Experimental Verification of the Application of Lateral Build-up Ratio on the 4 MeV Electron Beam

Abstract

The Lateral Build-up Ratio (LBR) used to estimate the depth dose distribution of electron beams for an irregular cutout field was obtained for a 4 MeV energy beam from a Varian 21 EX linear accelerator. The depth dose curves for a group of circular cutout fields starting from a 2 cm diameter were measured. Electron diodes were used in a large water tank, to measure the LBR values for 6, 9, 12 and 16 MeV electron beam energies and a 10 cm x 10 cm applicator. The results agreed with the published data. When the same equipment, setup and technique were used to determine the LBR values for the 4 MeV energy beam, the values were only reasonable, being within the clinical treatment range (i.e. LBR < 1) for the smallest 6 cm x 6 cm applicator. The calculated LBR values were unreasonable for the circular cutout fields with a diameter larger than 2 cm with the 10 cm x 10 cm applicator. The difficulty in the LBR measurement may be due to the significant contribution of scattered electrons from the beam defining system. This study also focused on how well the sigma values for the 4 MeV beam can predict depth dose curves for other field sizes and whether the values are applicator dependent.