DOSIMETRIC VERIFICATION OF INTENSITY-MODULATED FIELDS

The optimization of intensity distributions and the delivery of intens ity-modulated treatments with dynamic multi-leaf collimators (MLC) off er important improvements to three-dimensional conformal radiotherapy. In this study, a nine-beam intensity-modulated prostate plan was gene rated using the inverse radiotherapy technique. The resulting fluence profiles were converted into dynamic MLC leaf motions as functions of monitor units. The leaf motion pattern data were then transferred to t he MLC control computer and were used to guide the motions of the leav es during irradiation. To verify that the dose distribution predicted by the optimization and planning systems was actually delivered, a hom ogeneous polystyrene phantom was irradiated with each of the nine inte nsity-modulated beams incident normally on the phantom. For each expos ure, a radiographic film was placed normal to the beam in the phantom to record the deposited dose. The films were calibrated and scanned to generate 2-D isodose distributions. The dose was also calculated by c onvolving the incident fluence pattern with pencil beams. The measured and calculated dose distributions were compared and found to have dis crepancies in excess of 5% of the central axis dose. The source of dis crepancies was suspected to be the rounded edges of the leaves and the scattered radiation from the various components of the collimation sy stem. After approximate corrections were made for these effects, the a greement between the two dose distributions was within 2%. We also stu died the impact of the ''tongue-and-groove'' effect on dynamic MLC tre atments and showed that it is possible to render this effect inconsequ ential by appropriately synchronizing leaf motions. This study also de monstrated that accurate and rapid delivery of realistic intensity-mod ulated plans is feasible using a dynamic multi-leaf collimator. (C) 19 96 American Association of Physicists in Medicine.