Dose distribution considerations of medium energy electron beams at extended source-to-surface distance

Purpose : To determine the effects of extended source-to-surface distance (SSD) on dose distributions for a range of medium energy beams and cone sizes. Methods and Materials : The depth-dose cures and isodose distributions (of 6 MeV, 10 MeV, and 14 MeV electron beams from a dual photon and multielectron energies linear accelerator were studied. To examine the influence of cone size, the smallest and the largest cone sizes available were used. Measurements were carreid out in a water phantom with the water surface set at three different SSDs from 101 to 116 cm. Results : In the region between the phantom surface and the depth of maximum dose, the depth-dose decreases as the SSD increases for all electron beam energies. The effects of extended SSD in the region beyond the depth of maximum dose are unobservable and, hence, considered minimal. Extended SSD effects are apparent for higher electron beam energy with small cone size causing the depth of maximume dose and the rapid dose fall-off region toshift deeper into the phantom. However, the change in the depth-dose curve is small. On the other hand, the rapid dose fall-off region is essentially unaltered when the large cone is used. The penumbra enlarges and electron beam flatness deteriorates with increasing SSD.