DOSE DISTRIBUTION CONSIDERATIONS OF MEDIUM-ENERGY ELECTRON-BEAMS AT EXTENDED SOURCE-TO-SURFACE DISTANCE

Purpose: To determine the effects of extended source-to-surface distan ce (SSD) on dose distributions for a range of medium energy electron b eams and cone sizes. Methods and Materials: The depth-dose curves and isodose distributions of 6 MeV, 10 MeV, and 14 MeV electron beams from a dual photon and multielectron energies linear accelerator were stud ied, To examine the influence of cone size, the smallest and the large st cone sizes available were used, Measurements were carried 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 increas es for all electron beam energies. The effects of extended SSD in the region beyond the depth of maximum dose are unobservable and, hence, c onsidered minimal. Extended SSD effects are apparent for higher electr on beam energy with small cone size causing the depth of maximum dose and the rapid dose fall-off region to shift deeper into the phantom, H owever, the change in the depth-dose curve is small, On the other hand , the rapid dose fall-off region is essentially unaltered when the lar ge cone is used, The penumbra enlarges and electron beam flatness dete riorates with increasing SSD.