PRACTICAL APPROACH TO ELECTRON-BEAM DOSIMETRY AT EXTENDED SSD

This paper presents a practical approach to clinical dosimetry for ele ctron beams at extended source-to-surface distance (SSD). Characterist ics of electron beams from a Siemens MD-2 accelerator are presented at a nominal SSD of 100 cm and an extended SSD of 115 cm. Relative outpu t factors are measured at both 100 and 115 cm SSDs for a range of squa re field sizes for each beam energy. The change of output with SSD doe s not follow the inverse square law, if the nominal SSD is used. This deviation is larger for lower beam energy and smaller field sizes. An effective SSD, SSDeff, has been determined at d(max) for each beam ene rgy as a function of field size. A comprehensive approach to treatment time calculations is proposed.