Electron beam Therapy at extended diatances

The nominal distance for electron beam treatment is 100cm source to surface distance SSD. However, certain clinical conditions need an extended distance to increase the available irradiated field.
the study was performed on an electron beam produced from the Elekta Linear accelerator at four different energies: 6, 8, 10, and 15 MeV, both absolute dose and relative profiles were measured.
1- The absolute dose was measured with Roos Ion Chamber inserted in slap phantom at 100 cm SSD and gradually extended distances (100: 140)cm SSD, then compared to the calculated dose in Elekta Precise treatment planning system.
2- Electron beam fluence was measured with PTW 2D array to get the beam fluence within the irradiated field, evaluate the effect of extension in beam profiles, whether extended distance influence flatness, symmetry, penumbra and how to overcome this effect for suitable coverage when treated at extended distances.