variance around the average was minimum is desirable for Vadash,s factor. Vadash's factor for energy 6MV and 18MV Linac Varian 2100 CD are obtained 2.14 and 2.29 respectively. According to the collimator exchange effect, the Sterlling's formula needs a correction by applying the physical analysis of Sc for various energy, this correction is extractable.