Determination of the Collimator Scatter Factor by the Correction Sterling Formula for Asymmetrical Field Radiation Therapy with Megavoltage Photon beam

The output factor is an important factor in calculating the dose in the treatment of cancer patients with Megavoltage photon beams. Since in the most cases, the treatment field is asymmetric, therefore the calculation of this factor is very important.
Subjects and Measurements were done for Siemens Primus Plus accelerator, Golestan Hospital using Farmer ionization chamber 6cc. The output factors was measured for the symmetric fields and calibration curves were plotted for collimator scatter factor. For each symmetric square field, many asymmetrical fields were defined. Using analysis and comparison with measured values for Siemens Linac, a tuning parameter was calculated for each symmetric square field. Then, the tuning parameter was applied to calculate collimator scatter factor for the corresponding asymmetric field. Finally the calculation results were compared with experimental values.
The tuning parameter for the two symmetric fields 30 × 30 and 25 × 25 were 0.452±0.001 and 0.481 ± 0.001 respectively. Taking into account this tuning parameter for asymmetric fields, the maximum percentage difference between calculated and measured results, was 0.7%.
Results indicate a reduction in the asymmetric collimator scatter factor relative to the symmetrical field. In addition, with increase in distance from center of symmetry, it becomes more significant decline. Also according to the calibration curve, the equivalent square field side for every asymmetric field can be obtained.