Experimental verification of the application of lateral buildup ratio on the 4-MeV electron beam

The lateral buildup 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 x 10 cm2 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 x 6 cm2 applicator. The calculated LBR values were clinically unacceptable for the circular cutout fields with a diameter larger than 2 cm with the 10 x 10 cm2 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.