Localization of electron virtual SSD in a Siemens-Primus linear accelerator: Comparison of measurements with Monte Carlo simulation

Introduction: Because of importance of impact of the Source to Surface Distance (SSD) in determining of monitor unit for electron-therapy, it is essential to know the Virtual Source Position (VSP) for electron beam for a linear therapeutic accelerator for each energy and field size. , especially using the Khan method (photo distance squared method) And compare the results with Monte Carlo.   Materials and Methods: The VSP was measured for a Siemens- Primus linear accelerator for all available electron beams (5, 7, 8, 10 and 12 MeV) and for five applicator sizes (5×5, 10×10, 15×15, 20×20, 25×25 cm2) available at the department in Mahdieh Radiotherapy and Oncology Charity center in Hamadan. A theoretical method (known as Khan’s method) was applied to estimate VSP for all electron energies and for all potential applicators. A valid MCNPX Monte Carlo code was then used to evaluate the results achieved through the measurements.   Results:The results showed a significant difference between Monte Carlo those were found to be ~4% and 5.2% for F4 and F8 tally approaches, respectively.   Conclusion: MonteCarlo simulation code with correction factor can be used as an alternative approach to verify measurements acquired for VSD and other datasets applied for treatment Planning Systems.