Leaf position verification during dynamic beam delivery: A comparison of three applications using electronic portal imaging

The use of a dynamic multileaf collimator (MLC) to deliver intensity-modula ted beams presents a problem for conventional verification techniques. The use of electronic portal imaging to track MLC leaves during beam delivery h as been shown to provide a solution to this problem. An experimental compar ison of three different verification systems, each using a different electr onic portal imaging technology, is presented. Two of the systems presented are commercially available imagers with in-house modifications, with the th ird system being an in-house built experimental system. The random and syst ematic errors present in each of the verifications systems an measured and presented, together with the study of the effects of varying dose rate and leaf speed on verification system performance. The performance of the three systems is demonstrated to be very similar, with an overall accuracy in co mparing measured and prescribed collimator trajectories of approximately +/ -1.0 mm. Systematic errors in the percentage delivered dose signal provided by the accelerator are significant and must be corrected for good performa nce of the current systems. It is demonstrated that, with suitable modifica tions, commercially available portal imaging systems can be used to verify dynamic MLC beam delivery. (C) 2000 American Association of Physicists in M edicine. [S0094-2405 (00)00607-6].