Determination of therapeutic standard field for cancer patients including the medical megavoltage accelerator jaws movement.

Introduction: Accurate calculation of equal square field size is important in radiation therapy and A factor, in the Vadash formula must be determine by considering collimator exchange effect. Therefore, the purpose of this study was to determine the best A factor, for calculation of equal square field size in treatment planning   Materials and Methods: A new simple protocol method based on the selection of best dosimetric data was proposed to obtain A value in Vadash relation to predict the equivalent square field size, which compensates collimator exchange effect in output factors in air (OFair). Measurements performed with Farmer chamber 0.6 cc in SSD 100cm and build up cap of Plexiglas, 1.5 and 3.5 cm equals water, for 6 and 18 MV, respectively on a Perimus Plus linear accelerator. OFair measured for square and rectangular fields. MATLAB software (version R2014a) was used to calculations and curve fitting. Results: Power model with constant value applied to OFair as a function of square field size. OFair ranged from 0.983 to 1.038 in 6MV and from 0.731 to 1.05 in 18 MV and Y collimator had a greater effect on the OFair. The values of 1.42 and 1.55 were selected for "A" in 6 and 18 MV, respectively. Conclusion: The proposed A values minimize the collimator exchange effect, in calculating the equivalent squares which plays an important role in the patient dose calculation and treatment planning. The value of A should be calculated for each machine and energy.