The effects of collimation designs on the depth dose

OBJECTIVES: The purpose of the present study was to investigate the effects of collimation designs on the depth dose distribution of close energy electron beams for two different linear accelerators (LINACs) with different types of secondary collimation systems. METHODS: Saturne-42 (trimmer) and Oncor (cone) linear accelerators were used for the study. Depth dose measurements were done at constant SSD with solid water phantom and plane-parallel ionization chamber. RESULTS: Depth dose distributions for both LINACs with the same nominal energies (9-12 MeV) and same incident mean energies (8.1-10.8 MeV) on phantom surface were found to be different. The discrepancies of the depth dose distribution curves were increased, with an increasing difference in incident mean energy. CONCLUSION: The current study shows that the depth dose parameters may vary between two different LINACs for electron beams although the same nominal energy is used. If differences are not within acceptable ranges, the machines should not be used in place of each other.