COMBINING MULTILEAF FIELDS TO MODULATE FLUENCE DISTRIBUTIONS

Purpose: A method for modulating beam fluence from a linear accelerator is discussed. The beam modulation is accomplished remotely using a multileaf collimator and does not require entering the treatment room. Methods and Materials: The multileaf collimator is used to define a series of field shapes that are superimposed at a fixed gantry angle to produce any desired fluence pattern. A heuristic technique for deriving the field shapes and corresponding monitor unit settings is described. The technique has been tested on randomly generated fluence distributions and on distributions with a limited number of peaks and valleys. The second type of distribution more closely simulates fluence patterns obtained with dose optimization software. Estimates of the time required to use this approach to treat a four-field plan are given and compared to the technique of placing a physical compensator in each beam. Results: It has been demonstrated that complex fluence patterns within a 15 X 15 cm2 field can be achieved with less than 20 fields. Estimates show that this technique is faster than entering the treatment room to change physical compensators. Some limitations of the method are discussed. Conclusion: Optimized distributions that conform the dose to irregularly shaped target volumes that wrap around critical structures are possible using superimposed multileaf fields. A method for defining the field shapes is presented.