Dosimetric analysis of beam-matching procedure of two similar linear accelerators

Linear accelerators from the same vendor in a radiation therapy center are usually beam-matched following Vendors acceptance criteria. This protocol is limited to check the difference at particular points on the ionization curve for depth dose or beam profiles. This article describes different tests done after commissioning to evaluate the level of agreement between matched beams of two ONCOR Impression plus linear accelerators from Siemens. Total scatter factors, collimator scatter factors, wedge transmission factors were measured in water for 6-MV photon. All these factors for ONCOR2 were within +/- 1% of those values for ONCOR1. Along with these point dose measurements we have essentially used -index to compare the planar dose distribution from two beam-matched accelerators. For this study a set of ready packed EDR2 films was exposed on both accelerators. The set consisted of films for percentage depth dose, beam profiles, a pyramid shape, multileaf collimators positional and dose delivery accuracy, and a film to compare head scatter at tray level. To include treatment planning system calculations, a film kept in axial plane was exposed to 3DCRT and IMRT plans with actual gantry angles and monitor units. These films were analyzed for gamma in OmniPro IMRT software using different combinations of Delta dose and -Delta distances. All these films have shown good agreement for - Delta distance of 3 mm and dose of 3 %.