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Introduction: One of the important input factors in the commissioning of the radiotherapy treatment planning systems is the phantom scatter factor (Sp) which requires the same collimator opening for all radiation fields. In this study, we have proposed an analytical method to overcome this issue. Methods: The measurements were performed using Siemens Primus Plus with photon energy 6 MV for field sizes from 5×5cm2 to 40×40cm2 . Phantom scatter factor was measured through the division of total scatter output factors (Scp), and collimator scatter factor (Sc). Results: The mean percent difference between the measured and calculated Sp was 1.00% and -3.11% for 5×5, 40×40 cm2 field size respectively. Conclusion: This method is applicable especially for small fields used in IMRT which, measuring collimator scatter factor is not reliable due to the lateral electron disequilibrium