Effects of extended SSD on electron-beam depth-dose curves

The effects of extended source-to-surface distance (SSD) on the electron-beam depth dose curves were examined for five electron beam energies ranging from 6 MeV to 20 MeV. The depth dose curves were measured with the water phantom surface set at 100 cm, 110 cm, and 120 cm from the source. Except for the dose in the build-up region, negligible depth dose curve changes due to extended SSD were observed for electron energies less than or equal to 12 MeV. For electron beams greater than 12 MeV, the percent surface dose decreases and, additionally, the depth of maximum dose shifts deeper into the phantom as the SSD increases. Changes in the rapid falloff region of the depth dose curves were apparent only for the higher energy electron beams, in particular 20 MeV. Extended SSD has no observable effect on the tail portion of the depth dose curves. As such, the energies of the electron beams are not changed significantly by increasing the SSD.