An algorithm has been developed to calculate the collimator jaw motions required to generate intensity-modulated fields far use in conformal radiotherapy. The dynamic technique allows arbitrary intensity profiles to he generated using a single unidirectional sweep of the collimators. The collimator jaws have independent motion, so that an aperture of variable width is scanned across the field. The algorithm has the form of a constrained optimization problem and jaw motions are optimized such that the treatment time for the field at a given dose rate is minimized. The application and results of this technique are presented, and it is shown that this approach provides an efficient practical implementation of conformal radiotherapy plans based on the use of intensity-modulated fields. The technique can be extended to 3 D treatment plans and fields through the use of computercontrolled multileaf collimators.

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Highly conformal dose distributions can be generated by intensity-modulated radiotherapy. Intensity-modulated beams (IMBs) are generally determined by inverse-planning techniques designed to maximize conformality. Usually such techniques apply no constraints on the form of the IMBs which may then develop fine-scale modulation. In this paper we present a technique for generating smoother IMBs, which yields a dose distribution almost identical to that without the constraint on the form of the IMBs. The method applies various filters successively at intervals throughout the iterative inverse planning. It is shown that the IMBs so determined using a simple median window filter have desirable properties in terms of increasing the efficiency of delivery by the dynamic multileaf collimator method and may be 'more like conventional beams' than unconstrained, highly modulated IMBs.

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.

An algorithm to generate discrete beam-intensity modulation by dynamic multileaf collimation is presented which incorporates constraints on minimum allowed leaf separations. MLC positioning information is derived simultaneously for all leaf pairs and back-up diaphragms as they progress across the field. A feedback mechanism allows corrections to be applied to eliminate potential violations of minimum separation conditions and any underexposure in the interleaf tongue-and-groove region as they are encountered. The resulting motion correctly delivers the intended modulation and is physically realizable. Implementation of the algorithm is described. Results of the algorithm can also alternatively be interpreted as defining a series of static fields to deliver the same modulation.