Effective Source-Surface Distance in Various Field Sizes and Electron Beam Energies and its Effect on Cutout Factor in a Elekta Precise Linear Accelerator

Introduction: In electron beam treatment, because of the non-point electron beam source, inverse-square law cannot be applied for dosimetry in different treatment intervals. Therefore, providing source-surface distance (SSD) charts in all clinics is of paramount importance. This study aimed to determine the effective SSD for various electron beam energies and field sizes and to evaluate its effect on cutout factor in a linear accelerator.   Materials and Methods: We used Elekta Precise linear accelerator in Ayatollah Khansari Hospital, Arak, Iran, for various energy levels (10, 15, and 18 MeV). The measurement environment was MP3-M water phantom (PTW Co., Canada), and diode detector was utilized for dosimetry. The effective SSD and cutout factor was estimated for 100, 105, 110, 115, and 120 cm SSDs and 1.5×1.5 and 20×20 cm2 square fields.   Results: The effective SSD in the 1.5×1.5 to 20×20 cm2 fields altered from 29.95 to 93.95 cm, 50.40 to 96.50 cm, and 63.51 to 95.32 cm for different energy levels of 10 MeV, 15 MeV, and 18 MeV, respectively. The cutout factor increased along with the field size, but decreased by extending the SSD. These alterations were more significant for the energy level of 10 MeV.   Conclusion: Since the effective SSD is dependent on energy level and field size, it is recommended to independently compute the effective SSD considering these variables. Furthermore, for designing accurate therapies, cutout factor variations should be considered for small-sized fields, especially at low energy levels.