Start-up characteristics of beam generated by Siemens linear accelerator

Objective: Low MU may change beam characteristics, and therefore to investigate beam start-up characteristics is an important part of quality assurance system for IMRT implementation. Methods: The main parameters describing beam characteristics are dose linearity, dose uniformity, and radiation quality. These parameters were measured for different numbers of MU on an accelerator (Siemens Primus). Results: When few MU were delivered, beam was always unstable and could not meet clinical requirements. When MU increased, beam became stable, and the parameters reach their normal values. The detailed data are as follows: (1) Before the parameter D1G had been adjusted, the linearity of dose delivered was bad; After the D1G was adjusted, the linearity of dose delivered was better, and when delivered MU was more than 2, the difference of dose per MU was less than 0.1%. (2) When MU was more than 2, the beam flatness and symmetry were well in clinic use. (3) When low MU were delivered, the radiation quality became softer. Conclusion: The beam start-up characteristics should be checked periodically for IMRT implementation. (authors)