A novel method for sub-millimetric determination of laser-radiation isocenter coincidence by means of a computed radiography system

Abstract

Introduction A procedure has been devised to check the coincidence of room lasers with the radiation isocenter of a Linac, by performing a starshot test on a Computed Radiography System (CR). Purpose Currently Winston–Lutz test, or a starshot film are the gold standard. However, the accuracy of these procedures relies on the skill of the staff member performing the check to precisely position a phantom or marking the film. We propose an observer independent method for conducting this test. Materials and methods A Konica CR System is used. The CR plate sensitivity to visible light will be used to accurately determine the laser position. We set the plate without its cover, so light can reach the active area. The plate is irradiated with an open beam that covers all the active area, with a low dose setting, to create a homogeneous background. The starshot pattern is generated by delivering nine 6 MV photon beams, 1 × 40 field size. The room lasers are switched on during a short time. We put the cover and read the cassette. The resulting image will consist of a grey background, a darker starshot pattern, and a white cross formed by the pixels reached by laser light. The image can be imported and analysed in PTW Isocheck (TM) software. Results With this method coincidence can be determined with an accuracy of 0.1–0.3 mm. Conclusion This procedure not only can be used as a substitute of Winston–Lutz test, but also enables improved accuracy. Disclosure No conflicts of interest to declare.