Dynamic splitting of large intensity-modulated fields

The aims of this paper are to describe a method of splitting large intensit y-modulated fields that cannot be delivered as a single field and to verify the accuracy of our method. Some multileaf collimators may be operated in the dynamic mode to deliver intensity-modulated radiation treatments (IMRT) using the 'sliding window' technique. In this technique each pair of leave s sweeps over the treatment field while the beam is on. However, there are limitations on the width of the held that can be treated due to the limited length of the leaves. For instance, the leaf length of the Varian MLC is 1 4.5 cm. Since each leaf pair must travel from the left boundary to the righ t boundary of the beam aperture, the maximum width of the field aperture th at can be accommodated in one sweep of leaves is also limited to 14.5 cm, i n fact to a slightly smaller value. It has been shown that IMRT is more eff icient when used to plan and deliver the large and boost fields simultaneou sly. in such situations, the fields must be large enough to cover simultane ously the volumes of the gross tumour, microscopic disease and electively t reated regions. Such field sizes are often larger than 14.5 cm wide. In thi s paper, we present a dynamic 'feathering' technique to split the large int ensity-modulated fields into smaller fields. In this technique, the compone nt beams overlap each other by a small amount, and the intensity in the ove rlap region gradually decreases for one held component and increases for th e other. The sum of intensities remains the same as for the original field. This method eliminates the field matching problems associated with the con ventional step 'break' for static fields. The splitting process is integrat ed into the IMRT treatment procedure and the entire planning process is aut omated. Comparison of dose distributions calculated and measured in a phant om showed good agreement. Such a method can be applied to the 'step and sho ot' technique as well. IMRT fields of widths up to 25 cm can be delivered b y splitting only once, which is adequate for most treatments.