Determination of effective source to surface distance and cutout factor in small fields in electron beam radiotherapy: A comparison of different dosimeters

The main purpose of this study is to calculate the effective source to surface distance (SSD ) of small and large electron fields in 10, 15, and 18 MeV energies, and to investigate the effect of SSD on the cutout factor for electron beams a linear accelerator. The accuracy of different dosimeters is also evaluated. In the current study, Elekta Precise linear accelerator was used in electron beam energies of 10, 15, and 18 MeV. The measurements were performed in a PTW water phantom (model MP3-M). A Semiflex and Advanced Markus ionization chambers and a Diode E detector were used for dosimetry. SSD in 100, 105, 110, 115, and 120 cm SSDs for 1.5 × 1.5 cm to 5 × 5 cm (small fields) and 6 × 6 cm to 20 × 20 cm (large fields) field sizes were obtained. The cutout factor was measured for the small fields. SSDeff in small fields is highly dependent on energy and field size and increases with increasing electron beam energy and field size. For large electron fields, with some exceptions for the 20 × 20 cm field, this quantity also increases with energy. The SSD was increased with increasing beam energy and field size for all three detectors. The SSD varies significantly for different field sizes or cutouts. It is recommended that SSD be determined for each electron beam size or cutout. Selecting an appropriate dosimetry system can have an effect in determining cutout factor.