Independent verification using portal imaging of intensity-modulated beam delivery by the dynamic MLC technique

The use of intensity-modulated radiation fields in radiotherapy treatment has been shown to have the potential to deliver highly conformal dose distributions. One technique for delivering these intensity-modulated beams is a computerized dynamic multileaf collimator (MLC). A major current impediment to the development of dynamic MLC therapy is verification of these highly complex treatments. Electronic portal imaging is shown here to be a solution to this verification problem. Experimental results are presented showing that leaf penumbra measured with a portal imager can be used to accurately define the positions of moving leaves. The random error in these leaf positions is compared with mean leaf positions along each leaf bank and specified leaf positions at prescription control points to check mechanical performance. Individual leaves are also checked for systematic motion errors. All leaf positions are found to be well within the manufacturer's specifications at all times. Finally, integral intensity images are presented that can be related to the dose distribution delivered. Portal imaging is shown to have the potential to become a valuable tool for the verification of dynamic MLC irradiation.